Monitoring air pollution: Use of early warning systems for public health


  • The Authors: F.K. is Professor of Environmental Health and Director of the Environmental Research Group, King's College London. His main research interests are oxidative/antioxidant biology and the impact of ambient air pollution on public health. G.F. is Senior Lecturer in Air Quality Monitoring at King's College London and his main area of research is particulate matter source apportionment. H.W and J.F. are Senior Research Fellows in the Environmental Research Group. H.W.'s goal is the optimization of air pollution and health policies through a science led approach while J.F., a science communicator, focuses on disseminating the key research outcomes and findings of the Group.


Frank J. Kelly, MRC-HPA Centre for Environment and Health, School of Biomedical Sciences, King's College London, 150 Stamford Street, London SE1 9NH, UK. Email:


Research confirming the detrimental impact poor ambient air quality and episodes of abnormally high pollutants has on public health, plus differential susceptibility, calls for improved understanding of this complex topic among all walks of society. The public and particularly, vulnerable groups, should be aware of their quality of air, enabling action to be taken in the event of increased pollution. Policy makers must have a sound awareness of current air quality and future trends, to identify issues, guide policies and monitor their effectiveness. These attitudes are dependent upon air pollution monitoring, forecasting and reporting, serving all interested parties. Apart from the underlying national regulatory obligation a country has in reporting air quality information, data output serves several purposes. This review focuses on provision of real-time data and advanced warnings of potentially health-damaging events, in the form of national air quality indices and proactive alert services. Some of the challenges associated with designing these systems include technical issues associated with the complexity of air pollution and its science. These include inability to provide precise exposure concentrations or guidance on long-term/cumulative exposures or effects from pollutant combinations. Other issues relate to the degree to which people are aware and positively respond to these services. Looking to the future, mobile devices such as cellular phones, equipped with sensing applications have potential to provide dynamic, temporally and spatially precise exposure measures for the mass population. The ultimate aim should be to empower people to modify behaviour—for example, when to increase medication, the route/mode of transport taken to school or work or the appropriate time to pursue outdoor activities—in a way that protects their health as well as the quality of the air they breathe.


Global urbanization continues apace and with that, comes more intense energy consumption and increased emissions from transportation and industrial sources. As a result, people in both developed and developing countries are exposed to a more diverse variety of air pollutants and in many urban areas, unhealthy concentrations of many pollutants. Findings from epidemiological and toxicological research into the impact of ambient air pollution on public health have confirmed detrimental long- and short-term effects on mortality and morbidity from cardiopulmonary disease.1–6 Furthermore, an increasing number of studies are investigating the potential for air pollution to exert a wider threat, by, for example, negatively influencing reproductive outcomes7 and neurological health.8

Besides the health effects caused by day-to-day concentrations of urban pollution, premature death and morbidity are experienced during and following pollution ‘episodes’—periods of prolonged and abnormally high concentrations of one or more outdoor air pollutants. They arise as a consequence of poor atmospheric dispersion conditions generated by still air and/or unusually high emissions following incidents such as wildfires, dust storms, local traffic congestion and construction, as well as long-range (1000 km or more) trans-boundary air pollution. Wintertime episodes are characterized by elevated concentrations of particulate matter (PM), nitrogen dioxide (NO2) and/or sulphur dioxide (SO2). Notable examples are those experienced by London in 1952 and 1991 and parts of West Germany in 1985, claiming lives prematurely and increased morbidity from respiratory and cardiovascular causes.9–12 Examples of summertime episodes are the photochemical smogs, arising from the action of sunlight on oxides of nitrogen (NOx) and hydrocarbons released from vehicle exhaust. These episodes, characterized by elevated ambient ozone (O3) and PM concentrations, are also associated with excess death, as exemplified by the impact of the heat wave that affected much of Europe in 2003.13–15 Different types of pollution episodes are caused by wildfires and dust storms, which carry particulate pollution over several thousand kilometres and impact health across wide geographic areas. Associations have been reported between wildfire particulates and an excess of respiratory complaints and/or hospitalizations in Australia,16,17 Lithuania,18 United States19,20 and Southeast Asia.21,22 Reported links to mortality and cardiovascular outcomes are less consistent.16,17,22 Dust storms are brought about under certain weather conditions, where for example sand originating in the deserts of Mongolia and China is carried eastward by cold pressure systems, creating episodes of elevated PM in Taiwan.23,24 Studies investigating the health impact of these dust events in the Taiwanese capital of Tapai, have observed significant effects on emergency visits for cardiovascular disease25,26 and trends towards increased mortality27 and hospital admissions.28 As reviewed by Brunekreef and Forsberg,29 data originating from the United States and Europe on dust storms and wind-blown dust suggest an association with outpatient visits and hospital admissions for respiratory conditions.

The health response to increases in outdoor air pollution varies between individuals and subgroups of the population in a way that individual susceptibility may affect the level at which health effects are noticed and/or the rate of increase in symptoms as air pollution concentrations increase. Reasons why some individuals appear to be more susceptible may include a genetic predisposition, a chronic respiratory/cardiovascular condition,30,31 a metabolic disease such as diabetes32 or a suboptimal level of antioxidants in the diet.33 Age is also likely to affect response, in that children have a developing lung and immune system and the elderly are often faced with accumulating chronic disease and ageing body systems.34,35 Other factors that may contribute to a greater susceptibility include increased physical activity and with that, increased ventilation of the lungs,36 as well as social deprivation,37 probably owing to a number of factors such as higher levels of chronic disease, poorer diet and greater exposure.

Although poor air quality can have a significant impact on human health, research suggests that there is a lack of awareness among the public regarding the links between air pollution and ill health, and a lack of understanding concerning air quality information.38–40 Other reports, however, do at least acknowledge a significant amount of concern within the public over poor air quality.41 In addition, there is evidence of awareness of air quality warnings, and a positive relationship between the latter and a change in outdoor activities.42 In order to move towards a cleaner and healthier environment, where air pollution does not pose a significant risk to human health, there is a need for a much better understanding of air quality issues among all walks of society. For example, the public and particularly, vulnerable subgroups should be aware of their quality of air, enabling them to take action in the event of increased pollution. Policy makers must also have a sound awareness of current air quality and future trends, if they are to identify the issues, guide policies and monitor progress. This increased awareness is dependent upon optimal air pollution monitoring, forecasting and reporting, serving all interested parties. This article outlines air quality monitoring required to protect public health and overviews how these data should be utilized. We focus on the use of air quality data to create an air quality index (AQI) and proactive alert services, before discussing public perceptions of such systems and developments we can look forward to in order to increase their effectiveness and accuracy.


Monitoring and modelling air quality

Many countries have monitoring networks to measure the levels of different pollutants in the air. These networks are fundamentally structured around a country's regulatory obligation to report monitored air quality data and modelled predictions in accordance with requirements of national/European (in the case of members of the EU), regional and local legislation. For example, EU directives dictate the pollutants measured, quality control, monitoring technique and the number and location (roadside, urban background, rural) of sites. Outside of this regulatory framework, different monitoring networks have specific objectives, scope and coverage, with some providing near real-time data for the public, others providing details of pollution chemistry or composition, whereas some will measure concentrations over a day or month, thereby providing invaluable data to assess levels and impacts across a larger area.

Air quality modelling techniques compliment the monitoring networks by being able to predict concentrations of air pollutants and this in turn, enables air quality to be assessed across a greater geographical area than that possible with monitoring data alone. For example, air quality forecasting of long-range transport provides knowledge of pollution sources many hundreds of kilometres from the forecast location. In addition, air quality assessment across rural areas very often relies upon models, while a combination of monitoring and modelling can assist air quality forecasting in heavily trafficked urban locations. Various forecasting approaches, of varying complexities are in use around the world. These can be broadly divided into statistical approaches and deterministic models. The former utilizes human expertise and statistical links between meteorology and pollution episodes. The latter uses metrological and emissions information to model chemical and physical processes and these in turn, determine pollution concentrations. Recently, advances in computing power have allowed improved deterministic models to be developed such as the Prevair system that operates in France43 and the US AirNow system.44


The air pollutants that are monitored to safeguard short-term health effects are O3, PM, NO2, SO2 and carbon monoxide (CO). Others that are monitored to guard against long-term, mainly carcinogenic, risk such as benzene and heavy metals will not be discussed in this article.


Ground-level O3 is a secondary pollutant gas, generated at ground level by atmospheric reactions of UV light with NOx and hydrocarbons produced by motor vehicles, industry and plants. Concentrations are highest during the spring and summer and lowest in the winter, whereas a consistent diurnal pattern usually means that O3 reaches its peak concentration during the afternoon. Once generated, O3 and its precursors can travel long distances, for example to less polluted regions, where it can accumulate and reach high concentrations far away from the original pollution sources. Furthermore, as nitric oxide (NO) generated in cities can decrease local O3 through a reaction producing NO2, O3 concentrations are often higher in rural locations compared with urban environments.


NO2 is a gas generated when oxygen or O3 in the air oxidizes NO, although it is now also emitted directly from exhausts of certain vehicles. In due course, NO2 in ambient air is oxidized to nitric acid and nitrates, with the latter contributing to secondary PM. In outdoor air, the major source of NO2 is fossil fuel combustion, primarily from motor vehicles, and in addition, from power stations and factories. NO2 concentrations are generally higher in winter and urban areas.


PM is a general term that refers to a complex mixture of solids or liquids that vary in number, size, shape, surface area, chemical composition, solubility and origin. PM10 refers to the mass concentration (expressed in µg/m3) of PM that is generally less than 10 millionths of a metre (10 µm) in diameter.1 PM2.5 refers to the mass concentration of particles less than 2.5 µm in diameter. Primary particles are released directly from their source, whereas secondary ones are formed within the atmosphere as a result of other pollutants such as SO2 and NO2 undergoing chemical reactions. The main source of PM in urban areas is road transport in addition to the burning of fossil fuels in power stations and factories. Components of traffic-derived PM are engine emissions, brake and tyre wear and dust from road surfaces.45 Other major contributors of PM are industrial processes (production of metals, cement, lime, chemicals), construction work, quarrying and mining activities. Land and sea constitute additional sources, via wind-blown dust, sea salt, pollens, fungal spores and soil particles. PM pollution can be high at any time of day, particularly near busy roads during morning and evening rush hours. Seasonality is determined by many factors including emissions, dispersion, sunlight to drive secondary PM formation and temperature, which can partition volatile PM into the gas phase.


SO2 is produced during the combustion of sulphur-containing fuels such as coal and oil. SO2 either exists as a gas or dissolves in water, and is readily oxidized to produce sulphuric acid droplets in the atmosphere or sulphates, thereby contributing to secondary PM. In Europe and the United States, the burning of coal has declined, replaced by centrally generated electricity and the use of natural gas in commercial premises and homes. This shift, plus the fitting of emissions abatement measures in power stations and industry as well as the use of low sulphur fuels in motor vehicles, has dramatically reduced the once high concentrations of SO2 experienced in urban areas of the western world. Natural sources of SO2 include active volcanoes and forest fires.


CO is a colourless, odourless and tasteless gas produced by incomplete combustion of carbon and hydrogen fuels. The main exposure risk comes from indoor sources, such as incorrectly installed and/or poorly maintained/ventilated cooking and heating appliances. Petrol engines once emitted significant amounts of CO but the introduction of catalytic converters led to substantial reductions in ambient CO levels.46 Concentrations are typically high during cold weather, because of temperature inversions, trapping the pollution close to the ground. Within urban areas, concentrations are higher during rush hours, on busy roads and particularly in street canyons.


Apart from the underlying national regulatory obligation a country has in reporting air quality information, output from air monitoring and modelling serves several purposes. Data is used in a variety of ways by those with a vested interest, be it the chronic asthmatic living in a large polluted city, the policy maker in government, the scientist investigating a link between one or more pollutants and human health or simply, but just as importantly, the public, healthy or otherwise, but keen to be kept informed of the air quality around them.

Education and awareness

The general consensus within the environmental health community is that there is a greater need to engage and educate society as a whole, about what is undoubtedly the complex relationship between air quality and ill health. The theory behind this is that if people are made aware of: (i) day-by-day and month-by-month variations in the quality of the air in which they and others live; (ii) the effect of pollutants on health as well as the concentrations likely to elicit adverse effects; and (iii) practical actions to reduce pollution (through for example, responsible use of transportation), there is a greater likelihood of motivating changes in both individual behaviour and public policy. In turn this should contribute to reaching the ultimate aim of creating a cleaner environment populated by healthier people. An example of an initiative undertaken to address this issue was led by Common Information to European Air, an EU funded project that supports European cities and regions in their efforts to meet limit values and improve air quality for their citizens. The project recognized the need for ‘customer-friendly’ and high quality information on air quality that answers the questions that the public may have in a way that may change behaviour. Examples of reporting formats and public information on air quality were collected, existing technical air quality data was analysed and from this, a comprehensive communication guidebook for the public was produced containing explanatory information and practical air quality examples.47 The Aphekom Project is another European air pollution communication initiative, operating across 25 cities in 12 EU member states,48 with a focus on costs as well as health effects and a specific interest in traffic derived pollution. The project has delivered information to decision makers in order to facilitate the setting of more effective policies, to health professionals so that they can optimally advise vulnerable groups and to individuals with the aim of improving informed decisions.


At the policy level, within local communities where air quality management plans are drawn up, and in central government where objectives and limit values are created, air quality monitoring data and model predictions are used to establish links back to sources, explain episodes, identify emerging issues and guide/monitor the policies needed to address such issues. Furthermore, data is used by some cities—for example, Paris49 and previously Germany10—that follow guidelines calling for immediate reactionary measures (closing factories, imposing traffic restrictions and encouraging people to reduce domestic fuel use) when air pollution becomes a serious risk to health.


Over the years air quality monitoring data has been used to underpin a wealth of epidemiological research, investigating the effects air pollutants have on various health outcomes, be that mortality,2,5,50,51 cardiopulmonary disease3,6or other effects.7,8 High-quality monitoring data is also an indispensible component in an emerging area of research, evaluating the extent to which environmental regulations are succeeding in reducing ambient concentrations, pollutant exposure and adverse effects such as mortality, respiratory and cardiovascular morbidity.52 Examples include evaluations of a ban on the marketing, sale and distribution of coal in Ireland,53,54 a reduction of sulphur in fuel in Hong Kong,55 measures to reduce traffic in Atlanta during the 1996 Summer Olympic Games56 and the Congestion Charging Scheme in London.45

Information service

Output from measured concentrations of pollutants, air quality modelling systems and meteorological data is also used to provide real-time data and advanced warnings of potentially health-damaging events in the form of a public air pollution information and forecasting service, in line with national legislation. This data allows individuals and organizations (governments, health services) to instigate actions to respond to and ultimately reduce the health impacts of predicted air pollution. For example, groups sensitive to high levels of pollution may be prompted to take actions (reduce exposure and/or increase use of inhaled reliever medication) that alleviate their symptoms, whereas the general public may be encouraged to use public rather than private transport during periods of poor air quality. A discussion of this type of service, via a national AQI or a proactive alert service appears later in this article.

It should be noted that the type of monitoring, data processing and output required by the various users outlined earlier will vary. For example, while the data generated for AQIs is set for short-term averaging periods upon which an index is based, continuous pollutant monitoring is required to gauge progress towards air quality strategy objectives. Furthermore in addition to monitoring site data, public health researchers may use additional measures to assess public exposure, such as population density, estimated traffic exposure and emission inventories combined with air dispersion models.


By processing monitoring and modelling data with a variety of transformations, national AQIs translate complex air quality information into a system that communicates pollution levels and health effects likely to be experienced on the day described by the index or days soon afterwards (i.e. the short term). To be informative and of value to the lay person, they need to strike a careful balance between condensing scientific information into an easily understandable, clear and concise format, without losing valuable information. With increasing technology, there is a range of media currently available to deliver information on air quality measurement data, forecasts of pollution and alerts in the event of an episode of high pollution. AQIs can now be accessed via television, radio, newspapers, internet, freephone, teletext, and as different methods reach different audiences, it is important to provide a variety of ways for the public to access and keep up to date with their local air quality information.

The formulation of a country's AQI is based on the short-term health effects of the index pollutants, taking into account several aspects including current levels of pollutants, scientific evidence emerging from toxicological and epidemiological research, developments in national limit values and air quality objectives, and existing AQIs used in different countries. Indeed, there is no standardized approach to providing air quality information across the world, with variations from country to country in the pollutants included, number of bands and divisions within these, use of words to accompany the bands, maximum index values and comments on health effects (Table 1). Although AQIs do all have the common aim to prevent adverse health effects from short-term elevations in air pollution, some are more precautionary than others, whereas some may display a bias towards regulatory issues and others to health effects. Indices designed in the United States and the UK cover a wide range of concentrations and use information based on perceivable health effects, whereas in France, the index is linked to EU Limit Values and Alert Thresholds.62

Table 1.  Summary comparison of air quality indices
CountryScale/index valueNamed bandingsPollutantsSeparate health advice for at-risk groups and the general population
Australia572006CO, NO2, O3, PM10, SO2
Canada5810010 (4 named)CO, NO2, O3, PM10, SO2
France49106NO2, O3, PM10, SO2None
Hong Kong595005CO, NO2, O3, PM10, SO2
Taiwan603005CO, NO2, O3, PM2.5, PM10, SO2None
UK61 (soon to be adopted following a recent review)1010 (4 named)NO2, O3, PM2.5, PM10, SO2
United States445006CO, NO2, O3, PM2.5, PM10, SO2x

Features of AQIs

Table 1 summarizes the key features from a selection of national AQIs. The pollutants used to calculate the indices include those that are currently known to have short-term health effects, namely PM10, PM2.5, O3, NO2, SO2 and CO, although the latter has been removed by some countries owing to reductions in outdoor concentrations while PM2.5 is a relatively new addition.

Indices have a number, varying from country to country, of bands (named or otherwise) or categories, indicating either the quality of the air, the level of air pollution or the level of risk to human health. The number of bands adopted varies by country. A ‘Low’ band indicates air pollution concentrations where it is unlikely that anyone will suffer any adverse effects of short-term exposure, including people with lung or heart conditions who may be more susceptible, whereas at ‘Very High’ concentrations of air pollution even healthy individuals may experience adverse effects of short-term exposure.

The bands are further divided into a scale to provide a greater gradation of air pollution levels, and again the format and presentation of which varies by country. For example, France and UK use a simple 0–10 scale, whereas the United States has a scale or index value of 0–500. This approach allows people at-risk to calibrate their own sensitivity against the scale and interpret individually, the likelihood of experiencing adverse effects on a given day. The presence of sufficient gradations are also important to indicate variation in concentrations of air pollution and removes frustration that can be caused by having relatively constant index values and with that, the perception that air quality does not change or that the system is either too crude or not operating appropriately. With an increasing array of readily available media to keep people regularly updated with air quality, such a phenomenon is likely to gain further significance. These scales often incorporate colour coding to aid interpretation and the use of equivalent colours on maps that appear on AQI websites allows users to quickly determine their local air quality.

In addition to the index itself, accompanying information is sometimes included to provide health advice that can be taken to reduce impacts. Such advice may be to limit exposure to air pollutants, avoid exertion on high-pollution days or check that medication is being taken as advised by an individual's health practitioner. There is no common approach, in that some indices provide separate advice for non-risk groups and ‘at-risk’ groups as a whole (or specific ‘at-risk’ groups), whereas others provide combined advice or advice that varies by pollutant.

The general method for assigning an overall AQI is to take the highest pollutant index so that if the forecast or measurement for O3 is ‘Moderate’ and for PM it is ‘Low’, the overall index assigned will be ‘Moderate’.

The averaging times differ from pollutant to pollutant (and between different AQIs), reflecting the timescale of exposure over which adverse effects might be caused. As PM2.5, PM10 and O3 bandings relate to relatively long (24 or 8 h) averaging times, long delays between the onset of a pollution episode and an assignment of a band could occur. The recently reviewed UK AQI therefore proposes to provide ‘trigger values’.40 These are hourly PM2.5, PM10 and O3 measurements that indicate exposure is taking place that are likely to lead to significant health effects.

The UK AQI has recently undergone a review.40 The newly recommended AQI is shown in Table 2, together with the accompanying health advice, for the general population and susceptible individuals, in Tables 3 and 4, respectively.

Table 2.  Newly recommended UK AQI
Band (colour coding)IndexOzoneNitrogen DioxideSulphur DioxidePM2.5 ParticlesPM10 Particles
Running 8 hourly meanhourly mean15 minute mean24 hour mean24 hour mean
Pale green through to dark green10–260–660–880–110–16
Pale orange through to dark orange481–107201–267266–35436–4151–58
Bright red through to deep red7161–187401–467532–70854–5876–83
Very high
Black10241 or more601 or more1064 or more71 or more101 or more
Table 3.  Health advice for the general population to accompany the newly recommended UK AQI
Air pollution bandingValueAccompanying health messages for at-risk groups and the general population
At-risk individualsGeneral population
  • † 

    Adults and children with heart or lung problems are at greater risk of symptoms.

Low1–3Enjoy your usual outdoor activities.Enjoy your usual outdoor activities.
Moderate4–6Adults and children with lung problems, and adults with heart problems, who experience symptoms, should consider reducing strenuous physical activity, particularly outdoors.Enjoy your usual outdoor activities.
High7–9Adults and children with lung problems, and adults with heart problems, should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms. People with asthma may find they need to use their reliever inhaler more often. Older people should also reduce physical exertionAnyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing activity, particularly outdoors.
Very High10Adults and children with lung problems, adults with heart problems, and older people, should avoid strenuous physical activity. People with asthma may find they need to use their reliever inhaler more often.Reduce physical exertion, particularly outdoors, especially if you experience symptoms such as cough or sore throat.
Table 4.  Health advice for susceptible individuals to accompany the newly recommended UK AQI
The air quality index has been developed to provide advice on expected levels of air pollution. In addition, information on the short-term effects on health that might be expected to occur at the different bands of the index (Low, Moderate, High, Very High) is provided here.
Short-term effects of air pollution on health
Air pollution has a range of effects on health. However, air pollution in the UK does not rise to levels at which people need to make major changes to their habits to avoid exposure; nobody need fear going outdoors.
Adults and children with lung or heart conditions
It is known that, when levels of air pollutants rise, adults suffering from heart conditions, and adults and children with lung conditions, are at increased risk of becoming ill and needing treatment. Only a minority of those who suffer from these conditions are likely to be affected and it is not possible to predict in advance who will be affected. Some people are aware that air pollution affects their health: adults and children with asthma may notice that they need to increase their use of inhaled reliever medication on days when levels of air pollution are higher than average.
Older people are more likely to suffer from heart and lung conditions than young people and so it makes good sense for them to be aware of current air pollution conditions.
The general population At Very High levels of air pollution, some people may experience a sore or dry throat, sore eyes or, in some cases, a tickly cough even in healthy individuals.
Children need not be kept from school or prevented from taking part in games. Children with asthma may notice that they need to increase their use of reliever medication on days when levels of air pollution are higher than average.
Action that can be taken
When levels of air pollution increase it would be sensible for those who have noticed that they are affected to limit their exposure to air pollutants. This does not mean staying indoors, but reducing levels of exercise outdoors would be reasonable.
Older people and those with heart and lung conditions might avoid exertion on High pollution days.
Adults and children with asthma should check that they are taking their medication as advised by their health practitioner and may notice that they need to increase their use of inhaled reliever medication.
Adults with heart and circulatory conditions should not modify their treatment schedules on the basis of advice provided by the air quality index: such modification should only be made on a health practitioner's advice.
Some athletes, even if they are not asthmatic, may notice that they find their performance less good than expected when levels of a certain air pollutant (ground level ozone) are High, and they may notice that they find deep breathing causes some discomfort in the chest. This might be expected in summer on days when ground level ozone levels are raised. This does not mean that they are in danger but it would be sensible for them to limit their activities on such days.

Issues associated with AQIs

The development of an AQI, designed to convey short-term variations in air pollution for use in a public information delivery system, is associated with a number of scientific challenges that translate into a number of technical and communication issues. This compares with, for example, UV (sunburn) and heat indices, which are a lot less complex. These complexities may contribute towards perceptions that air quality is less readily understandable to the general public and indeed, a call from the public for focused, jargon-free advice.40

Thresholds for effect

The setting of the break points between bands has to be an arbitrary process owing to inter-subject variability of sensitivity to pollutants. Indeed, research has shown that, at a population level, no thresholds of effect can be identified for the common air pollutants and thus one can expect an impact in some individuals even at low levels of exposure.

Long-term and cumulative effects

Variations in air quality from one day to another can have a delayed impact on health. This means that a high concentration of one or more pollutants on a given day could produce observable effects not only on that day or in the following few days, but also for a month or more afterwards. This has been shown to be the case for mortality12,63 and, less commonly, for hospital admissions64 although the methodological analysis used to ascertain such effects continues to be debated.65 Furthermore, exposure to a series of days exhibiting higher-than-usual concentrations of pollutant(s) could feasibly produce greater health effects compared with equivalent exposures on days that are interspersed with lower concentrations. This may, however, depend on the specific health effect in that repeated exposure to O3 results in attenuation of lung function decrements, while inflammatory damage may persist.66 The restraint an AQI has in providing information about real-time and forecast levels of air pollution in the short-term, means that it cannot provide guidance on long-term or cumulative effects as days are treated by an index as discrete events rather than a cumulative series. As an example, confusion would ensue if an index ascribed to a day when the PM10 value was, or was predicted to be between 50 and 100 µg/m3 varied depending on the concentration on the previous day/few days.

Spatial and temporal variations in air pollution

Owing to a host of factors outlined below, indices can only provide a guide to the levels of air pollution that an individual will experience rather than a precise concentration occurring at an individual address.

  • • While pollutants have a regional distribution, others are influenced to a greater degree by local sources.
  • • During the course of a day, people travel around, spending time in areas with varying levels of air pollution.
  • • Not all pollutants follow the same spatial distribution at ground level. Primary pollutants from traffic will be highest near busy roads, whereas O3 will tend to be lowest near roads and highest in rural areas.
  • • Pollution concentrations also vary over hours and days depending on the temporal pattern of sources (such as traffic), as well as meteorological factors that influence the dispersion of pollutants and atmospheric chemistry.

Mixtures of pollutants

The majority of pollution episodes are not confined to a single pollutant. For example, during photochemical episodes, pollutant emissions and atmospheric chemical processes that lead to abnormally high levels of O3, can also lead to elevated concentrations of PM10 and NO2. AQIs are not, however, designed to take into account the possible effects of a mixture of pollutants. If, both NO2 and PM are assigned ‘Moderate’, the overall index assigned is ‘Moderate’ rather than being increased to allow for a potential additive or interactive effect of pollutants. This reflects the lack of clear (with respect to size of effect) and relevant (pollutant concentrations used) evidence of synergism from studies of mixtures of air pollutants,67 as well as the difficulty in disentangling correlations between pollutants during multi-pollutant epidemiological analyses.68


Air pollution alert services are systems that proactively alert registered users of impending pollution events, rather than leaving it up to the user to find the information elsewhere. As would be expected, these information tools are targeted towards susceptible groups in a community or simply individuals who have limited access to media that routinely reports AQI information. Examples of such systems are airALERT69 and airTEXT70 operating in the UK, Luftkvalitet in Sweden71 and the American-based EnviroFlash.72 Those who register to this service can choose to receive the alerts via a home phone (voice message), mobile phone (short message service, smart phone application) or computer (Really Simple Syndication feed or email). Another method of information delivery is via smart phone apps, an example of which is ‘London Air’, developed by the Environmental Research Group at King's College London for the iphone and Android market.73‘London Air’ displays pollution concentrations in real-time from 100 monitoring stations located across Greater London and is fully integrated with Google Maps enabling user-friendly ‘locate-me’ and postcode finder features. Users can also subscribe to receive notifications when pollution exceeds ‘Moderate’ concentrations at a site(s) of their choice, and be informed as to how a site has performed each year in relation to the UK air quality objectives (Fig. 1).

Figure 1.

‘London Air’ iPhone app. (a) Maps including ‘locate me’, satellite and hybrid views and London postcode search. (b) Notification alert when pollution levels change. (c) Performance in relation to UK air quality objectives.

Use and effectiveness of air pollution alert services

The underlying rationale for designing and subscribing to these services is the evidence supporting associations between short-term changes and effects on health.1,2,50,74 Therefore, by supplying preventative information, such services aim to empower people by allowing them to make decisions to improve the management of their health. The most obvious actions would be to reduce exposure and/or increase medication to lessen or prevent the onset of symptoms. In turn they have the potential to reduce hospital admissions and general practitioner visits and, as a result, health-care burden and costs. Experience from other public health interventions, however, suggests that the expected benefits may or may not transpire. Examples include the ineffectiveness of house dust mite impermeable bedding in the management of perennial allergic rhinitis75 and increased risk of lung and gastric cancer following beta-carotene supplementation.76 Furthermore, if benefits do occur, they may not for the expected reasons. For example, lower cardiovascular risks in people encouraged to increase physical activity may be explained by a lower fat intake among those who are already physically active.77 In the current economic climate it seems prudent therefore to evaluate the real benefits of these proactive services. This could be done by addressing qualitative measures such as how is the information perceived? What is the proportion of users that take action in response to receiving an alert? Alternatively, more challenging quantitative measures could be addressed such as: (i) ongoing verification of evidence supporting a link between short-term changes in air pollution and effects on health; (ii) defining whether such a relationship is causal; (iii) quantifying the size of the health impact, within the general population and within those registered to receive the alerts, following pollution increments that trigger an alert system; and (iv) determining the magnitude of any reductions in adverse health outcomes in those users of the service who take action in response to the alert.


Studies into public awareness of air quality information and how to access it have produced mixed results with some research concluding that messages do not get into the public domain during potentially hazardous air quality conditions39,40 and others reporting both awareness and compliance.42,78 Evidence indicates that public understanding can be localized within an individual's immediate social and geographical environment and personal experiences, which then do not reflect the spacing of monitoring sites within the vicinity.38 Trust is another issue that has been raised, and the fact that it can stem from personal experience of local air quality rather than the accuracy of validated data.79 Indeed Semenza et al.39 not only reported a low (10–15%) level of personal behavioural changes during an episode, but that the personal perception of poor air quality rather than the advisory service, was driving the change. It is conceivable that the complexity of air pollution and its science, contributes towards any confusion and apathy to respond to pollution event notification. Not only are there many primary pollutants to consider, but also secondary products owing to atmospheric transformation. For example, rural areas are very often considered safe places to escape from pollution, when at times with respect to O3, they are as likely to be as polluted as urban locations or even more so, owing to the presence of lower concentrations of NO to sequester rural O3. Where research has indicated that individuals are aware of air quality warnings and take responsive actions, larger responses were observed for more susceptible groups or carers thereof.42,78 Not only do findings suggest that parents of asthmatic children appear to be aware of the hazard of outdoor air pollution and of official alerts designed to protect them and their children, but also the majority comply with the recommendations of the alerts at least some of the time.78 Among the adult population, a cross-sectional study of 33 888 individuals taking part in the 2005 Behavioral Risk Factor Surveillance System, reported a 31% change in outdoor activity due to media alerts among adults with lifetime asthma and 16% without asthma. The prevalence of outdoor activity change increased to 75% among those with lifetime asthma and to 68% without asthma, when the combined the effects of media alerts and individual perception were examined.42 These results are not surprising, in that ‘healthy’ individuals would not be motivated by personal experience of the benefits that increasing medication and/or lessening exposure may bring. As such, they are less likely to be committed to behavioural changes that, in fact, may not be necessary in the absence of a vulnerable condition.

In an ideal world people, and especially susceptible individuals, should make themselves aware of outdoor air quality by regularly checking the AQI or targeted notifications for real-time data before going to work, school or pursue leisure activities, in the much the same way as our inherent habit to keep abreast of daily weather forecasts. At risk individuals should regularly check the current index value, be aware of personal symptoms and self-calibrate to the reported AQI value. For example, if symptoms are triggered when an index is 6 (in the case of the UK or French AQI), precautions should routinely be taken (by following the corresponding health messages) when the index is 6 or higher. A balance is also required to ensure that the precaution provided is clear and focused, and does not create exaggerated concern and alarm, leading perhaps to over-medication. Similarly, the advice should not over-play the proportion of the population likely to be susceptible to the effects of air pollution. For example, children with no known respiratory disease are unlikely to be particularly susceptible to moderate levels of air pollution, and should not be discouraged from taking exercise outdoors. Even when air quality is poor, exercise can be resumed indoors. This message is emphasized in the US Environmental Protection Agency initiative entitled Air Quality and Outdoor Activities: Recommendations for Schools,80 which takes the form of an easy to understand colour-coded AQI chart for O3 with a clear and concise set of accompanying instructions. The chart is used to modify plans for outdoor activities in the presence of elevated concentrations of ground-level O3 pollution and cross-references both the US AQI44 and air pollution alert service.72


The monitoring, forecasting and reporting of air quality has become increasingly sophisticated and accurate and this will undoubtedly continue into the future with the use of more individualized measures of exposures. AQIs and alert systems sourced by monitoring sites are always going to be limited by location, spacing and density. Within urban areas, the reliability of forecasts will improve by increasing the number of sites but monitoring networks will, however, rarely achieve a density that reflects the special distribution of pollutants in a city. Added to this, an increasing trend among the public for more information probably means that the type of data provided by current systems will need to progress. For example, we should expect a greater use of sophisticated mapping incorporated into proactive alert services, enabling people to gain feedback as to the outdoor activities appropriate to be undertaken during a given day or what route their children should take to and from school. Although urban walking route websites81 already provide some advice, this information needs to reach the user in a proactive way and be linked into real-time air quality measurements.

Apart from the introduction of more sensors within communities, combining current data with parameters such as proximity to roads with known traffic frequencies is a possible way forward. Alternatively, portable instruments worn by subjects, or biomarkers of pollutants found in blood, urine or breath have the potential to glean more information on personalized exposure. These, however, are currently labour-intensive, costly and only effectively deployed in small samples for brief observation periods, thereby limiting their widespread use. As air pollution levels can vary dramatically over short distances and time scales, there is a need for more precise and dynamic measures of time-activity patterns in relation to exposures. An obvious answer is the use of smart phone technology integrated with low-cost air quality sensors. Smart phones are a ubiquitous source of computing platforms with rich internal devices and a communication infrastructure capable of capturing data interactively or autonomously. Added to these features, they are invariably located with a user, and at the same time could be gathering air quality data from not only monitoring stations, vehicle traffic patterns and locations of industrial facilities, but also the multitude of places where people commute and reside within a community. In this way, such a device has the potential to give dynamic, temporally and spatially more precise exposure measures for the mass population. Indeed, the penetration of the mobile phone is unrivalled in demographics, geographic coverage, acceptance and presence in everyday life, opening up new possibilities in the communication of individual exposure and activity data. Such developments will need the support of innovative research, linking health effects with the more precise pollution measurements so that health advice and effective traffic management policies are up to date and relevant to the appropriate scale of exposure.82 Indeed, such research, combining realistic, flexible and sophisticated approaches to exposure assessment, has recently begun in London by a multidisciplinary team representing exposure science, air quality and noise modelling, toxicology, statistics and epidemiology.83 By using a number of rich data sources of journey patterns, the work aims to describe and understand the patterns of exposure of the London population to traffic pollution and their relationships to health.


Clear and concise communication of the complex relationship between air pollutant exposure and ill health is essential in providing effective air pollution information systems, be they national AQIs or more localized proactive alert services. Added to this, output must be accessible, easy to interpret, informative and engaging. For example, increased use of visual tools incorporating mapping applications will encourage people to check an index on a regular basis and thus achieve a durable change in public attitude and in turn behaviour. The latter may include practical actions among the general public to reduce pollution and protective ones among susceptible individuals in order to lessen or prevent the onset of symptoms. The ultimate goal is to secure a cleaner environment, a healthier population and with that, a reduction in health-care burden and costs.

It is encouraging that studies designed to gain an understanding of how the public receive air quality and the information systems designed to communicate the subject have reported not only awareness, but also behaviour changes with respect to reducing the amount of time spent outdoors during periods of adverse pollution. This is particularly so among susceptible groups and parents thereof. Indeed, Neidell and Kinney84 hypothesized that ambient air quality from monitors may not reflect personal exposure if individuals intentionally limit their exposure as air quality worsens. These researchers went onto to estimate the statistical association between ambient O3 concentrations and hospitalizations in Southern California, while accounting for potential avoidance behaviour in response to forecasted air quality. They found that accounting for potential responses to information about pollution, leads to significantly larger estimates of the relationship between O3 concentrations and asthma hospital admissions, especially for susceptible groups. In relation to this, Bell et al.85 previously hypothesized that variation in time spent outdoors could even be a driving factor behind the considerable heterogeneity in O3-induced mortality across communities.

Despite the degree to which our current air pollution information services are effective, the need to continually evaluate performance is critical. This is not often an easy exercise, but a valuable one to ensure the time, effort and cost involved in development and maintenance is worthwhile. Systems are already sophisticated and accurate, but there is no doubt the future holds further promise. In particular, everyday mobile devices such as smart phones, armed with sensing applications have real potential to bring about an increased quantity and quality of data. Importantly, the latter tends to generate more significant action and better understanding. Ultimately, these devices may provide a personal platform, generating air quality information at a spatial resolution, to empower people to modify behaviour in a way that improves their health as well as the quality of the air they breathe.


The funding for our work on the health impact of air quality was provided by: Guy's and St Thomas' NHS Foundation Trust and King's College London NIHR comprehensive Biomedical Research Centre, the MRC-HPA Centre for Environment and Health and Santander Universities.


  • 1

    Strictly, particles that pass through a size-selective inlet with a 50% efficiency cut-off at 10 µm aerodynamic diameter (or 2.5 µm for PM2.5).