Human exposure to air contaminants in sports environments

The aim of this review was to investigate human exposure to relevant indoor air contaminants, predictors affecting the levels, and the means to reduce the harmful exposure in indoor sports facilities. Our study revealed that the contaminants of primary concern are the following: particulate matter in indoor climbing, golf, and horse riding facilities; carbon dioxide and particulate matter in fitness centers, gymnasiums, and sports halls; Staphylococci on gymnasium surfaces; nitrogen dioxide and carbon monoxide in ice hockey arenas; carbon monoxide, nitrogen oxide(s), and particulate matter in motor sports arenas; and disinfection by-products in indoor chlorinated swimming pools. Means to reduce human exposure to indoor contaminants include the following: adequate mechanical ventilation with filters, suitable cleaning practices, a limited number of occupants in fitness centers and gymnasiums, the use of electric resurfacers instead of the engine powered resurfacers in ice hockey arenas, carefully regulated chlorine and temperature levels in indoor swimming pools, properly ventilated pools, and good personal hygiene. Because of the large number of susceptible people in these facilities, as well as all active people having an increased respiratory rate and airflow velocity, strict air quality requirements in indoor sports facilities should be maintained.

particulate matter with an aerodynamic diameter of <10 µm and <2.5 µm, respectively) emitted by the ice resurfacers, 3,11,12 and indoor swimming pools, with a focus on high concentrations of disinfection by-products (DBPs) (eg, trihalomethanes (THMs)). 13,14In addition to these pollutants, also for example, ultrafine particles (UFPs), volatile organic compounds (VOCs), aldehydes (eg, formaldehyde (HCHO)), ozone (O 3 ), and bioparticles (fungi and bacteria)) could be cause for health effects in some sports facilities.Besides, important comfort parameters-temperature (T) and relative humidity (RH)-may affect for example material emissions [15][16][17] and occupant's perception of the indoor air quality (IAQ) 18 and should be taken into account when evaluating exposure issues in sports facilities.
However, the knowledge of the exposure to these pollutants in different sports facilities is very scattered and limited.
Like other indoor places, IAQ in indoor sports facilities is affected by the type of ventilation, building materials, and building maintenance, [65][66][67][68] but what makes indoor sports facilities different is the higher human occupancy and the type of activity taking place inside. 68In addition, unlike other indoor areas, many different plastic and rubber materials (mats, exercise equipment, cushions, etc) are used in indoor sports facilities, and there is a lack of knowledge about their effects on IAQ and human health.0][71] Indoor exercise facilities may be located in buildings not originally designed as sports facilities; thus, the dimensioning of ventilation relative to the activity in the facility may be insufficient.Chemical emissions from materials used in indoor sports facilities as well as the amount and quality of chemicals used will be further emphasized in the future through the Energy Performance of Building Directive 2010/31/ EU (EPBD). 72The directive requires energy savings for ventilation, which can increase the level of indoor air pollution (eg, chemical and particulate contaminants).
In contrast to working environments and ambient air, legally binding regulations as well as other guidelines are only partially established for indoor pollutants 20,21,73,74 (see Table S1 in the Supporting Information (SI)), and those proposed regulations and guidelines do not take into account specific actual exposure and risk during sport activities.Professional athletes and amateurs can be at special risk when they are exercising in polluted environments because 1) their respiratory rate increases proportionally to the quantity of inhaled pollutants; 2) the increased airflow velocity carries gaseous pollutants deeper into the respiratory tract; and 3) most of the air is inhaled through the mouth, bypassing the normal nasal filtration of large particles. 3,6,68,75e to that the most important international comfort standards, ASHRAE 55 (given by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE)) 76 and ISO 7730 (given by the International Standards Organization (ISO)) 77 are mainly applicable to sedentary activity, such as office work.Own comfort recommendations for T and RH in sport facilities are needed and recently proposed by the American College of Sports Medicine (ACSM) and the International Fitness Association (IFA) (see Table S1 in the SI).
The aim of this study is to collect and summarize information on the exposure to relevant indoor air contaminants in different types of sports facilities, predictors affecting the occurrence and concentrations of those contaminants, and the means to reduce harmful exposure to air contaminants in sports facilities.

| Search strategy and eligibility criteria
Web of Science, SCOPUS, Google Scholar, and PubMed were primarily used to search for literature published between 1999 and 2020.Other electronic databases available from Aalto University and Queensland University of Technology were also consulted.
The literature search was conducted from October 2019 to May 2020.Altogether, different combinations of 76 search terms were used (see Table S2 in the Supporting Information (SI)).The search strategy included combinations of at least four terms simultaneously, and each combination included at least two of the following terms each time: indoor air, sports facilities, environment,

Practical Implications
• Elevated levels of CO 2 , PM 10 , and PM 2.5 cause problems in fitness centers, gymnasiums, and sports halls.
• Exposure to CO and NO 2 is the main problem in ice skating arenas.
• Exposure to disinfection by-products (DBPs) such as CHCl 3 and NCl 3 is the main concern for indoor swimming pools.
• Air temperature and humidity should be in the thermal comfort range.
• Air quality guidelines are required for DBPs.exposure, physical exercise, and concentration.Quotation marks were used between search terms.Original peer-reviewed scientific journal articles, literature reviews, and conference articles (full papers) were included.The search was then extended to the reference lists of relevant articles, based on their abstract and/or full text.The decision to examine certain articles in more detail was based on the article titles.From the more than 400 publications identified in the initial search, 288 were selected for inclusion in the analysis.All publications reporting indoor air quality (IAQ) in different indoor sports facilities, the measured IAQ parameters, and information on the location of the studied facility (city and country) are listed in Table S3 in the SI.

| Definition of the different type of facilities
We classified the publications about different indoor sports facilities into nine categories: fitness centers, gymnasiums, ice hockey/ice skating arenas, indoor climbing facilities, indoor golf courses, indoor horse riding arenas, indoor motorsport arenas, indoor swimming pools, and sports halls.
Fitness centers have specific characteristics, such as types of occupants and activities, layout, equipment, daily patterns of use, and even frequent operation compared to other types of sports environments, like school or university gymnasiums and competitive sports arenas. 79,80The term "fitness" identifies a range of activities conducted every day in fitness centers or gymnasiums, and these activities can be grouped into resistance training activities, group fitness activities, and functional fitness activities. 81Typical spaces in a fitness center are classrooms (eg, for indoor cycling), studios (eg, for yoga and pilates), and gymnasiums (eg, for strength training). 82A gymnasium is a large room with equipment for exercising the body and increasing strength. 83A "gymnasium" is the same as a "gym" and it is equipped with bars, weights, ropes, climbing walls, etc, for physical training. 84Gymnasiums in our study are mainly school or university gymnasiums.An ice hockey/ice skating arena is a hall with an ice rink.An ice rink (or ice skating rink) is a frozen body of water and/or hardened chemicals where people can ice skate or play winter sports.Arena's uses include ice skating, ice hockey, bandy, rink bandy, ringette, broomball, speed skating, figure skating, ice stock sport, and curling as well as exhibitions, contests, and ice shows.The rinks are mechanically frozen or artificial, where a coolant produces cold temperatures in the surface below the water, causing the water to freeze. 85Indoor climbing facilities are for sport climbers who train and compete on an artificial climbing wall with safety points for hanging ropes or on a boulder wall.Indoor golf courses provide a virtual golf experience.Indoor horse riding arenas are buildings that are specially designed for equestrian sport with or without spectators.
The floor of the arena usually consists of mixtures of different particulate materials.Indoor motorsport arenas, also known as kart facilities, are small racetracks with asphalt surfaces for the amateur and leisure sector.Motorsport events, like monster truck competitions, tractor pulls, motorcycle stunt shows, and races, are often presented in multipurpose arenas with capacities of 10,000-25,000 people.
Usually, the engines of the vehicles are modified to achieve high power.Monster trucks, for example, run on methanol.They must pass a technical inspection before each show, but there is almost no public information about the composition of the exhaust gas. 86swimming pool is an artificial area of water for swimming 87 and indoor swimming pools are located inside, under a roof and insulated by at least three walls. 88Indoor pools are built for the purpose of year-round swimming or training, and they are common in all climate types.A sports hall is a building or part of a building in which sports are played. 89It offers spaces for example for basketball, badminton, volleyball running, long-jumping, and high-jumping.Different competition events are often organized in sports halls.

| RE SULTS AND D ISCUSS I ON
This section begins with a general treatise on the performancedependent formation of carbon dioxide in exhaled human breath (Subsection 3.1).Exhaled CO 2 potentially affects all sports facilities and may result in elevated indoor levels of the gas.In Subsection 3.2, the different sports facilities are discussed separately with regard to the respective pollutants and possible exposure.All relevant references are summarized in Table S3 in the SI.

| Carbon dioxide in exhaled human breath
Depending on their physical condition and their level of activity, humans exhale more or less carbon dioxide.The problem of high levels of carbon dioxide indoors is well known in school classrooms 90 where respective guidelines 73 are often exceeded within a short period of time.A similar situation arises in workout rooms, where a large number of people exercise.In exercise physiology, the metabolic equivalent rate M (unit MET) corresponds to the consumption of 3.5 ml of oxygen per kg of body mass per minute.McArdle et al 91 provide a five-level classification of physical activity ranging from 1.6 to 3.9 MET (light) to >10 MET (unduly heavy) for men and from 1.2 to 2.7 MET (light) to >7.6 MET (unduly heavy) for women.Another important parameter is the basal metabolic rate (BMR), which defines the required energy to keep a body functioning at rest.According to Schofield, 92 the BMR is 7.25 MJ/day for a 75 kg 30-to 60-year-old male and 5.58 MJ/day for a 60 kg 30-to 60-year-old female.For calculating the carbon dioxide exhalation rate VCO 2 (l/s) from M and BMR, Persily and de Jonge 93 derived Equation (1).
The respiratory quotient (RQ), which often equals 0.85, defines the ratio of the volumetric rate at which carbon dioxide is produced to the rate at which oxygen is consumed.T is the temperature (K) and p is the air pressure (kPa).For a 75-kg male (RQ = 0.85, M = 6 MET, BMR in the exhaled breath, which is 3-4%. 91In the range between 0 Watt (W) and 200 W, Hollmann 94 found a linear relationship between the exercising power and AMV.

| Exposure to indoor air contaminants in different sports facilities
Reported indoor air parameters in different sports environments from published studies are summarized in Table 1.More detailed information about the studies is available in Table S3 in SI.In the following subsections, each type of facility is discussed in detail.

| Fitness centers
We found sixteen studies reporting the indoor air quality parameters in fitness centers.The most often studied parameters were particles (62% of the studies), CO 2 (31% of the studies), and temperature (31% of the studies).Concentrations of PM 10 and respirable dust in fitness centers as well as other sports environments are presented in Figure 1.
In fitness centers, the levels of particles were highly influenced by the level of occupancy, the type (intense) of indoor activity, and the type of ventilation. 68,95The highest particulate matter concentrations exceeding mean target values for the 24 hours (50 μg/m 3 for PM 10 and 25 μg/m 3 for PM 2.5 ) 74 and for the annual mean (40 µg/ m 3 for PM 10 and 25 µg/m 3 for PM 2.5 ) 28 were found especially during classes with elevated numbers of occupants, revealing a relation between PM concentration and the resuspension of dust caused by the physical activity practitioners. 68ncentrations of PM 10 and PM Cleaning can also affect the PM 10 concentrations. 68,97For example, in a study by Ramos et al, 68 increased PM 10 levels by six to eight times were monitored for floor cleaning activities.In some cases, increased median PM concentrations were found during non-occupied periods (during the night when empty). 96That finding could be explained by the formation of new aerosols caused by oxidation of volatile organic compounds 98 emitted from late-afternoon cleaning or the effect of outdoor emissions accumulating due to the motionless conditions preventing mixing. 99ezakova et al 96 monitored higher (by two times) indoor concentrations of ultrafine particles during occupied periods with larger temporal variations noted in general fitness areas than in classrooms and studios. 96They also found that women exhibited 1.2 times higher UFPs intake than men, suggesting the need for gender-specific studies about UFP exposure in indoor sports environments.
This finding was possible due to larger limitation of expiratory flow in female subjects and increased efforts to breath during intense physical exercise.Occupants in indoor sports facilities may be exposed to wet or dry aerosols (eg, metal and oxide nanoparticles) from nano-enhanced products, such as health and fitness products, which are likely to lead to inhalation exposure. 100Those nanoparticles interact with other components, such as semi-volatile organic compounds (SVOCs), in indoor air, 33 possibly causing mixed aggregates exposure. 100pecially in fitness centers, elevated human occupancy promotes the increase of CO 2 concentrations.People are the dominant source of indoor CO 2 , and its production rate depends primarily on the number of people in the room and on their metabolic level 68,75,101 (see Section 3.1).Ramos et al 75 found that in fitness centers, CO 2 was the gas most inhaled by physical exercise practitioners, and even high concentrations of CO 2 during physical exercise did not affect psychomotor performance.Ramos et al 68,75 also concluded that in a fitness center, the type of activity and physical intensity determined the CO 2 concentration; the CO 2 concentration was lower in a yoga class compared to a high-energy fitness class (body attack).The high levels of CO 2 concentrations indicated insufficient ventilation in those spaces.Slezakova et al 79 reported that CO 2 levels correlated well with relative humidity (r s 0.534-0.625)and occupancy due to human exhalation and perspiration during exercising.They also found that the concentration of CO 2 was higher typically during periods of high frequent occupancy around midday (at ~12:00-13:00) and in early evening hours (at ~20:00-21:00).The  early afternoon (at ~08:00-09:00 and ~15:00-16:00).Temporal maximum values of CO 2 exceeded the recommended level of 1000 ppm (1800 mg/m 3 ) given by the American National Standard Institute (ANSI) and American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). 102drade et al 103 measured CO 2 concentrations in three fitness centers in Florianopolis, Brazil, and concluded that all three fitness centers recorded CO 2 concentrations significantly above (P <.01) the recommended maximum level of 750 ppm (350 ppm above the outdoor level of 400 ppm) given by the European Committee for Standardization. 104The mean CO 2 concentration levels in their study varied between 1000 and 3752 ppm (median 914-3526 ppm).
They noticed that the mean CO 2 concentrations were higher in the evening than in the morning or afternoon.
In the fitness center study, Ramos et al 75 revealed that except for CO, all other measured pollutants (TVOC (total volatile organic compounds), CO 2 , PM 10 , and PM 2.5 ) given limits 105 were sometimes exceeded.CO never exceeded the limit of 10 mg/m 3 (= 8.73 ppm). 20,105OC exceeded the limit of 0.6 mg/m 3 (=600 µg/m 3 ) 105 both in aerobic and holistic classes. 75High TVOC levels were also reported by Slezakova et al. 79 In their study of four health facilities, TVOCs highly exceeded the limit of 600 µg/m 3 in all spaces, even when unoccupied, indicating possible risks for the respective occupants.TVOC levels (both medians and range) were higher in the larger workout areas (a joint space with free weights, bodybuilding machines, and cardiovascular equipment) rather than in rooms or studios for group classes.In fitness centers, the high concentration of VOC may originate from alcohol-based hand disinfectant distributed throughout the facilities. 68In addition, humans (exhaled breath, perspiration), personal-care products (perfumes, hair sprays, hand disinfectants), 68,79,[106][107][108] and reactions between the ozone and human skin (secondary oxidation reactions) 109,110 can also be relevant sources of VOCs.In recently opened fitness centers, the high VOC and formaldehyde concentrations are probably associated with emissions from new building material, furniture, and equipment. 68,111It is suggested to use a system composed by an activated carbon filter calcined with a copper oxide catalyst to remove VOCs from indoor environments controlled by a heating, ventilating, and air conditioning (HVAC) system. 112Furthermore, care should be taken with the use of alcohol-based hand disinfectant or other types of cleaning products in environments used for physical exercise and sporting events because they are recognized as risk factors for respiratory health. 68veral studies have reported frequent or occasional daytime temperatures over the recommended values of 20-27˚C [113][114][115] making the sport environment uncomfortable and fatiguing.For example, Onchang and Panyakapo 116 reported the temperature and humidity range of 20.9-36.6˚Cand 49.7%-99.8%,respectively, in three fitness centers.In two of those centers, the mean temperature value was over 30˚C.Lower levels of RH in fitness centers were observed when non-occupied. 79During exercise, breathing and perspiration generate a substantial amount of water vapor, which impacts RH. 117 The number of occupants is a predictor of room temperature; occupants generate heat, 117 as they are a source of internal heat, and body temperature is typically much higher than room temperature. 118l recommended T and RH values for sports facilities are summarized in Table S1 in the SI.The means to maintain comfort parameters within recommended levels include the proper use of air conditioning systems, room insulating, and sun or heat reduction. 79 should also be noted that women generally prefer a warmer thermal environment. 119Zhai et al 120

| Gymnasiums
We found 21 studies reporting indoor air quality in 65 gymnasiums.
The majority of the gymnasiums (74%) located in the context of the school or university.The most commonly studied contaminant was particulate matter, reported in the 67% of the studies.High particulate matter concentrations in school gymnasiums were found during classes with elevated numbers of occupants and particle resuspension.Elevated temperature in a gymnasium makes the sports environment uncomfortable and fatiguing and can led to occupants, especially children, suffering serious illness. 2 For example, in the study by Alves et al, 2 the relative humidity values in the gymnasium during the occupancy periods were within the comfort limits, but frequent daytime temperatures, over 30°C, exceeded the recommended values of 18-27°C [113][114][115]128 (see Table S1 in the SI).
Considering the cleaning done in the different sports facilities used for physical exercise and athletic events, it is recommended the use of powerful vacuum cleaners with multi-stage HEPA filtration systems and graduated filters.Other measures to prevent likely health outcomes are a regular renewal of tatami (a type of mat used as a flooring material) and foam cubes (in gymnastics foam pits), the use of liquid chalk instead of the common magnesia alba, and the installation of indoor multi-stage filtration systems. 97

| Ice rinks/ice arenas used for skating or hockey
Indoor air quality in enclosed ice skating arenas became a public concern in the 1990s and 2000s due to the use of propane-or gasoline-powered ice resurfacers and edgers. 129According to the United States Environmental Protection Agency, 48 a primary source of indoor air concerns is the release of combustion pollutants, such as carbon monoxide (CO), nitrogen dioxide (NO 2 ), and particulate matter (PM), into the indoor air from the exhaust of fuel-fired ice resurfacers.In several studies, the highest CO levels were recorded in the ice rink with gasoline-fueled resurfacers, and the highest NO 2 levels in the ice rink were recorded where propane-fueled ice resurfacers were used. 129Later, it was tested and suggested that an electric resurfacer is the best solution to abate indoor air pollution in ice arenas, 12 and during the last few years, new ice resurfacers that meet the most stringent US EPA standards (given by the United States Environmental Protection Agency (US EPA)) has been reported to reduce hydrocarbon, nitrous oxide, and carbon monoxide emissions by about 70%, 80%, and 60%, respectively. 48Concentrations of CO, NO, and NO 2 in ice skating rinks as well as other sports environments are presented in Figure 2.
We found 31 studies reporting air contaminants in ice skating rink or arenas (see Table S3 in the SI), and in 48% of those studies, the CO concentrations were measured.For example, Guo et al 129 reported the average CO concentration ranged from 2.78 ppm to 5.89 ppm (3190 μg/m 3 to 6749 μg/m 3 ) in Hong Kong ice arenas.
Cox et al 130 measured average concentrations of CO for all activities ranging from 0 ppm to 60 ppm and reported that elevated CO exposure measured while using the resurfacer may have residual CO from prior edging activities (activities using the ice resurfacer).
Minimal CO exposure was recorded when resurfacing occurred without edging.The maximum CO concentrations observed were during edging (202 ppm) and exceeded recommended levels for short-term exposure (ie, TLV Excursion = 125 ppm and NIOSH Ceiling = 200 ppm) 130,131 (see Figure 2).Macnow et al 132  ranging from 21 µg/m 3 to 1176 µg/m 3 (see Figure 2).In Sweden, Thunqvist et al 133  was associated with rhinitis (18.3%) and a cough (13.7%) prevalent in hockey players during or after training or a game, and those symptoms were reported to possibly decrease performance.There is also evidence that exposure to high levels of NO 2 in an indoor ice arena may be associated with an increased occurrence of airway symptoms several years later. 134e concentration of SO 2 was reported in 6% of the studies re- PM 10 : 138 µg/m 3 to 157 µg/m 3 ). 129asured PM 10 values and some measured PM 2.5 values were above the 24-hour recommended mean values, 50 µg/m 3 for the PM 10 and 20 µg/m 3 for the PM 2.5, given by the WHO. 60It has been concluded that the PM values were not affected by the electrical resurfacer but were caused by an inadequate ventilation system and accumulation of PM 136 and that the increased concentrations of both PM 2.5 and PM 10 were probably related to the location of the nearby heavily traveled roads.Based on those reported PM values and the general use of filtering systems in ice arenas, it can be concluded that the resurfacers had little effects on the levels of PM in the ice rinks.
However, in studies with low ventilation rates, high FP (fine particles, diameter <2.5 µm), UFP, 137,138 and CO concentrations were associated with the resurfacing process. 137Rundell 138 concluded that acute exposure to PM 1 generated by ice resurfacing may aggravate an asthmatic reaction, while the precise effect of years of chronic exposure at high ventilation rates during ice sport activity from an early age may modify the morphology of the peripheral airways.
Microbiological contaminants were studied in only one ice arena in a Canadian study conducted by Game and Bell. 135In their study, Eurotium amstelodami was the only species to show an increase over the entire hockey season inside the ice arena, but these levels were lower than in the outdoor sample.Additionally, the only species recorded at a concentration of 50 CFU/m 3 (colony forming units per cubic meter) or higher in the arena were Eurotium amstelodami and Alternia alternata, the concentrations being at the same level as the levels in outdoor air.
In addition to chemical and microbiological indoor air pollutants, comfort parameters for CO 2 and temperature affect the indoor air quality and well-being of occupants, as reported in several studies.
For example, Toomla et al, 139 Grande and Cao, 136 and Guo et al 129 reported CO 2 values exceeding the upper limit guideline value of 1200 ppm. 140In Finland, Toomla et al 139 demonstrated that although the measured concentration levels of CO 2 were below 1200 ppm for the majority of the time, some measurements exceeded that value briefly, especially during the weekend when more people occupied the space.In Norway, Grande and Cao 136 found that the CO 2 concentration increased from 870 ppm to almost 1400 ppm in just under three hours with stabile high activity (during the training on the ice rink).In Hong Kong, Guo et al 129 found that the average CO 2 concentration ranged from 851 ppm to 1329 ppm and concluded that the high CO 2 levels recorded in the ice arenas were attributed to overcrowding and an insufficient supply of fresh air. 141 Finland, the reported average temperature and humidity inside the arenas were 3.5°C-8.8°Cand 64.5%-82%, respectively. 139,142Salonen et al with poor maintenance practices if combustion engine resurfacers are used; 5) if the CO concentration is >20 mg/m 3 (17.46ppm) or the NO 2 concentration is >150 µg/m 3 , ventilation should be increased; 6) at CO concentration >60 mg/m 3 (52.37ppm) or NO 2 concentration >2000 µg/m 3 , ice arena users and spectators should be evacuated if the elevated pollutant level cannot be effectively reduced within 15-30 minutes.

| Indoor climbing facilities
The fact that climbing and bouldering are Olympic disciplines demonstrates the increasing popularity of this sport.Climbing and bouldering can now be practiced in countless facilities, and the number of facilities is still increasing.In the beginning, standard gyms were equipped with climbing walls.Later, special halls became necessary in order to be able to practice the different climbing techniques.We found 5 studies (see Table S3 in the SI) reporting the indoor air quality parameters in indoor climbing facilities, and all of those include measurements of particulate matter.For the athlete, it is essential to keep the hands dry, and MgCO 3 (magnesia or magnesia alba) is the material of choice.However, magnesia is a strong source of particulate matter.Weinbruch et al 143 studied the particle mass concentrations of PM 1 , PM 2.5 , and PM 10 in nine indoor climbing facilities and in five sports facilities.For periods of high activity, PM 10 values between 1000 µg/m 3 and 4000 µg/m 3 were monitored (see Figure 1).

| Indoor golf courses
Little is known about the indoor air quality of indoor golf courses.
Most popular is screen golf in which a computer, connected to sensors and cameras, provides a virtual golf experience in a projected landscape.Moreover, most indoor arenas offer training facilities for teeoff and putting.Goung et al 148 published the only comprehensive study on indoor air quality in sixty-four Korean screen golf courses.
Concentrations of PM 10 , carbon monoxide, carbon dioxide, nitrogen dioxide, ozone, formaldehyde, TVOC, bacteria, asbestos, and radon were measured.The average concentrations of the target pollutants did not exceed the respective pollutant standard set by the Korean law.However, some of the facilities showed increased concentrations of PM 10 , formaldehyde, and bacteria.

| Indoor horse riding arenas
The riding surface is an important component in the equestrian discipline.The surface should support the performance of the horse but should also help prevent injuries.Materials like sand, sawdust, and synthetic fiber are commonly used in riding arenas. 149e constant walking, trotting, and cantering of heavy horses on such grit causes the resuspension of particulate matter and, consequently, exposure to different types of airborne pollutants.We found six studies (see Table S3 in the SI) reporting the indoor air quality parameters in indoor horse riding arenas.Particles (83% of the studies) and dust (67% of the studies) were the most commonly studied contaminants.Lühe et al 150 investigated the distribution of grain sizes in four German riding arenas: 80-90% were between 125 µm and 500 µm and 3-7% were smaller than 63 µm.
The mean particle mass in the air of indoor riding arenas, averaged over one year, was between 0.022 mg/m 3 and 0.233 mg/m 3 , but extreme peak concentrations up to 3 mg/m 3 were observed (see Figure 1).Venable et al 151 showed that recycled rubber material can reduce particulate matter in the air during an indoor riding event when applied over the layer of sand.The personal exposure of an equestrian worker to crystalline silica and respirable dust was studied over 16 days by Bulfin et al 152 The concentrations measured over 8 hours per day resulted in time-weighted averages of <0.01-0.34mg/m 3 for respirable dust (see Figure 1) and <0.01-0.09mg/m 3 for crystalline silica.The concentrations were lower on days when the arena was watered.Claußen et al 153 studied the release of PM 10 from footing materials, including pure sand, sand-wood chips, and sand-fiber in dependence of their moisture content, density, and particle size distribution.It was found that the density of the sand-fiber footings had a significant influence on the release of PM 10 .The authors considered regular watering a suitable measure to lower emissions of particulate matter.In horse stables, Samadi et al 154

| Indoor motorsport arenas
We found five studies (see Table S3 in the SI) reporting the indoor air quality parameters in indoor motorsport arenas, and carbon monoxide was the most commonly studied contaminant (80% of the studies).Particulate matter was reported in 40% of the studies.
Morley et al 86  For many people, go-kart driving is also a popular activity.Today, most go-kart arenas are organized as multi-event complexes and offer other facilities, like bowling, restaurants, and business centers.Reports about acute cardiovascular events among drivers at an indoor go-kart arena prompted a study by Kim and Wagner 157 on carbon monoxide and PM 2.5 inside an indoor go-kart facility.For drivers, the peak exposures to carbon monoxide ranged from 52 ppm to 130 ppm (see Figure 2).In the case of PM 2.5 ,peak concentration ranged from 39 µg/m 3 to 350 µg/m 3 , and the maximum 5-minute average PM 2.5 concentrations was between 28 µg/m 3 and 42 µg/ m 3 .Sysoltseva et al 158 studied particulate matter in air at eight indoor go-kart facilities.The mean PM 10 concentrations were 4.9 µg/ m 3 to 34.9 µg/m 3 for workplaces and 5.6 µg/m 3 to 28.4 µg/m 3 for spectator areas (see Figure 1).The mean PM 2.5 concentrations were 2.3 µg/m 3 to 29.2 µg/m 3 for workplaces and 2.4 µg/m 3 to 27.4 µg/ m 3 for spectator areas.The authors point out that not only motor emissions but also brake and tire debris contribute to air pollution in kart arenas.In parallel measurements, Wolf et al 159 found increased concentrations of carbon monoxide, NO x , C1-C3-benzenes, naphthalene, and benzo[a]pyrene dependent on the fuel (regular, special, and liquid gas).The lowest pollutant concentrations were measured for electrically powered karts.

| Indoor swimming pools
We found 62 studies (see Table S3) reporting the indoor air quality of indoor swimming pools and aquatic centers.Since chlorine (Cl 2 ) and sodium hypochlorite (NaOCl) are the most common disinfectants used in swimming pools, air quality in light of disinfection by-products (DBPs) is unique in that kind of sport environment. 160Chlorine and hypochlorite react with natural organic matter (eg, sweat, skin cells, cosmetics, and urine from swimmers) in the water of swimming pools. 161,162Hypochlorite can also be produced from alternative disinfectants like trichloroisocyanuric acid and bromochlorodimethylhydantoin. 163[166] Chloroform (CHCl 3 ) is the dominant THM by mass in swimming pools, and as chloroform is a highly volatile compound, in addition to absorption via dermal uptake, it can be also inhaled. 167 indoor chlorinated swimming pool facilities, both air and water qualities are relevant issues with respect to the health of the facility users. 168Chiu et al 57 concluded that work-related eye and respiratory symptoms were about five times more common among employees working in the waterpark than employees working in the other parts of the resort (non-waterpark employees consisted of maintenance staff, an arcade attendant, a bartender, housekeeping staff, hotel front desk staff, office staff, and managers).These symptoms were consistent with exposure to chemicals formed when chlorine used to disinfect pool water reacts with materials from swimmers' bodies.
In Italy, Fernández-Luna et al 169 reported that the mean chlorine level in the air of twenty-one swimming pools was 4.3 ± 2.3 mg/m 3 , and in 85% of the facilities, the concentration of 1.5 mg/m 3 , the suggested limit for the risk of irritating effects, was exceeded.They also found that the concentration of chlorine in indoor swimming pool air had a direct effect on self-perceived health problems among swimming pool workers.
In Portugal, 14,170 the reported THM concentrations in the pool air varied between 28 µg/m 3 and 906 µg/m 3 .Gouveia et al 14 used the measured values to predict multi-pathway chronic daily intake (CDI), cancer risk (CR), and hazard index (HI), and they highlighted the need to develop comprehensive guidelines to safeguard the health of individuals involved in elite swimming.Sa et al 170 found that THM concentrations at 150 cm above water surface were about 30% lower than the corresponding values found at 5 cm.Their data confirmed that swimmers are exposed to higher concentrations of THMs by inhalation than lifeguards.In Italy, 56,165 the reported mean levels of THMs in ambient air at swimming pools in Modena and Emilia Romagna region were 58 µg/m 3 and 81 µg/m 3 , respectively, being lower than the mean THMs values of 119 µg/m 3 (ranging from 58 µg/ m 3 to 552 µg/m 3 ) measured in Canada 171 and the mean THMs values of 146 µg/m 3 (±117.8µg/m 3 ) measured in France. 1724][175][176][177][178] For example, in Europe, the reported mean chloroform concentrations in the air of indoor swimming pools ranged between 35 µg/m 3 and 55 µg/m 3 . 4,165,179It should be noted that the presence of chloroform indicates the formation and potential for exposure to additional chlorine disinfection by-products, such as chloramines (CAMs). 57The possible factors affecting chloroform concentrations in the air in indoor swimming pool facilities include ventilation rate, bather load, and free chlorine concentration. 1802][183][184][185] The formation of trichloramine in swimming pools from urea is presented in Figure 3.However, other nitrogen compounds (amines, amides, and amino acids) also act as precursors. 186Due to the low Henry constant of H = 9.9•10 -4 mol/(m 3 •Pa), 187 the compound is volatile in water.Chu et al 188 found both the concentration of free available chlorine in water and the number of swimmers to be significantly associated with the airborne concentration of NCl 3 (P < .05).Bessonneau et al 176 found that indoor trichloramine (geometric mean 190 µg/m 3 ) as well as THM (geometric mean of 74.9 µg/m 3 ) concentrations were associated with the number of swimmers.They also reported that trichloramine was linked to air temperature and pH value.
In Switzerland 189 and France, 190,191 the suggested exposure limit for trichloramine in indoor swimming pool air is 300 µg/m 3 .However, frequent pool users who may be exposed to trichloramine for longer periods might be more sensitive to lower concentrations, and respiratory symptoms with concentrations as low as 17 µg/m 3 were reported. 188In Switzerland, Parrat et al 189 measured a 114 µg/ m 3 mean concentration of trichloramine and concluded that there is an increased risk of irritative symptoms up to a level of 200-300 µg/ m 3 of trichloramine.Rundell et al 192 observed significant ocular and respiratory symptoms for lifeguards and trainers when they were exposed to 500-1300 µg/m 3 of trichloramine.Fantuzzi et al 193 recognized that irritative symptoms become significant at trichloramine concentrations over 500 µg/m 3 .Those studies by Rundell et al 192 and Fantuzzi et al 193 confirmed that the recommended values given by the WHO can be considered protective in occupational exposure to airborne trichloramine in indoor swimming pools.
There are several studies in which the suggested exposure limits for trichloramine were frequently exceeded.For example, in Belgium, the reported mean value for trichloramine in air varied from 300 to 660 µg/m 3 . 182,194,195In Spain, the mean concentration of trichloramine ranged from 170 to 858 µg/m 3 . 179,196In Sweden, reported mean concentrations of trichloramine in the pool air varied between 23 µg/m 3 and 210 µg/m 3 (range 1-770 µg/m 3 ). 181,184,197,198r personal exposure to trichloramine in indoor swimming pool facilities in Sweden, the reported mean values varied between 36 µg/ m 3 and 71 µg/m 3 (range < 1 µg/m 3 to 240 µg/m 3 ). 198,199A swimming school teacher and a lifeguard presented the highest exposures to trichloramine of 240 µg/m 3 and 220 µg/m 3 , respectively. 199In the United States 168 and in Canada, 200 the reported mean concentrations of trichloramine in the air were 150 µg/m 3 and 380 µg/m 3 , respectively.In indoor chlorinated swimming pool facilities located in Southern Europe, the reported mean concentration of indoor bromoform (CHBr 3 ), dibromochloromethane (CHClBr 2 ), chloroform (CHCl 3 ), and bromodichloromethane (CHBrCl 2 ) was 0.4 µg/ m 3 -11.2µg/m 3 ; 1.9 µg/m 3 -13.2µg/m 3 ; 35.0 µg/m 3 -54.5 µg/m 3 ; and 4.2 µg/m 3 -14.6 µg/m 3 , respectively. 165,179In indoor seawater swimming pools located in France, the reported mean concentration of indoor bromoform (CHBr 3 ) and dibromochloromethane (CHClBr 2 ) were 200 µg/m 3 (max 1600 µg/m 3 ) and 13 µg/m 3 (max 150 µg/m 3 ), respectively. 164,172Bromoform is the most abundant trihalomethane (THM) compound in seawater swimming pools, and measured levels of bromoform (CHBr 3 ) in those kinds of pools are particularly alarming.Consequently, it seems important to determine the occupants exposure to bromoform (and to the other THM), not only in water but also in air. 164The measured values of bromoform exceeded frequently the recommended value of 350 µg/m 3 . 201udies concerning pollutants other than DBPs in swimming pool facilities are rare.Tolis et al 4  (for one-hour exposure). 20In their study, indoor concentrations of NO 2 showed an increased pattern when the swimming pool was fully occupied (around hours 16:00 to 21:00).The concentrations of ozone outdoors (mean 81.96 µg/m 3 ) was higher than ozone concentrations inside the facility (mean 59.90 µg/m 3 ).This indoor/outside ratio of 0.73 is high but not uncommon. 42In a swimming pool, the surfaces are covered with a film of water, which slows down the surface reaction of ozone. 202The Henry constant of ozone is small with H = 1.0.10 -4 mol/(m 3 •Pa). 187Therefore, the pool water is not a significant sink.The diurnal variation of ozone indoors followed the diurnal variation of the ozone outdoor concentration (a clearly increasing trend, starting from 11:00 and ending by 21:30).The explanation for this may be that swimming pool areas started to be ventilated during these hours, while for the rest of the day, these areas remain unventilated or closed.One possible explanation for the lower concentrations of ozone indoors may be that if there are not indoor sources of ozone present, gas phase reactions, and deposition, which might result in lower indoor ozone. 42 Italy, Brandi et al 203 investigated the occurrence of mycotic species in 10 swimming pools, and they found moderate mycotic titers and high biodiversity.Penicillium spp.; Aspergillus spp.; Cladosporium spp.; and Alternaria spp.were continually detected in air and surface samples by the swimming area, while pathogenic yeast Candida albicans was never detected.Fusarium spp. was the most common taxon isolated from surfaces.In Iran, Mansoorian et al 204 found that E coli; Actinobacteria; Pseudomonas alcaligenes; Pseudomonas aeruginosa; and Klebsiella pneumonia were the most important isolated bacteria types in swimming pool environments, and bacterial contamination was observed in about 17% of the samples.In waterpark studies conducted in the United States, the reported mean endotoxin levels 22,57 in the pool area were 45 EU/m 3 (endotoxin units per cubic meter of air volume) (range: not detectable to 84 EU/m 3 ).
Indoor CO 2 concentrations in swimming pool facilities usually meet the recommendations.For example in Greece, the reported mean concentration of CO 2 was 502 ppm (range: 259-1322 ppm). 205e air temperature and RH levels measured in the swimming pool facilities and waterparks did not always meet the given guidelines 206,207 (see Table S1 in the SI).For example, in the Southern European swimming pool studies, the daily average air temperature and humidity were 26.5°C (range: 19.2-37.5°C)and 64.5% (range: 23.9%-95.7%),respectively. 165,172,205,208In a US waterpark study by Dang et al, 22 daily average air temperature and relative humidity ranged from 27.8°C to 31.7°C, and 41% to 69%, respectively.ASHRAE 206 recommends that air temperatures be 2˚C to 4˚C above the water temperature to reduce evaporation and avoid chilling effects for waterpark users.For buildings containing swimming pools, ASHRAE 206 guidelines require an RH of 50-60%, exceeding of which (over 60%), can lead to mold growth. 57ficient means to reduce indoor exposure in swimming facilities include carefully regulated chlorine levels and temperatures in swimming pools, properly ventilated pools, and information transmitted to bathers about the importance of personal hygiene to reduce irritants in swimming pool environments. 209Using indoor pools treated with combined chemical treatments (eg, ozone) can reduce direct exposure to disinfection by-products and their negative effects on respiratory function compared to chlorinated pools. 210

| Sports halls
We found nine studies reporting indoor air contaminants in sports halls (see Table S3 in the SI).The most often studied parameters were carbon dioxide (78% of the studies), temperature (78% of the studies), and relative humidity (67% of the studies).2][213][214][215] This indicates that inside closed spaces, where physical activity is practiced, 215 recommended optimum levels and guidelines of CO 2 are very often exceeded. 102,104,105,140,216(see Table S1 in the SI).For example, in Spain, Accili 213 found that CO 2 concentration increases significantly during the evening, and the recommended comfort levels of CO 2 217 (see Table S1 in SI) are not met starting from 19:00.The study establishes that the users of the sports hall experience periods of discomfort due to poor air quality when the occupation is maximum.
In Europe, 211,212,214 the reported mean values for T and RH in the sports halls varied between 15.9 and 22.1°C and 48.5 and 49.9%.In several cases, the temperature were under the recommended range of 20°C and 22°C 115 (see Table S1 in the SI), while the relative humidity in those facilities were in accordance with the recommendations (under 60%). 115In Australia, Rajagopalan and Luther 218 investigated (by using the comfort cart designed according to the ASHRAE standard 219 ) the thermal and ventilation performance of a naturally and hybrid ventilated sports hall within an aquatic center located in Victoria, Australia.They found a high level of thermal discomfort during warm weather, with high solar radiation.
Particulate matter in sports halls were measured in 33% of the studies.The concentration of PM 10 (mean 400 ± 310 µg/m 3 ) were measured only in three sports halls 211 and the range of concentration is presented in the Figure 1.In all those sports halls, the recommended values of 20-50 µg/m 3 21,28,74,114  The VOC in sports halls were found at very low levels, both at indoor and especially at outdoor air, and toluene was the compound with the highest concentrations both indoors and outdoors. 212The I/O ratio was below 1 for most of the compounds, indicating the major source to be outdoor air.Styrene and d-limonene with the maximum observed I/O ratio value of 4.7 and 3.7, respectively, seems to have both indoor sources and an outdoor influence through the air exchange.Possible indoor sources of those compounds in sports halls are the cleaning process and the plastic substrate of the floor for the volley playing field.It was also found that athletic events influence the VOC concentrations in indoor and outdoor air. 212e sports hall results have revealed that outdoor sources of O 3 significantly affected indoor air quality inside the halls. 5,212The measured O 3 indoor air concentrations (mean 12-36 µg/m 3 ) in sports halls has been lower than concentrations in outdoor air (50-64 µg/ m 3 ) both in the "event period" as well as "no event period." 5,212For example, in Greece, Stathopoulou et al 5 studied two large differently ventilated sports halls and found that both in the naturally ventilated as well as in the mechanically ventilated sports halls, the concentration of O 3 is much higher at the outdoor air than at indoor air (I/O ratio 0.3 in the naturally ventilated sports hall and 0.6 in the mechanically ventilated sports hall) during the event and at a period when there are no events.
For NO 2 , the findings are contradictory.For example, in the study by Stathopoulou et al, 5 the NO 2 concentrations found at outdoor air (33-41 µg/m 3 ) were lower than that found at indoor air (mean 38-61 µg/m 3 ).This is opposite to that observed by Tolis et al 212 In their study, the mean NO 2 concentration at indoor and outdoor was 73 µg/m 3 and 95 µg/m 3 , respectively.Microbial contamination was studied in three sports hall studies. 211,215,220The measured concentrations of fungi and bacteria in those studies ranged between 0-1649 CFU/m 3 for fungi and 0-6872 CFU/m 3 for bacteria.After intense sporting activities, hemolytic bacteria were present in high concentration.The correlation between the microorganisms (fungi and bacteria) and the number of persons showed that intense athletic activities influenced the variation of microorganisms. 215 order to ensure comfort and energy efficiency as well as achieve and maintain sport performance in sports halls, many parameters must be considered in an integrated approach. 5,215The studies have revealed that the outdoor pollution, the type of the ventilation, and the operating patterns of the HVAC system when ventilation is mechanical, the indoor materials, the location of the sports hall, and the location of its physical openings as well as the different indoor activities with the varied number of occupants are important factors affecting and controlling IAQ in sports halls as well as in other large enclosures where large number of spectators are present during athletic events.

| CON CLUS I ON S AND RECOMMENDATIONS
This study provides a review on human exposure to indoor air contaminants in different types of indoor sport facilities, the contributing factors of various levels of those contaminants, and the means to reduce the exposure.The respective people can be active themselves or be a spectator.
Combustion engines are used in many sporting events with or without an audience.Here, for example, the available indoor guide values for CO, NO x , and TVOC can be used as a criterion (see The air quality in large closed arenas with spectator capacities of up to 25,000 people has rarely been examined (see Table S3 in the SI).In contrast, there is much work on outdoor air quality in open stadiums 221 and at major sporting events such as the Olympic Games 222,223 and World Championships. 10 The same applies to the health effects of air pollutants on athletes in the amateur and professional sectors.Brunekreef and colleagues 7,224 examined the effects of photochemical air pollutants and particles on cyclists.As early as 2001, Carlisle and Sharp 6 discussed adverse effects of ambient air pollutants in sporting activities.Various works deal with the influence of air pollution on marathon runners. 8,225In 2018, the Clean Air Initiative of the International Association of Athletics Federation's (IAAF) has started a project for the regular measurement of air pollution at major sporting events. 226On the other hand, there is a lack of risk assessments for indoor sport activities.
The evaluation of measured pollutant concentrations shows that trained and recreational athletes as well as children and all asthmatic individuals could also be at risk when they are practicing in indoor environments.The effect on athletes is apparent as metabolic demands of exercise increase the alveolar minute ventilation (AMV) and thus the rate of inhalation of pollutants.At moderate levels of activity (approx.100 W), the AMV of a standard person is quadrupled with approx.30 l/min compared to the resting volume.At maximum power of athletes, the AMV is even increased tenfold (see Figure 4).Therefore, the question arises, whether published indoor air guidelines 20,73 can also be used during physical activity.The results are contradictory.In various studies, no adverse response during exposure to ozone, 227 NO 2 , 228 or VOCs 229 was found under light training conditions.On the other hand, the combination of terpenes and ozone caused irritation even at low concentrations. 230 addition, occupants working in the facilities may be at risk.The

PE E R R E V I E W
The peer review history for this article is available at https://publo ns.com/publo n/10.1111/ina.12718.

F i t n e s s c e n t e
monitored in the early morning and studied fifteen ice arenas with either propaneor electric-powered resurfacing machines and reported the following indoor mean concentration of NO 2 : 276 µg/m 3 (propane) and 11 µg/m 3 (electric).In Hong Kong, Guo et al129 studied three large indoor skating rinks and reported that the average NO 2 concentrations ranged from 58 to 242 µg/m 3 .In Utah, United States, Cox et al130 reported that the NO 2 concentrations were negligible for all rinks and all activities in their study.However, they concluded that the peak concentration level (0.4 ppm = 750 µg/m 3 ) was nearly one-half the short-term recommended exposure limit (REL), indicating that exposures to NO 2 are still possible in these rinks despite the use of gasoline-powered equipment.In a study by Salonen et al,12 the exposure to NO 2 (mean 228 µg/m 3 ) in ice rinks viewed.Game and Bell135 reported surprisingly high concentrations of SO 2 in ice arenas, ranging from 0.3 to 4.5 ppm throughout the season.In Hong Kong, Guo et al129 measured much lower values.The average concentrations of SO 2 ranged between 10 ppb and 13 ppb (0.01-0.013 ppm) and corresponded to the SO 2 concentration in outdoor air.The cold, dry air, the presence of molds inside and outside the ice arena, the presence of SO 2 in the arena air, combined with intense exercise during practices or competition, as well as living in a cold environment may trigger pulmonary impairment and possibly negatively affect athletic performance.135Concentrations of PM or UFP were reported in 24% of the studies on ice arenas.For example, in Norway reported PM 10 values were between 20 µg/m 3 and 180 µg/m 3 and PM 2.5 values up to 30 µg/ m 3 .136In Hong Kong, the average PM 2.5 (28 µg/m 3 to 62 µg/m 3 ) and PM 10 (50 µg/m 3 to 79 µg/m 3 ) concentrations inside the arenas were lower than those measured outdoors (PM 2.5 :97 µg/m 3 to 134 µg/m 3 ;

F I G U R E 2
12 listed the following means to reduce exposure in ice arenas: 1) the use of electric resurfacers instead of combustion engine powered resurfacers; 2) retrofitting emission control technology in propane-fueled resurfacers as an efficient temporary option to reduce engine emissions; 3) mechanical ventilation at a reasonable air exchange rate (0.25-0.5 h -1 ) during opening hours; 4) personnel training to understand the risks associated Concentrations of CO, NO, and NO 2 in different sports environments.Go-kart facility I: eight facilities, range of CO, NO, and NO 2 maximum concentrations in the track areas159 ; go-kart facility II: one facility, maximum of one minute average CO concentrations during driving157 ; ice rink: multiple facilities, range of CO and NO 2 concentrations.12,130The arrows indicate that the minimum concentration is below the limit of quantitation G o k a rt fa c il it y I G o k a rt fa c il it y II Ic e ri n k /a re n a G o k a rt fa c il it y I G o k a rt fa c il it y I Ic e ri n k /a re n

PM 2 .
5 reached concentrations up to 500 µg/m 3 .The size distribution and the total particle number concentration (3.7 nm-10 µm electrical mobility diameter) were determined in one climbing facility.The highest number of concentrations were approximately 12,000/cm 3 , and the authors concluded that the use of magnesia alba was not a strong source for ultrafine particles.Weinbruch et al144 also studied different application techniques of magnesia and found that suspensions in ethanol led to similar low-mass concentrations as the prohibition of magnesia alba.Today, the use of liquid chalk is common in indoor climbing facilities.However, air ventilation was considered as the most effective way of reducing dust concentrations in indoor climbing gyms.Other studies produced analogous results.Almand-Hunter et al 145 measured up to 597 µg/m 3 PM 10 (6 hours on average) in a gym.Alves et al 146 measured PM 10 concentrations in the indoor environment of climbing venues and found distinct differences in the chemical composition of indoor and outdoor particles.The use of magnesia alba caused a significant increase in the mass and number of coarse mode particles.Moreover, MgCO 3 was the dominant component of indoor particles.Moshammer et al 147 examined 109 climbers before and after a climbing activity and found acute and subacute adverse effects in lung function.
measured high concentrations of airborne particulate matter.The maximum value was 9.6 mg/m 3 .Moreover, increased concentrations of endotoxins and ß (1 → 3) glucan were found.The problem of extreme particle concentrations in horse stables were also investigated by Hessel et al,155 who measured the generation of PM 20 , PM 10 , PM 2.5, and PM 1 from horse feed.The maximum concentrations were 35.31 mg/m 3 (PM 20 ), 9.58 mg/ m 3 (PM 20 ), 0.34 mg/m 3 (PM 2.5 ), and 0.13 mg/m 3 (PM 1 ).
investigated the exposure of employees and spectators to carbon monoxide at a monster truck and motocross show.The peak concentrations were between 75 ppm and 340 ppm, and the time-weighted average during the show was between 42 ppm and 80 ppm.Levesque et al 156 investigated the air quality during an indoor monster truck and car demolition show in a Canadian arena.For carbon monoxide, extreme peak concentrations between 219 ppm and 1645 ppm were found.Time-weighted averages ranged between 33 ppm and 100 ppm.The nitrogen dioxide concentrations were below the detection limit of the measuring device (0.5 ppm).
did a comprehensive air quality investigation at an aquatic center in Greece, measuring different indoor air parameters.They found that, in general, the mean concentrations of VOCs were very low, except in the morning.Chloroform was the most abundant compound found in the atmosphere of the aquatic center due to the disinfection of the water.In addition to chloroform, m-xylene, p-xylene, octane, and toluene were three compounds with the highest concentrations found indoors.Limonene, commonly contained in cleaning products and F I G U R E 3 The formation of NCl 3 in swimming pools (see also Schmalz et al 186 ) NCl 3 (H = 9.9•10 -4 mol/(m³•Pa)) partitioning personal hygiene products, used inside the swimming pool was characterized by a high indoor/outdoor value.The concentrations of NO 2 indoors (mean 113.07 µg/m 3 ) was higher than outdoors (mean 63.31 µg/m 3 ), and the hourly concentrations of NO 2 for indoor areas sometimes exceeded the WHO guideline of 200 µg/m 3

high CO 2
concentrations and the calculated ventilation rates in several studies demonstrated that, in general, fitness centers and gymnasiums have inefficient ventilation, considering the large number of people, and relatively small room sizes for the types of activities conducted in those facilities.High CO 2 concentrations have also been found to be a problem in sports halls, especially during the evenings with elevated number of occupants.When exercising at 100 W, a standard person exhales approx.80 l CO 2 per hour.In a 400 m 3 room, an occupancy of 25 people would require a minimum air change of 6 h -1 to keep the CO 2 concentration permanently in the range of 1000 ppm.The location of air intakes and air filtration is also essential for the maintenance of good IAQ.As cleaning practices affect chemical exposure and cleaning chemicals are recognized as risk factors for respiratory health, low emitting cleaning agents and cleaning practices should be used.In addition, attention should be paid to the number of people attending the facility as well as the occupant's behavior.Comfort parameters (T and RH) should be maintained within the recommended ranges (eg, by the proper use of an air conditioning system, room insulation, or sun and heat reduction).At indoor swimming pools, the most concerning pollutants are DBPs, such as CHCl 3 and NCl 3 .Appropriate ventilation to minimize chloramine accumulation, control of water chlorination and temperature, and adequate hygiene of swimmers should be enforced to reduce irritants in swimming pool environments for the occupants.In conclusion, better and more efficient ventilation concepts for sport facilities are required, especially for fitness centers and climbing halls due to the potentially high concentrations of CO 2 and particles.In general, there is a lack of knowledge regarding IAQ in sports facilities and additional studies representing different climatic areas are urgently needed.The respective measurement strategy should be based on the individual circumstances.In climbing halls, it can make sense to measure the personal concentration of particles instead of the air concentration.Air quality guidelines are already available for many types of pollutants.33,129However, for several important compounds like DBPs such guidelines are still missing.Moreover, a critical discussion on stringent air quality requirements for sport facilities is advisable, thus young people as well as vulnerable groups spend a lot of time in these facilities at high pulmonary ventilation rates.Benefits of physical activities can be strengthened by reducing the exposure to pollutants and by minimizing the risk of possible adverse health effects in different indoor sport environments.ACK N OWLED G EM ENTSTS is grateful to Prof. C. Zwiener, Tübingen for providing very helpful information on DBPs in swimming pool water.This research was supported by the Finnish Work Environment Fund (LIIKU-project, 200068).CO N FLI C T S O F I NTE R E S TThe authors declare no conflict of interest.AUTH O R CO NTR I B UTI O N Heidi Johanna Salonen: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (lead); Investigation (lead); Methodology (equal); Project administration (lead); Resources (equal); Software (equal); Visualization (supporting); Writing-original draft (equal); Writing-review & editing (equal).Tunga Salthammer: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (supporting); Investigation (supporting); Methodology (equal); Project administration (supporting); Resources (supporting); Visualization (lead); Writing-original draft (equal); Writing-review & editing (equal).Lidia Morawska: Conceptualization (supporting); Funding acquisition (supporting); Methodology (supporting); Project administration (supporting); Resources (supporting); Writing-original draft (supporting); Writing-review & editing (supporting).

Sports environment/facility Reported indoor air parameters
MgCO 3 , magnesium carbonate; NO, nitrogen oxide; NO 2 , nitrogen dioxide; O 3 , ozone; PM 1.0 , particulate matter (PM) with diameters that are 1.0 micrometers and smaller; PM 2.5 , particulate matter (PM) with diameters that are 2.5 micrometers and smaller; PM 10 , particulate matter (PM) with diameters that are 10 micrometers and smaller; PM 20 , particulate matter (PM) with diameters that are 20 micrometers and smaller; PM 0.1 -PM 10 , particulate matter (PM) with diameters between 0.1 and 10 micrometers; PM 0.3 -PM 10 , particulate matter (PM) with diameters between 0.3 and 10 micrometers; PM 211,1360 , particulate matter (PM) with diameters between 1 and 20 micrometers; RH, relative humidity; Rn, radon; SO 2 , sulfur dioxide; T, temperature; TSP, total suspended particles; TVOC, total volatile organic compounds; UFP, ultrafine particulate matter, particulate matter of nanoscale size (<0.1 μm or 100 nm in diameter); VOC, volatile organic compounds; VVOCs, very volatile organic compounds.TA B L E 1 (Continued)F I G U R E 1 Concentrations of PM 10 and respirable dust in different sports environments.Fitness center: eleven facilities, range of PM 10 during the morning program68; gymnasium: one facility, different types of activity, range of PM 10 ;97indoor climbing facility: nine facilities, range of PM 10 during periods of high activity143; indoor riding arena: one facility, range of total suspended particles (TSP)150; equestrian center: one facility (sixteen measurements), eight-hour time-weighted average (TWA) of respirable dust152; go-kart facility: eight facilities (spectators area), range of PM 10 maximum values during activity158; indoor golf: sixty-four facilities, range of PM 10 during activity148; ice skating arena: four facilities, range of PM 10 concentrations during activities, including resurfacing129,136; sports hall: three facilities, range of PM 10 concentrations during activities211 1177,124For example, Alves et al2found that during human occupation, PM 10 concentrations in a gymnasium ranged from 154 µg/ MgCO 3 ).Average PM 10 concentrations of over 440 µg/m 3 were reached due to the use of climbing chalk and the constant resuspension.The type of activity also affects the level of inhaled pollutants.Žitnik et al117concluded that an exercising person in the gymnasium would receive about six times higher levels of PM 10 inside than she/he would have received at rest outside. This cnclusion was based on their finding that the intensity of breathing 127o 198 µg/m 3 , and on the weekend (without occupation) the PM 10 concentration was 17 µg/m 3 .Gymnasiums are often equipped with climbing walls (se also Section 3.2.4) and the high particle levels were mainly due to the use of climbing chalk and constant resuspension.Castro et al97reported that the air quality in the gymnasium in their study was strongly influenced by the number of gymnasts training and the use of magnesia alba (Viegas et al126studied the prevalence of fungi in gymnasium, specifically containing swimming pools, and found the most commonly isolated fungi as follows: Cladosporium sp (37%); Penicillium sp (19%); Aspergillus sp (10%); Mucor sp (7%); Phoma sp; and Chrysonilia sp (3%).For yeasts, three different genera were identified, namely Rhodotorula sp (70%), Trichosporon mucoides, and Cryptococcus uniguttulattus (10%).Mntgomery et al127found that significant exposure to Staphylococcus aureus (MRSA) exists in gymnasiums.
Table S1 in the SI).In ice hockey arenas, NO 2 and CO exposure are