Effective exhaust hoods can mitigate the indoor air quality impacts of pollutant emissions from residential cooking. This study reports capture efficiencies (CE) measured for cooking-generated particles for scripted cooking procedures in a 121-m3 chamber with kitchenette. CEs also were measured for burner produced CO2 during cooking and separately for pots and pans containing water. The study used four exhaust hoods previously tested by Delp and Singer (Environ. Sci. Technol., 2012, 46, 6167–6173). For pan-frying a hamburger over medium heat on the back burner, CEs for particles were similar to those for burner produced CO2 and mostly above 80%. For stir-frying green beans in a wok (high heat, front burner), CEs for burner CO2 during cooking varied by hood and airflow: CEs were 34–38% for low (51–68 l/s) and 54–72% for high (109–138 l/s) settings. CEs for 0.3–2.0 μm particles during front burner stir-frying were 3–11% on low and 16–70% on high settings. Results indicate that CEs measured for burner CO2 are not predictive of CEs of cooking-generated particles under all conditions, but they may be suitable to identify devices with CEs above 80% both for burner combustion products and for cooking-related particles.
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This study reinforces previous findings that exhaust hoods can be much more effective in capturing pollutants when cooking occurs on the back burners, compared to the front cooktop burners. Results indicate that capture efficiency (CE) measured for burner produced CO2 is not predictive of CE for cooking-generated particles under all conditions, but results of a CO2-based CE test may be suitable to identify devices with CEs above 80% for both burner combustion products and cooking-related particles. Development of a standard test method for pollutant capture efficiency is an important step to providing effective kitchen ventilation in residences.
Pollutant emissions from cooking burners and the cooking of food can substantially and adversely impact air quality in homes. Natural gas burners commonly emit nitrogen dioxide (NO2) and under some conditions emit substantial quantities of carbon monoxide (CO), formaldehyde (HCHO) and ultrafine particles (UFP) (Dennekamp et al., 2001; Moschandreas and Relwani, 1989; Singer et al., 2010; Wallace et al., 2004). Electric coil resistance burners produce UFP (Dennekamp et al., 2001). Cooking activities produce fine and ultrafine particles and a wide range of irritant and other potentially harmful gases including acrolein (Abdullahi et al., 2013; Buonanno et al., 2009; Fortmann et al., 2001; Fullana et al., 2004; Seaman et al., 2009; Zhang et al., 2010). Gas burners and cooking also release substantial quantities of water vapor that can contribute to moisture-related indoor air quality problems (Parrott et al., 2003). Individual cooking events can produce short-term PM2.5 concentrations exceeding 300 μg/m3 and UFP concentrations exceeding 105/cm3 in homes (Abdullahi et al., 2013; Afshari et al., 2005; Buonanno et al., 2009; He et al., 2004; Wallace et al., 2004; Zhang et al., 2010). A recent study estimated that among Southern California homes that cook with natural gas on a weekly basis, >5% have acute CO and >50% have 1-h NO2 concentrations that exceed the corresponding concentration thresholds for health-based ambient air quality standards (Logue et al., 2014).
Indoor concentrations of pollutants generated during cooking can be reduced through use of an exhaust hood positioned above the cooktop or an exhaust fan in the kitchen. Exhaust hoods remove some fraction of the emitted pollutants before they mix into the general air volume of the kitchen with additional removal as air from the kitchen is exhausted outdoors. Exhaust fan effectiveness can be described as a capture efficiency (Li and Delsante, 1996; Li et al., 1997; Singer et al., 2012) that quantifies the fraction of generated pollutants removed either directly or over the duration of exhaust fan operation. Alternately, effectiveness can be framed as the reduction in pollutant concentrations in the kitchen or other location in the home (Rim et al., 2012; Zhang et al., 2010). Direct removal without mixing into the kitchen is termed first-pass CE, including removal from the exhaust provided by the fan gives total CE.
Published studies conducted in laboratories, test homes and in the field have reported CE or other metrics of exhaust hood removal effectiveness for gases, particles, or moisture produced by natural gas burners (Delp and Singer, 2012; Farnsworth et al., 1989; Rim et al., 2012; Singer et al., 2012). Performance has been shown to vary with airflow, hood geometry, and height with respect to the cooktop; whether front, back, or oven burners were used; and hood design, which is most prominently differentiated by the collection volume offered by the hood. A few studies have reported on the effectiveness of range hoods at reducing concentrations of particles generated by cooking activities (Sjaastad and Svendsen, 2010; Zhang et al., 2010).
Currently in the US, there is no direct information available to consumers about the pollutant removal effectiveness of cooking exhaust hoods. The Home Ventilating Institute (HVI) provides third-party certification of airflows and sound levels determined using standard test methods (HVI, 2013a,b). Rated airflows and sound levels are listed in a catalog that is updated monthly by HVI (http://www.hvi.org/proddirectory/). Standard IEC-61591 (IEC, 2005) quantifies grease removal effectiveness for an oil-drop heating event by pre- and post-weighing of the hood and filters and includes a test for odor removal, but this test is not commonly used to rate products sold in the US.
A relatively simple test method was used recently by Delp and Singer (2012) to characterize CE of seven exhaust hoods in a controlled laboratory study and by Singer et al. (2012) to quantify CE for 13 exhaust hoods and two downdraft systems installed in residences. The method calculates CE as the ratio of incremental CO2 mass removal through the exhaust hood to CO2 generation by the burners. Incremental CO2 exhaust flow is calculated as the product of the measured airflow rate through the exhaust hood and the measured increase in CO2 concentration in the exhaust flow during burner use. The method looks at incremental CO2 flow because there is a baseline flow of CO2 from ambient air and from additional CO2 exhaled by the cook and other building occupants. The method uses CO2 as a surrogate for all combustion products emitted by a gas burner. The test method incorporates pots of water (POW) on the cooktop burners to simulate the impact of cooking vessels on the dynamics of the exhaust plumes.
The work described in this paper was initiated with the dual objectives of (i) quantifying CE for particles generated during typical cooking events and (ii) conducting a preliminary assessment of the applicability of the simple, CO2-based test method as an indicator of capture efficiency for cooking-related particles.
Materials and methods
Capture efficiencies (CE) were determined for four under-cabinet exhaust hoods under carefully controlled conditions in an experimental room. CE for burner pollutants was determined directly by comparing the CO2 mass flow through the exhaust hood to the CO2 produced at the burner. CE for cooking particles was determined indirectly by comparing particle concentrations in the room when a hood was in use to concentrations measured during the same cooking activity with no hood installed. The indirect approach was required because particle losses in the hood and ductwork would bias the concentrations seen in the hood exhaust stream and consequently bias the calculated particle CE. The room was supplied with particle-free air through HEPA filters, and the room was maintained at a positive pressure of 1 Pa relative to the outdoors. The flow rate of the main exhaust air pathway – through a blower door – was modulated to maintain a constant total exhaust flow as airflow through the cooking exhaust hoods was varied.
Particle CEs were determined for four hoods previously investigated by Delp and Singer (2012). The tested hoods included a low-cost model (L1), an Energy Star qualified model (E2), a premium hood (P1), and a combined microwave exhaust hood (M1) that represent common geometries and ranges in airflow rates. The microwave was mounted at the same height as the other hoods, but the bottom of the unit was closer to the stovetop, as occurs in typical installations. The microwave drew air from both the bottom and top front of the unit.
A schematic of the laboratory layout is shown in Figure 1. The volume of the room was 121 m3 (5.8 m by 7.2 m by 2.9 m high). A simulated residential cooking area was affixed to a 2.4 m by 2.4 m section of wall located approximately 1/3 of the length of the room closest to the supply air. Each range hood was mounted on this wall with the top of the hood positioned 76 cm above the cooktop of a 76-cm-wide cooking range (Delp and Singer, 2012). The hoods were mounted between drywall boxes installed to simulate a kitchen with wall cabinets adjoining the hood. The cooking range was installed between drywall boxes topped with steel sheeting to simulate side cabinets and countertops. The top of the room consisted of structural 22.9 cm I-beams, and the bottom of the steel roof decks with fiberglass batt insulation. Semi-rigid plastic sheeting was attached across the bottoms of the I-beams over the cooking area to keep the rising exhaust plume out of the shallow channels formed by the I-beams. Baseline experiments were conducted with no hood in place above the stove.
The cooktop had one nominal 12 000 BTU/h (12.7 MJ/h) burner at the front right position and three nominal 9500 BTU/h (10.0 MJ/h) burners. The range was supplied 99.97% methane from certified cylinders (Airgas). Fuel flow was measured using a mass flow meter (Model MLD-20SLPM-D/5M; Alicat, www.alicat.com), factory calibrated for methane with an accuracy of 1%. Flow was reported at a reference condition of 1 atm and 25°C and logged at 1 Hz. Fuel flow was controlled using the burner adjustment knobs on the appliance.
Upon installation, a range hood was connected to a 0.6 m long section of 15.2 cm diameter smooth galvanized ducting followed by 6 m of 15.2 cm diameter aluminum flexible ducting to vent to the outside. An Energy Conservatory Minneapolis Duct Blaster flow measurement device comprising a calibrated fan and throttling ring was placed in-line approximately 2.4 m downstream of the hood. The Duct Blaster quantifies airflow based on the pressure difference measured across the throttling ring; it reports flow at a reference air density of 1.2014 kg/m3. An Energy Conservatory Automated Performance Testing (APT) measurement and control unit measured and recorded airflows, and the Teclog software provided with the APT maintained a constant airflow rate. The flow rate through the hood was provided using both the hood fan and the Duct Blaster. An additional in-line fan (PM-150X; Soler & Palau, www.solerpalau-usa.com) was installed in the ducting to boost airflow when the exhaust hood and Duct Blaster together were not able to achieve the nominal airflow rates of the exhaust hood. Pressures and airflows were logged at 2 s intervals using Teclog software. The APT also measured temperature and relative humidity in the duct.
The room was operated at the same overall ventilation airflow rate for all experiments. Particle-free air was supplied to the room through HEPA filters (Airhandler, Terminal module 3EJY1, www.grainger.com) connected to in-line blowers (Dayton, model 5TCK9) using 30-cm-diameter aluminum flexible ducting. Room air was exhausted using an Energy Conservatory Minneapolis Blower Door. The pressure difference across the blower door was monitored using an Energy Conservatory DG-700 pressure and flow gauge with Teclog software to control the fan speed. The combined airflow supplied to the room was 765 L/s (1620 cfm) as measured by constant injection tracer gas and confirmed by the measured SF6 tracer decay rate of 23/h. Two household axial fans were added to improve mixing and reduce directional airflows around the range (Figure 1). Estimated airflows induced by Fans A and B were 292 and 552 L/s. The pressure across the building shell was monitored and recorded. When a range hood was used, the flow out of the blower door was lowered by the amount of flow through the hood while maintaining a positive pressure of approximately 1 Pa in the room.
Preliminary analysis of experimental results raised concerns about the accuracy of the recorded hood flow rates when the in-line fan was used. The exhaust airflow rate for each range hood that was operated with the in-line fan was checked using a second Duct Blaster connected to the inlet of the hood with a custom-fabricated connector. This is a routine field method, often called powered capture hood, used to measure flows for non-standard air system registers as described in the Duct Blaster manual (Energy Conservatory, 2012). For two of the three hoods used with the in-line fan, the actual flow rate was higher than the set point. Adjustment factors were used to obtain accurate airflows for the hoods operated with the in-line fan.
Airflow and mixing verification experiments
During the setup phase, we conducted tracer release experiments to investigate mixing and airflow patterns in the room. In one experiment, SF6 was released at the supply air inlet and concentrations were measured at the exit and at other locations within the room using an infrared gas analyzer (MIRAN SapphIRe Model 205B-XL; Thermo Scientific, www.thermoscientific.com). The flow in the region in front of the stove was visualized using a smoke machine. The number, placement, and settings of mixing fans were adjusted to achieve generally consistent concentrations around the room and to minimize short-circuiting from the stove area or from the supply to the outlet.
The supplemental information provides details of experiments conducted to explore two key mixing questions: (i) whether there was any short-circuiting recirculation from the area above the hood – where pollutants would rise after not being captured on first pass – to the room air being drawn into the hood from the nearby surroundings and (ii) whether concentrations measured at the sampling location used throughout the CE tests accurately reflect concentrations in the air leaving the room through the blower door. Another experiment was conducted to compare CE values calculated using the direct method (based on mass flow of CO2 through the exhaust hood) and the indirect method (based on particle flow out of the room). During a stir-fry cooking procedure, SF6 was released into the wok through copper tubing formed into a Y and SF6 concentrations were measured in both the hood (E2) and the room exhaust. Another experiment examined the impact of ventilation and mixing fans on CE of burner pollutants: the blower door, supply and mixing fans were all turned off, and CO2 CEs were measured for hood E2. Results for these experiments are provided in Figures S4 and S5.
Carbon dioxide concentrations in the range hood exhaust were measured each 2 s using a PP Systems EGM-4 infrared analyzer. The flow in the exhaust was turbulent. The sampling location was 2 m downstream of the hood connection, corresponding to 12 duct diameters. Radial uniformity at this location was verified by moving the sampling probe through a full transverse during pilot experiments.
Particle number concentrations in room air were measured using two instruments. A condensation particle counter (CPC, Model 3781; TSI Incorporated, www.tsi.com) measured all particles ≥6 nm. An optical particle counter (OPC, Model BT-637S; MetOne Instruments, www.metone.com) measured particles in six size bins: 0.3–0.5, 0.5–0.7, 0.7–1.0, 1.0–2.0, 2.0–5.0, and above 5.0 μm. Measurements were recorded every 10 s.
Particle instruments sampled approximately 1.2 m away from the blower door fan 0.6 m off of the floor. This location was selected as the exhaust flow through the blower door ensured that all cooking emissions passed through this general location. The particles were sampled through a 1 m length of 0.64 cm ID conductive tubing that was divided using flow splitters within a few centimeters of each instrument.
Measuring CE for cooking-related particles requires cooking procedures with suitably repeatable particle generation rates; these depend on both the food being cooked and the cooking procedure. Carefully scripted cooking procedures were developed with the aim of repeatability.
We sought to develop at least two cooktop procedures and one oven procedure with distinct plume characteristics that vary in the challenge they present for capture. We developed a procedure for pan-frying a hamburger on medium heat on the back burner because the lower pan height, lower plume velocity, and the location on the back burner fully underneath all of the range hoods present a geometry that facilitates high capture efficiency for exhaust gases. A stir-fry procedure using high heat with a wok on the front burner was developed as a more challenging, lower capture efficiency condition due to the wider and higher pan, more disruption of the plume by the activity of the cook (Huang et al., 2010), faster plume rise from higher energy input (Kosonen et al., 2006), and the location of the pan on the front burner. Efforts to develop an oven cooking activity are described in the Supporting Information.
Hamburgers were cooked on medium heat in vegetable oil in a nominal 11-inch (28 cm) stainless steel frying pan (Winco SSFT-11 Master Cook). The hamburgers were 85% lean/15% fat, pre-formed, pure ground beef patties purchased at a local store of a nationwide specialty grocer. Individual patties were separated, wrapped in foil, and stored in a freezer. Patties were fully thawed and weighed before cooking. The pan was weighed and 3.5 ± 0.05 g of canola oil was added. The burner was lit and adjusted to a gas flow rate of 1.7 ± 0.1 lpm using the appliance burner control. The pan with oil was placed on the burner and the hamburger was added after 2 min. The burger was lightly pressed with a spatula for 5 s at each 1 min interval, flipped at 3 min, and then pressed again at 1-min intervals until the burner was turned off after 6 min. The pan was allowed to cool for 30 s, after which it was covered, weighed, and removed from the room.
Green beans were selected for the stir-fry because they maintained structural integrity for the desired duration of cooking. Green beans were stir-fried using peanut oil in a nominal 15-inch (38 cm) carbon steel wok (Thunder Group Inc., www.thundergroup.com). Green beans were purchased frozen in 680 g bags at the same store as the burgers. Twenty bags were purchased together and mixed to produce a homogeneous supply for experiments. Beans were measured into 150 ± 0.5 g portions, sealed in new plastic bags, and returned to the freezer. For each experiment, the wok was weighed and 10 ± 0.1 g of peanut oil added. The burner was lit and adjusted to a fuel flow rate of 4.4 ± 0.1 lpm. The wok was placed on the burner, and the beans added after 90 s. The beans were stirred continuously using a silicone spatula. After 3 min of cooking, the burner was turned off and the wok was allowed to cool for 30 s; it then was covered, weighed, and removed from the room. Stir-fry experiments were conducted on the front burner for all hoods. The stir-fry procedure was conducted on the back burners to evaluate the effect of cooking burner position, using Hood E2 operating on low and high. A nominal 12-inch (30 cm) wok was used for these experiments because the 38-cm wok did not fit over the middle of the back burner.
CO2-Based capture efficiency
Capture efficiency for burner combustion products was measured using the CO2-based method outlined in Delp and Singer (2012). Two burner configurations were used: (i) both back burners and (ii) both front burners. Covered 5-l stainless steel pots filled with approximately 3 l of water were placed on the cooktop burners to simulate use. The pots were placed on the stovetop; the burners were ignited and operated for 3 min and then turned off. The researcher moved away from the range after placing the POW to minimize activity-based air currents that can affect CE. This approach will be referred to as the POW CE test. Fuel flow rates were 8.5 ± 0.3 lpm for the two front burners and 7.6 ± 0.6 lpm for the two back burners.
The CO2-based method was also used to calculate burner combustion product CE during the cooking activities; results of these calculations are identified in Figures 4–6 as ‘Cook’.
There were four important differences between the POW procedure and the scripted cooking activities: (i) the POW procedure used two burners, whereas the cooking activities each used a single burner, resulting in different fuel flows and heat generation rates; (ii) the water in the POW procedure provides a heat sink that can impact the burner plume; (iii) the pans used in the cooking protocols have a different shape than those used in the POW procedure; and (iv) a technician stood in front of the range to execute each cooking activity but avoided this area during the POW protocol. All four are expected to affect CE. Differences in vessel geometry, heat generation and removal rates, and the use of two vs. one burner all should impact both the geometry and fluid dynamic properties of the plume rising from the cooking burner(s). The activity of the cooking technician may intermittently disrupt the plume.
To explore the extent to which these factors affect CO2-based CE, the CE of the frying pan and wok each was measured using a modified version of the POW test. The wok or pan was half-filled with water and covered with foil. Fuel flow rates and burner position were the same during the pan tests as during the cooking activities, and CE was quantified similarly as the POW test.
One of the supply fans providing HEPA-filtered air operated continuously to maintain room air particle concentrations below ambient levels. The second supply fan and the exhaust fan(s) were started at the beginning of each experimental day. Cooking experiments did not begin until the total particle concentrations in the room were stable and low, typically around 500/cm3.
Experiments were conducted June through October 2013. Five experiments each were conducted for most combinations of range hood, fan speed, and cooking procedure. Four experiments each were conducted for the hamburger and green beans on hood L1 at low fan speed, and six were conducted with the hamburger on hood L1 at high speed. Experiments without a hood were conducted between the series of tests on the first and second hoods and toward the end of the experiments for a total of 18 experiments for both the pan- and stir-frying. The range top was cleaned between experiments.
Capture efficiency calculations
As noted in the Introduction, total capture efficiency (CE) or pollutant removal effectiveness by a range hood is simply the mass exhausted through the hood divided by the mass emitted from the source:
Total CE includes pollutants captured by the hood before they mix into the room as well as those that escape into the room and are then removed with the room air that is exhausted from the hood,
The mass that is captured directly from cooking and does not escape into the room can be used to calculate a ‘First-Pass’ capture efficiency CEFP,
In theory, the CEFP offers a more useful measure of the performance of the hood as it can be combined with whatever general exhaust benefit provided by the extant space and ventilation conditions. In practice, the airflow dynamics of the room can impact first-pass capture efficiency, so neither metric is uniformly applicable across installations.
CE for the exhaust of a combustion-based cooking burner can be determined by relating the mass flow of exhaust gases through the hood to the generation rate at the cooking surface. Carbon dioxide (CO2) can be used as a surrogate for the exhaust gas mixture. For the typical case of complete (or nearly complete) combustion, the mass generation rate of CO2, S, can be calculated (estimated) from the fuel rate of the burner and knowledge of the carbon content of the fuel. The total mass flow of CO2 through the hood can be calculated from the measured concentration of CO2 in the exhaust air stream and an independent measurement of the exhaust airflow rate. To focus on the direct capture of CO2 from the burner, the concentration of CO2 from the room, Croom, is subtracted, as shown in Equation (4):
In calculating CE, we accounted for a density difference of 1.5% in the reference conditions used by the Duct Blaster compared to those of the Alicat fuel flow sensor.
If a person is present in the immediate vicinity of the cooking surface and range hood, the contribution of exhaled CO2 must be considered in the analysis. Exhaled CO2 from occupants of the room is included in the room air CO2 measurement and requires no special attention when quantifying first-pass CE. For our calculations, we assumed a CO2 exhalation rate of 0.33 ± 0.15 lpm (Emmerich and Persily, 2007). This value is based on the assumption of a single female cook of 163 cm height with a metabolic activity factor of 1.3 (ASHRAE, 2013). The ±0.15 lpm incorporates uncertainty in the rate for an individual female, the potential contribution of a cooking assistant coming close enough to contribute to CO2 seen in the hood and some fraction of the cook's exhaled CO2 not being drawn directly into the hood.
Determining the CE – either total or first pass – for pollutants generated by an actual cooking process is more challenging because the generation rate is typically not calculable. This challenge may be addressed by generating a reproducible quantity of pollutants. In principle, one could volatilize or release an inert tracer into a cooking vessel and release a suitable quantity of heat at the cooking surface to simulate the buoyant plume that is characteristic of the represented cooking activity. The inert tracer generation approach was deemed unsuitable for the current study, which had the objective of evaluating CE for particles relevant to actual cooking activities.
The approach employed in this study was to attempt to generate a reproducible aerosol through execution of the tightly controlled cooking protocols described above. Instead of tracking particle mass, we tracked particles by number concentration in specified size ranges. The number of particles generated from the cooking event was quantified by measuring the airflow rate and concentration of particles leaving the chamber through the blower door, which was the route of all air exiting the room when no range hood was used. Since particle loss in the room is expected to be low due to the low residence time (high air exchange rate), the number of particles being removed by ventilation when no hood was operating is a good estimate of the emission rate and thus a valid reference point for calculating CE when hoods were operating:
This equation uses the symbol M for consistency with prior equations even though we have switched from talking about mass to number concentration. The equations could be used for either quantity. Similar to the CO2-based calculation above, there is a need to account for particles in the room air that are not associated with the cooking activity, Cbkg.
The total CE associated with range hood use is calculated as the difference in the number of particles exhausted through the blower door when no hood was used relative to the number exhausted through the blower door when a hood was used:
The number of cooking-associated particles not captured by a range hood – that is, those exiting through the blower door when a hood was used – is calculated as shown in Equation (7):
The particle concentration in the room in Equation (7) (Croom@exh,with-hood) reflects the effects of particles being captured directly from cooking as well as some particles being removed with the room air exhausted through the hood during cooking. To calculate CEFP, the room air particles removed by the hood must be added to the count of particles not captured on the first pass:
Substitution of Equations (5) and (8) into Equation (6) results in the first-pass capture by the hood
The resulting equation for CEFP for particles is
The difference in particle concentrations in the numerator and denominator of Equation (10) is calculated by integrating the particle concentration measured from the time when cooking begins, t0, to when the particle concentration returns to background, tb,
Ci,room@exh is the particle concentration measured at the room exhaust for particle size bin i, and Ci,bkg is the background when no cooking is occurring. Particle concentrations vary between individual cooking events. As a result, the CEFP is calculated using the average emissions over multiple cooking events
The calculated CE represents the average CE over the measurement period.
The formulas for first-pass capture efficiency for the direct and indirect methods use the concentrations measured in the room, but in two different places: near the hood and near the exhaust. The two values may not be equivalent depending on the mixing conditions in the room. For a well-mixed room, the two concentrations will be equal. Any difference between these two values will be important when comparing resulting CEs. All CEs reported in the Results section of this paper are first-pass CEs.
Uncertainty in calculated capture efficiencies
Uncertainty in the calculated CE for burner pollutants is a function of uncertainties in (i) the measured hood airflow rate, (ii) the measured CO2 concentrations in the hood exhaust and background room air, and (iii) the calculated mass emission rate of CO2. Uncertainty in the hood airflow rate is estimated to be 5% based on verification tests with a second Duct Blaster. The uncertainty in CO2 concentration measured in the hood exhaust was calculated to be 3% based on the standard deviation of the EGM4 calibrations; uncertainty in the background concentration was estimated as 2%. The mass emission rate of CO2 depends on both gas flow and breathing rate. Uncertainty in fuel flow rate was 1%. Uncertainty in the breathing rate is estimated to be 45%, as noted earlier. Because these uncertainties are combined in quadrature, the fractional uncertainty of the breathing rate increases as the gas flow rate to the burner decreases, and the contribution of breathing to the emitted CO2 becomes greater. Total uncertainty in the CO2-based CE ranged from 6.6% to 14.4%.
Uncertainty in particle CE was estimated using the standard error of CEs from replicate experiments combined in quadrature with the instrument measurement uncertainty. We estimate day-to-day instrument repeatability to be 3%. In general, the standard error across replicate experiments was larger than instrument accuracy and dominated uncertainty in particle CE.
Results and discussion
Figure 2 shows typical experimental results for panfrying and stir-frying. Each figure shows time resolved measurements of CO2 (ppm) in the hood exhaust, fuel flow (lpm), and size-resolved number concentrations of particles measured in the room: ≥6 nm (#/cm3) as measured by the CPC and five of six particle ranges measured by the OPC (#/l). There was no substantial response for the largest particle size (>5 μm). Gray vertical lines on the figures mark the following experimental events: pan with oil placed on the stovetop, food added to the pan, fuel turned off, and pan covered and removed from the room. The top two plots show CO2 in the hood and fuel flow increasing sharply just after the burners are ignited and remaining roughly constant throughout the experiment.
Stir-frying emitted more particles than pan-frying, and the difference was more pronounced for particles larger than 0.5 μm. Particle emissions from pan-frying were predominately in the 0.3 to 0.5 μm size range and below, as indicated by the OPC and CPC responses. The stir-fry plot shows particle concentrations (≥6 nm) starting to increase after the pan and oil were added and before the beans were added, whereas the other particle size channels increased sharply only after the beans were added. Adding the pan and oil to the medium flame did not produce such a clear increase in particles as adding the wok and oil to the larger flame did. This reflects a more rapid heating of the oil in the 1-mm steel wok over high heat compared to the pan with 5-mm aluminum bottom over medium heat. For the stir-fry, the OPC response shows that after the quick initial increase in all particle sizes when beans were added, particles larger than 0.3 μm increased only gradually through the remainder of the cooking event. The continuing, steep increase in total particle concentration (CPC) throughout the experiment indicates ongoing emissions of particles smaller than 0.3 μm, including ultrafine particles. Concentrations of all particle sizes followed an exponential decay (at the room AER) after the fuel was turned off, and the pan or wok was covered and removed from the room.
An example of the variability in measured particle concentrations across replicate cooking experiments is displayed in Figure 3, which shows concentrations of 6 nm and larger particles for pan-frying hamburgers without a hood and for E2 operating on low and high speed. These data demonstrate a core challenge in conducting performance assessments using cooking-generated particles: despite precisely defined cooking protocols, there was substantial variability in particle concentrations across replicate implementations for many of the conditions. The relative standard deviation (RSD) of the time-integrated concentration for the no-hood condition was 23%, while those measured with the hood were 10% and 50% for the low and high fan speeds. Particle concentrations returned to the same range of background concentrations for both hood and no-hood conditions. Plots analogous to Figure 3 are shown for each hood and cooking condition in Figures S6 and S7.
The calculated total number of particles leaving the room through the blower door and RSDs for each hood and flow condition are provided in Table 1 for pan-frying and Table 2 for stir-frying. RSDs for specific combinations of hood and airflow rates ranged from approximately 10% to 90%. The mean RSD for the CPC and the five OPC channels with no-hood operating was 48% for stir-frying and 35% for pan-frying. The mean RSDs across all combinations of hood and fan speed were 36% for stir-frying and 36% for pan-frying. Variability in emissions reflected by the RSDs is similar for the two cooking activities both with and without hood use, indicating that the variability in emissions is primarily from the cooking rather than airflow. A previous study that used a scripted procedure for pan-frying of steak (Sjaastad and Svendsen, 2010) reported similar variability in particle emissions in the 0.3 to 0.5 μm size range: RSDs ranged from 12% to 62% with an average value of 37%.
Table 1. Summary results for hamburger pan-frying experiments
Mean total number of particles removed through the blower door and relative standard deviation (RSD) of measurements by the condensation particle counter (CPC) and optical particle counter (OPC).
Capture efficiency results
Figure 4 shows the burner pollutant (CO2) CEs calculated for POW, pan with water (Pan), and cooking (Cook), along with cooking particle CEs calculated for hamburger pan-frying on a back burner. These CEs and all others presented in this section are first-pass CEs. CEs are shown for particles ≥6 nm (CPC) and for five size ranges measured by the OPC. CEs for panfrying on the back burner were mostly above 90% for high fan speeds and mostly above 80% for low fan speeds. CO2-based CEs measured during the cooking events were slightly lower than those measured by POW and pan with water for most combinations of hood and fan speed. Possible contributions to this result are the cook disrupting flow near the burners and the POW and pan with water drawing more heat than the pan with burger, producing less energetic plumes. With the exception of the POW test for E2 on low speed (62% CE), CEs were 75% or better for both CO2 and particles for all the hoods and fan settings.
Figure 5 displays CO2-based CEs calculated for POW, wok with water, and cooking, as well as particle CEs for stir-frying of green beans on front burner(s). CO2-based CEs were similar for POW (2 pots on 2 burners) and wok with water (1 burner), suggesting that neither cooking vessel shape nor the number of burners had a large effect (unless the effects happened to balance).
CO2-based CEs measured during cooking were substantially lower than those measured during POW and wok with water experiments for most combinations of hood and fan setting.
In stir-frying experiments, CEs for particles >0.3 μm were lower than CEs determined for CO2 for all but one combination of hood and setting (M1 on high). The difference between particle CE and CO2 CE during cooking varied across hoods. For example, P1 (only one fan speed) had particle CEs of 56–69% across the particle size bins and a cooking CO2 CE of 72%. By contrast, Hood E2 on high speed had size-resolved particle CEs of 15–38% and a CO2 CE of 54% during cooking. When hoods L1, E2, and M1 were operated at low speed, particle CE values for the stir-fry were extremely low for the particles in the OPC size range: CEs for 0.3 μm and larger particles were 3–16%. CEs for all particles ≥6 nm were 34% for M1 on low, 39% for E2, and 12% for L1 on low fan speed. CO2-based CEs measured during the stir-fry experiments with hoods on low speed were all in the range of 35–38%.
Figure 6 compares capture efficiencies for stir-frying on either the front or back burners with hood E2. This plot further illustrates the sharp contrast between particle CEs at the two locations. While the particle capture efficiencies measured on the front burner are low – varying between 4% and 39% by hood, fan setting, and particle size – those on the back burner were much higher, varying between 70% and 99%. The results for cooking on the back burner are similar to those measured during pan-frying for the same hood. In addition, like the pan-frying results, CEs for the different particle size bins were similar to the CO2 CE measured using POW. At the high flow rate, CEs for both cooking activities on the back burner for hood E2 are close to 100%. At the low fan setting, particle CEs averaged 83% for pan-frying and 78% for stir-frying. It is clear that the burner location has a much more significant effect on capture efficiency than the cooking activity. Cooking on the rear burner means that the hood fully covers the area over which the emissions occur, resulting in higher capture efficiency.
Airflow and mixing verification experiments
Results of the airflow and mixing experiments are presented in the Supporting Information; here we note only the summary findings. One set of experiments indicated a short-circuit loop that increased the apparent first-pass capture efficiency relative to the ideal pattern of pollutants mixing evenly throughout the room if they are not captured on the first pass. The same experiments found variations of roughly 15% in SF6 concentration in the vicinity of the blower door during an experiment. These variations are much smaller than the variance in room air particle concentrations across replicate experiments. These two features are displayed in Figures S2 and S3. In a set of experiments in which SF6 was released into the wok during stir-fry procedures, capture efficiencies calculated based on SF6 measurements in the hood exhaust were roughly 5% higher than CEs calculated from measurements in the room. This result, displayed in Figure S4, is consistent with short-circuiting causing a bias in the apparent first-pass CE. Supplemental Figure S5 shows that CO2-based CEs measured with the blower door supply and room air mixing fans off were indistinguishable from CEs measured under the standard mixing conditions that applied through the experiments measuring CE.
Comparisons to published studies on cooking hood effectiveness
The new results for burner exhaust pollutants, obtained using the CO2-based POW, pan with water, and cooking tests, in some cases indicate higher CEs than previously reported for the same hoods executing the same POW procedures in a different laboratory (Delp and Singer, 2012). At this time, we cannot explain the differences. Experiments exploring the potential effects of airflow patterns were conducted with the configuration reported in this new study but not in the earlier study.
The particle CEs presented in Figures 4–6 expand on the published record of range hood effectiveness for particles. Rim et al. (2012) quantified exhaust hood effectiveness for ultrafine particle concentrations measured in the bedroom of a test house, examining burner-generated particles in the size range of 2–20 nm. That study found a strong size dependence on effectiveness over the range studied with higher effectiveness for higher hood airflow rates and back burners compared with oven or front burners. Yet even for the lowest airflow hood with oven or front burner use, effectiveness exceeded 70% for the largest particles (14–20 nm) observed. Sjaastad and Svendsen (2010) measured concentrations of 0.3–0.5 μm particles 1.3 m to the side of the burner during and just after pan-frying a beefsteak in a 56-m3 chamber. The scripted cooking procedure was repeated for 9–13 replicates of each condition, examining cooking area location (wall, corner, or middle of room), variations in exhaust airflow, and two installation heights for three hoods. The study did not report effectiveness per se and hood airflow variations had a large effect on overall chamber air exchange. These factors and the single measurement location to the source limit the generalizability of the reported results.
Capture efficiencies (CE) were measured for cooking-generated particles during scripted cooking procedures and for burner produced CO2 during the same cooking activities and separately for pots and pans containing water. CEs were determined for four exhaust hoods including a basic low-volume hood, an energy-efficient device with a flat bottom, a microwave exhaust hood, and a high-performance hood with large capture volume. For pan-frying a hamburger over medium heat on the back burner, CEs for particles were similar to those for burner combustion products, as indicated by CO2, and mostly above 80%. For stir-frying green beans in a wok on the front burner over high heat, CEs for burner produced CO2 during cooking varied by hood and airflow: CEs were 34–38% for low (51–68 L/s) and 54–72% for high (109–138 L/s) settings. CEs for 0.3–2.0 μm particles during front burner stir-frying were 3–11% on low and 16–70% on high settings. High CE was obtained when stir-frying on a back burner. Results indicate that CO2-based CEs measured for combustion pollutants are not predictive of CEs for cooking-generated particles under all conditions, but they may be suitable to identify devices with CEs above 80% for both burner exhaust gases and cooking-related particles.
Funding was provided by the California Energy Commission PIER-Environmental Program via Contract 500-09-024; the U.S. Department of Energy Building America Program via Contract DE-AC02-05CH11231; the U.S. Department of Housing and Urban Development, Office of Healthy Homes and Lead Hazard Control via Agreement I-PHI-01070; and the U.S. Environmental Protection Agency Indoor Environments Division via Agreement DW-89-92322201-0. We thank Tosh Hotchi and Doug Sullivan for chamber set-up and Marcella Barrios and Omsri Bharat for carefully conducting the cooking experiments.