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School bus air quality Brauer, Michael; Hsieh, Julie; Copes, Ray 2000

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School Bus Air Quality  Michael Brauer Julie Hsieh Ray Copes School of Occupational and Environmental Hygiene, The University of British Columbia  Final Report July 3, 2000  Address all correspondence to:  Dr. Michael Brauer The University of British Columbia School of Occupational and Environmental Hygiene 2206 East Mall, Vancouver, BC V6T1Z3 Canada tel: 604.822.9585 fax: 604.822.9588 e-mail: brauer@interchange.ubc.ca  Summary In response to concerns regarding indoor air quality inside school buses, we conducted a survey of several indoor air quality parameters. The objective of the survey was to measure concentrations of several potential indoor air contaminants and air quality indicators inside a random sample of school buses during normal operation. Measurements were conducted in 25 randomly selected school buses from School District 34, serving the community of Abbotsford, British Columbia. All measurements were conducted in the period November 17 - December 15, 1999 and were made during normal school bus runs while under normal occupancy loads. At the same time that measurements were collected, data regarding bus characteristics and use were also recorded. Carbon dioxide (CO2), temperature, relative humidity, carbon monoxide and nitrogen dioxide were measured with direct-reading instruments. Additional measurements for carbon monoxide (CO), nitrogen dioxide (NO2) and hydrocarbons were made with a (Drager) Chip Measurement System which uses detector tube technology. NO2 was also measured with passive samplers. Measurements of NO2, hydrocarbons and CO were low or near background levels in most instances. Occasional short-term elevated CO concentrations reaching approximately 30 ppm for 1-2 minutes were observed inside buses. These elevated CO concentrations were highly transient and were likely associated with infiltration of CO from other vehicles. CO2 concentrations were elevated above the 1000 parts per million (ppm) (the indoor air quality guideline value) in most instances, with the highest concentrations reaching 5000 ppm. Elevated CO2 concentrations were found to be associated with the number of passengers on the bus, but not with any other predictor variables. For these measurements, the amount of time spent on the bus was not a significant predictor of indoor CO2 concentrations. Measurements of air exchange rates indicated levels of ventilation on the order of one air exchange every 4-6 minutes. Taken together, these observations suggest that elevated CO2 concentrations are mainly a result of high passenger levels and that concentrations decrease quickly once occupant levels decrease. For significantly longer bus runs it is likely that CO2 concentrations above 1000 ppm will be reached and sustained, therefore control measures may need to be implemented in these situations.  1  Introduction  In response to concerns regarding indoor air quality inside school buses1, a survey was conducted to measure several selected indoor air quality parameters inside school buses during normal operation. The objective of the survey was to measure concentrations of several potential indoor air contaminants and air quality indicators inside a random sample of school buses during normal operation. School District 34 (SD34) (Abbotsford, BC) kindly agreed to have the survey performed in their bus fleet. It should be noted that there had been no prior complaints regarding indoor air quality inside any SD34 buses and this fleet was chosen for the survey due to the willingness of SD34 officials to participate. The SD 34 fleet was selected because it is located in a Lower Mainland rural region and was thought to have reasonably lengthy transit distances. It was acknowledged beforehand that these distances were shorter than those experienced in more rural communities. The following parameters were chosen for monitoring: Carbon Dioxide Carbon dioxide (CO2 ) is an indicator of the buildup contaminants generated inside school buses. CO2 is produced by human metabolism. There is rarely concern about toxic levels of CO2 indoors. For example, the Workers’ Compensation Board (WCB) of British Columbia exposure limit2 for CO2 is a concentration of 5000 parts per million (ppm) in an eight-hour period; this level is considered hazardous to adult workers. The Canadian ‘Exposure Guidelines for Residential Indoor Air Quality’ recommend an acceptable long-term exposure limit in 3 residential air of 3500ppm . Measurement of CO2 is useful as the indoor concentration of CO2 is often used as an indicator Measurement of CO2 is useful as the indoor concentration of CO2 is often used as an indicator of ventilation. Indoor CO2 concentrations are often elevated in environments with high occupant density and/or reduced ventilation. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) guideline for indoor air quality4 specifies a CO2 concentration of less than 1000 ppm. The WCB Occupational Health and Safety Regulations for Indoor Air Quality (section 4.7) suggests that measurement of indoor CO2 concentrations above 1000 ppm may be indicative of poor indoor air quality. Carbon Monoxide Carbon Monoxide (CO) is an odorless gas that interferes with the delivery of oxygen in the blood to the rest of the body. It is generated as a result of incomplete combustion of carbon-containing fuels and can cause fatigue, headaches, dizziness, weakness, nausea, and vomiting. In this survey, CO was measured as an indicator of the emission of engine exhaust into the passenger cabin of the buses. The WCB exposure limit is 25 ppm CO for an eight-hour average, and less than 100 ppm for a fifteen-minute average. It should be noted that WCB exposure limits are 1  Environmental Health Assessment of Indoor Air Quality in School Buses. Part 1, Part 2 - Student Passengers. Wisdom Consultants. 1999  2  Occupational Health and Safety Regulation. Workers' Compensation Board of British Columbia. October 1, 1999.  3  Health Canada, Health Protection Branch. Exposure Guidelines for Residential Indoor Air Quality. Ottawa 1987 (rev. 1989)  4  ASHRAE Standard 62-1989. Ventilation for Acceptable Indoor Air Quality. American Society of Heating, Refrigeration and Air Conditioning Engineers.  2  developed for health adult workers, and are not necessarily applicable to children. The World Health Organization (WHO) has developed Air Quality Guidelines5 which are more appropriate to the general population, including children. The WHO Guideline is 26 ppm for a 1-hour average and 9 ppm for an 8-hour average. . The Canadian residential indoor air guidelines are 11 ppm (8hr average) and 25 ppm (1 hr average)6.  Nitrogen Dioxide Nitrogen dioxide (NO2) is a colorless, odorless gas that is produced as a combustion byproduct. It can cause irritation of the respiratory tract, shortness of breath, and increased incidences of respiratory illness. Children and individuals with asthma and other respiratory illness are thought to be especially sensitive to exposure to NO2. NO2 was measured as an indicator of the emission of engine exhaust into the passenger cabin of the buses; unlike CO which is a product of incomplete combustion, NO2 may be produced in appreciable concentrations during normal combustion and may be elevated with propane-fueled vehicles or in exhaust of engines specifically tuned (rich air:fuel ratio) to limit CO emission . The WCB exposure limit for NO2 is a concentration less than 1 ppm at any time during the exposure period. The WHO guideline is 0.106 ppm for a 1-hour average and the Canadian residential indoor air guideline is 0.25ppm (1 hr average).  Hydrocarbons Hydrocarbons (HC) consist of a large group of chemicals composed of hydrogen and carbon. This group as a whole is used to indicate the emission or leakage of any fuel or fuel emission products into the passenger cabin of the bus. Petroleum mixtures contain a range of hydrocarbons and can be separated into different petroleum products by fractional distillation. Propane contains light hydrocarbons, gasoline is made up of C5 to C11 hydrocarbons, and diesel contains hydrocarbons with form 8 to 26 carbon atoms. The health effects depend on the specific hydrocarbon in question. There are no general regulations for total hydrocarbons, although gasoline has a WCB exposure limit of 890 mg/m3 (8-hr average) (approximately 237 ppm as Toluene equivalent) and a 15 minute limit of 1480 mg/m3 (approximately 394 ppm as Toluene equivalent). Temperature and Relative Humidity Temperature and Relative Humidity (RH) were monitored as indicators of occupant thermal comfort. ASHRAE standard 55-19927 defines comfort ranges for indoor temperatures. In general, acceptable ranges are 19 to 24.5°C in winter and 22 to 26.5°C in summer. Because the testing for this study was carried out in November and December, the winter range is most appropriate. ASHRAE 55-1992 defines a relative humidity comfort zone that ranges between 20 to 60 %. 5  http://www.who.int/peh/air/airguides2.htm  6  Health Canada, Health Protection Branch. Exposure Guidelines for Residential Indoor Air Quality. Ottawa 1987 (rev. 1989)  7  ASHRAE Standard 55-1992. Thermal environmental conditions for human occupancy. American Society of Heating, Refrigeration and Air Conditioning Engineers.  3  Methods Survey procedures The 47-vehicle fleet School District No. 34 was chosen for this survey. The fleet includes vehicles that are fueled by propane, diesel or gasoline. Twenty-five buses from this fleet were randomly selected for indoor air quality monitoring as the vehicles traveled on typical school bus runs. Three of the 25 chosen buses were tested twice. The purpose of the repeat measurement was to evaluate measurement reproducibility. In total, 28 tests were conducted from November 17 to December 15, 2000. In general, two buses were tested on each sampling day; once in the morning and once in the afternoon. The occupants of the tested buses normally included one adult bus driver, one individual conducting air sampling, and children at various ages ranging from 5 - 18 years old. All instruments used in this study were direct reading monitors, except passive samplers which were used as a supplementary measurement for NO2. Within each bus, two sets of CO and CO2 / RH / Temperature monitors were used and placed at two different sampling locations within each bus, wherever possible. Both sets of instruments were placed at randomly selected locations inside the buses. In some instances, however, the second set of instruments had to be placed next to the first set, at a non-random location, due to limited space within the bus. The individual conducting the air sampling sat in the seat next to one set of air sampling instruments. Outdoor air quality was measured in front of an office building near the bus yard before morning bus runs on each day. Four contaminants were measured: CO2, CO, NO2 and hydrocarbons using. Afterwards, all testing instruments were brought onto the bus being tested on that day and the equipment was set up inside the bus while the bus engine was idling to warm up in the yard. Usually, morning bus runs began at 7 am and finished at 9:15 am. After the morning rides all buses returned to the yard and were unoccupied until the afternoon runs. Some buses were also operated for kindergarten transport or local field trips in the morning and therefore had three runs per day. After the morning bus rides, all instruments were taken into an office and recharged before the afternoon trips. Three of buses being tested in the afternoon were scheduled for morning kindergarten trips; the testing equipment was then turned off after the kindergarten run and remained on the bus for the next trip. At the end of each sampling day, data were downloaded and saved. Sampling was done on 3-4 days per week and instruments were calibrated weekly (on Tuesday, when no sampling was scheduled). Measurements of Carbon Dioxide, Temperature, and Relative Humidity The CO2 content of the air was monitored with a continuous Indoor Air Quality Monitor (YES204, Young Environmental Systems). The monitor uses a non-dispersive infrared detector for measurement of CO2. The concentration is expressed in parts per million (ppm). The YES monitor also measures temperature (thermistor) and relative humidity (thin film capacitance). Chemical Measurements Sampling for various indoor air components, such as NO2, hydrocarbons (HC) and CO was completed. A Chip Measurement System (CMS) device (Drager) was used to monitor HC, CO  4  and NO2. This device uses colorimetric detector tube technology in combination with an optical length of stain reader and measurement of reaction time to provide measurements with accuracy and precision that are superior to traditional detector tubes. CO and NO2, expressed in parts per million (ppm), were monitored with Toxilog (Biosystems) electrochemical monitors. NO2 was also collected by a passive sampler, the Palmes Tubes8, for 47 ~ 221 minutes depending on the bus ride duration. These samples were analyzed by ion chromatography with ultraviolet detection. The detection limits (LOD) of each monitor are listed in Table 1. Measurements which were below the respective detection limits were substituted with (½)½ * LOD (0.707*LOD) in order to calculate summary statistics. Monitor Toxilog CO Toxilog NO2 Drager CMS CO NO2 HC Palmes Tube  Detection Limit 1 ppm 0.25 ppm 5 ppm 0.5 ppm 20 ppm 0.32 ppm*  Table 1. Limits of detection for measurement devices used in survey. * Based on 1/2 concentration of lowest standard used in ion chromatography analysis.  Observational measurements The following parameters were also recorded during each sample collection period by visual observation: Bus identification number, Bus manufacturer, bus age, fuel, the number and time of open windows/roof hatch, the number of passengers and each time a person got on or off the bus.  Results Operational parameters Table 2 summarizes the operating parameters of the buses that were included in the survey. The mean age of the buses was 6.8 years old (Standard deviation [sd] =3.8, range:1-13 years). 13 of 28 buses (46.5%) were fueled with propane, 13 (46.5%) with diesel and the remaining 2 (7%) with gasoline. The older buses (pre 1993) tended to be fueled with propane while the newer buses were fueled with diesel or gasoline. The majority of buses (68%) that were sampled were considered large capacity (36 or 48 adult passengers), with the remainder made up of buses with adult capacities of 8-24 personas. Adult capacities can be multiplied by 1.5 to determine the capacity for primary school children. 54% of the surveyed buses had an exhaust pipe located in the back of the bus on one side of the emergency exit, while 39% had exhaust in the back corner and the remaining 7% exhausted from the back corner to the side. The mean total number of passengers on the bus during each sampling period was 46 (sd = 40, range: 4 - 180). The mean sampling duration was 135 minutes (sd = 62, range: 46 - 346). Because passengers were 8  Palmes ED, Gunnison AF, DiMattio J, Tomczyk C. Personal sampler for nitrogen dioxide. Am Ind Hygiene Assoc J. 1976: 37: 570-577.  5  continuously getting on (in morning runs TO school) and off (in afternoon runs FROM school) we calculated the total persons*minutes for each sample collection period, by summing #of passengers times the number of minutes each passenger was on the bus to provide an indicator of the bus occupancy for each run. The mean person*minutes was 6547 (sd = 5167, range: 271 23325). A similar variable was constructed for assessing the extent of open windows (# of open windows times the amount of time each window was open) amount of time that windows were open (window*minutes). The mean windows*minutes was 86 (sd = 293, range: 0-113). In nearly all instances, students were not allowed to open windows of the buses, therefore windows remained close, except for occasional circumstances when a parent riding on the bus, or the driver opened one or more windows. Most buses also contained roof hatches that could be opened. The roof hatch was only found to be open during one of the bus runs. For all others it was kept closed. Outdoor measurements Outdoor CO concentrations were very low with 81% of the measurements below detection limit. Substituting 0.707*the LOD for those measurements below LOD, we estimated a mean outdoor concentration of 0.91 ppm (sd = 0.69, range: 0.71 - 3.75). All outdoor NO2 measurements were below the electrochemical detector LOD of 0.25 ppm as well as the CMS and Palmes Tubes LODs of 0.5 and 0.32 ppm, respectively. All outdoor hydrocarbon measurements were below the LOD of 20 ppm. For CO2, the mean outdoor concentration was 433 ppm (sd = 31, range: 373 490) which is slightly higher than the expected outdoor CO2 concentration of 350 ppm. This discrepancy may be due to CO2 from nearby engine exhaust or a slight bias in the instrument response due to inadequate temperature compensation. It is reassuring to note that the outdoor concentrations were quite consistent. The mean outdoor temperature was 12.8 C (sd = 2.5, range: 7.4 - 17.5) and the mean outdoor relative humidity was 50% (sd = 10.3, range: 31-66). In-bus measurements A summary of the in-bus measurements is presented in Table 3. The mean in-bus temperature was 19.5 C (sd=1.7, range: 16.8 - 22.3). The mean in-bus relative humidity was 48% (sd=10.2, range 31-69). Measurements of NO2 were low and in most cases below detection limits. The exceptions were occasional NO2 concentrations which reached as high as 0.6 ppm for 1-2 minutes only. Review of continuous monitoring plots did not reveal any systematic association between operational parameters (time of run, etc.) and the occurrence of these short-duration NO2 peaks. All hydrocarbon measurements were below the 20 ppm detection limit of the CMS. Carbon monoxide Mean CO concentrations were also quite low, although there were occasional short-term high CO concentrations measured inside the bus by the electrochemical detector and confirmed by CMS measurements. Substituting 0.707*LOD for those measurements below the LOD, we estimated an average of all sample-period mean CO concentrations of 1.1 ppm (sd = 0.54 ppm, range: 0.71-2.7 ppm). 46% of the mean CO measurements were below the LOD. The average of all sample-period peak (1 minute) CO concentrations was 4.6 ppm (sd=6.6, range: 0.71-29). Peak concentrations were examined in more detail by looking at time-series plots from the continuous monitoring data. For example, Figures 1 and 2 show elevated CO levels during the initial first few minutes of the sample period, coinciding with the period in which the bus was in  6  the bus yard and near other idling buses. These concentrations dropped quickly, indicating high air exchange rates. Figure 1. CO concentration measured during initial November 17 morning bus run.  Bus #3025 (November 17)  CO Concentration (ppm)  30 25 20 15 10 5 0 7:12 AM  7:40 AM  8:09 AM  8:38 AM  Time  9:07 AM  9:36 AM 2nd set  7  CO Concentration (ppm  Bus #3203 (November 24) 9 8 7 6 5 4 3 2 1 0 7:12 AM  7:40 AM  8:09 AM  8:38 AM  9:07 AM  9:36 AM  Time Figure 2. CO Concentration measured during initial November 24 morning bus run.  Figure 4 indicates an occurrence of high CO peaks. In this example, for the first peak in the CO concentration the monitor which was placed closest to the front of the bus measured higher concentrations, suggesting infiltration of CO from external sources, probably other vehicles, as the main source of these elevated CO concentrations. The likelihood of external source is also consistent with the very short duration of the elevated CO levels. Figure 3 indicates an example of a CO peak in which the two monitors were placed side by side in the second row of seats and are in good agreement.  8  CO Concentration (ppm)  Bus #7342 (November 29) 35 30 25 20 15 10 5 0 6:43 AM  7:12 AM  7:40 AM  8:09 AM  8:38 AM  9:07 AM  9:36 AM 10:04 AM  Time Set 1  Set 2  Figure 3. CO concentration as measured by two monitors located side by side on bus.  CO Concentration (ppm  Bus #3202 (November 24) 40 35 30 25 20 15 10 5 0 1:26 PM  1:55 PM  2:24 PM  2:52 PM  3:21 PM  3:50 PM  4:19 PM  Time Set 1  Set 2  Figure 4. CO concentration as measured by two monitors in different locations on bus. Set 1 was located in row 2 of the 4 row bus and Set 1 was located next to the driver.  9  Air exchange rates Air exchange rates were evaluated for the two samples in which CO concentrations were elevated at periods other than the beginning of the bus runs and exhibited a smooth decay curve. In such instances, the CO concentration decay can be used as an internal tracer gas to estimate the air exchange rate (the time required for the air in the bus to be replaced with fresh air). For the November 29 run of bus #7342 (Figure 3) the estimated air exchange rate was 10.3 hr-1, meaning that the air within the bus is replaced with fresh air every 5.8 minutes. For The November 24 run of bus #3202 (Figure 4) the estimated air exchange rate was 13.5 hr-1, meaning that the air within the bus is replaced with fresh air every 4.4 minutes. These air exchange rates are based on the average of the two CO monitors in each bus; differences in the air exchange rates estimated by the CO concentrations measured by the two monitors were within 30% of each other. These air exchange rates are much higher than those measured in cars with closed windows and recirculated air (1.8 – 3.7 hr-1) but significantly lower than air exchange rates measured in cars with open windows (13-26 hr-1) or operating ventilation fans 36-37 hr-1.9 Carbon dioxide CO2 concentrations were somewhat higher, and sometimes exceeded the ASHRAE guideline value of 1000 ppm. The mean of all sample-period mean CO2 concentrations was 1061 ppm (sd = 321, range: 653 - 2279). The mean of all sample-period peak (1 minute) CO2 concentrations was 2115 ppm (sd = 1109, range: 1039- 5309 ppm). Examination of continuous monitoring data (Figures 5-10) indicates that at these occupancy levels the CO2 concentrations would be expected to continue to increase during longer duration bus runs.  9  Park J, Spengler JD, Yoon D, Dumyahn T, Lee K, Ozkaynak H. Measurement of air exchange rate of stationary vehicles and estimation of in-vehicle exposure. J Expo Anal Environ Epidemiol 1998; 8(1):65-78.6  10  CO2 Concentration (ppm  Bus #9342 (November 17)  2500 2000 1500 1000 500 0 2:09 PM 2:24 PM 2:38 PM 2:52 PM 3:07 PM 3:21 PM 3:36 PM 3:50 PM  Time Figure 5. 24 passengers boarded bus at 2:43 and then steadily exited until 3:18 when an additional 28 passengers boarded the bus. These passengers steadily exited until 3:44  CO2 Concentration (ppm)  Bus #7341 (November 19) 3000 2500 2000 1500 1000 500 0 2:09 PM  2:24 PM  2:38 PM  2:52 PM  3:07 PM  3:21 PM  3:36 PM  3:50 PM  4:04 PM  4:19 PM  Time Figure 6. Sharp increase in concentration at 2:50 corresponds to 29 passengers boarding the bus. 23 of these passengers exited between 2:55 and 3:13 when 27 additional passengers entered the bus.  11  CO2 Concentration (ppm)  Bus #7342 (November 29) 2000 1500 1000 500 0 6:43 AM  7:12 AM  7:40 AM  8:09 AM  8:38 AM  9:07 AM  9:36 AM 10:04 AM  Time  Figure 7. Increase in concentration corresponds to steady stream of passengers boarding the bus. 27 passengers exited at 7:56, followed by another steady stream of passengers boarding bus until 31 exited at 8:58.  CO2 Concentration (ppm  Bus #2446 (November 29) 2500 2000 1500 1000 500 0 1:55 PM  2:09 PM  2:24 PM  2:38 PM  2:52 PM  3:07 PM  3:21 PM  3:36 PM  3:50 PM  4:04 PM  4:19 PM  Time Figure 8. 18 passengers boarded the bus at 2:36 and steadily exited until 3:24 when an additional 33 passengers boarded. These passengers steadily exited until 3:51.  12  CO2 Concentration (ppm  Bus #2722 (December 8) 1500  1000  500  0 7:12 AM  7:40 AM  8:09 AM  8:38 AM  9:07 AM  Time  9:36 AM 2nd set  Figure 9. 6 passengers entered between 7:26 and 7:55 and exited at 8:03. 16 additional passengers entered between 8:20 and 9:01 and all exited at 9:07.  CO2 Concentration (ppm  Bus #2371 (December 15) 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 7:12 AM  7:40 AM  8:09 AM  8:38 AM  9:07 AM  9:36 AM  Time  10:04 AM  10:33 AM  11:02 AM  11:31 AM  2nd set  Figure 10. 21 passengers steadily entered between 7:32 and 7:45. At 7:51 an additional 14 passengers entered. All passengers exited at 7:55. Bus was idling and unoccupied until 8:34 when new passengers began steadily entering; these passengers exited at 9:06. At 9:37, 55 passengers entered bus and exited at 9:49. Bus then remained with engine off and door open until these 55 passengers re-entered at 10:39 and exited at 10:49.  13  In practice, CO2 concentrations dropped sharply as soon as occupancy levels decreased, as suggested by the high air exchange rates. The single long duration run where samples were collected was during a field trip – as such this was not typical because windows were allowed to be opened – normally this is restricted (Figure 11). During this trip, the concentration rose to a much higher level than on other bus runs, but this appeared to be more due to the high occupancy level than the trip duration. During this trip windows were open during the latter portion of each period. 7 windows were open between 9:28 and 10:32 at the end of the morning run, and 6 windows were open between 2:56 and 4:11 at end of the afternoon bus run.  CO2 Concentration (ppm)  Bus #2959 (December 7) 6000 5000 4000 3000 2000 1000 0 7:12 AM  8:24 AM  9:36 AM  10:48 AM  12:00 PM  1:12 PM  2:24 PM  3:36 PM  4:48 PM  6:00 PM  Time  Figure 11. Period between 10:30 and 2:45 with no data refers to period during which bus was not operated nor occupied during field trip. Measurements were not made during this period. Passengers steadily entered bus between 7:34 ad 7:45, when all 37 exited. Bus began steadily loading passengers at 8:37 until 8:53;all passengers exited at 9:01. Bus sat with engine off until 9:18 when 47 passengers boarded the bus. These 47 passengers exited at 10:32 and re-entered at 2:50 and remained on bus until 4:11. As there was some tendency for elevated CO2 concentrations we evaluated the observational variables (model, fuel, age, size, exhaust, time of bus run, person*minutes, window*minutes, sample duration, # of passengers) as potential predictors of high CO2 concentrations. For both mean CO2 and peak CO2, only the number of passengers was a significant predictor (multivariate regression, p<0.005): Maximum CO2: (Adjusted R2=0.59): Maximum CO2 (ppm) = 21.7 * #of passengers + 1081 Mean CO2 (ppm) = 4.1 * #of passengers + 866 Mean CO2: (Adjusted R2=0.23): With the exception of passenger age (see below) there was no evidence for a relationship between any of the other variables (bus age or size, model, fuel, etc.) on the CO2 concentrations. Elevated concentrations of CO and NO2 were rare and very transient and therefore it was not possible to evaluate any relationships between the occurrences of elevated concentrations of these gases and any bus or operational parameters.  14  We also evaluated the impact of the age of passengers on the indoor CO2 concentrations. Differences in CO2 concentrations between bus runs with different passenger ages were not statistically significant, and passenger age was not a significant predictor of indoor CO2 concentrations. However, there was a trend in the expected direction such that bus runs with older passengers had generally higher maximum and mean indoor CO2 concentrations (Figure 12ab10).  Maximum CO2 concentration (ppm)  6000  5000  4000  3000  2000  1000 0 N=  16  28  High school  3  2  Kindergarden  Elementary School  Elementary + High Sc  Passenger age Figure 12a. Maximum CO2 concentrations for different passenger age categories.  10  Number below x-axis indicates the number of samples from each age group. Top and bottom of box indicate the 25th and 75th percentiles, length of box is interquartile distance and box whiskers extend to measured values that are 1 interquartile distance from 75th and 25th percentiles, respectively. Line inside box indicates the median value.  15  Mean CO2 concentration (ppm)  4000  3000  2000  1000  0 N=  16  28  High school  3  2  Kindergarden  Elementary School  Elementary + High Sc  Passenger age  Figure 12b. Mean CO2 concentrations for different passenger age categories. Multiple measurements within the same bus As two CO2 monitors were used during most sample collection periods, we evaluated the agreement between the two measurements. We compared 10 sampling sessions where both monitors operated continuously without gaps and where both were placed at different randomlyselected locations within the bus. For these sessions, the mean slope was 0.91 (mean R2 = 0.83) and the average difference of sampler period mean concentrations was 128 ppm, suggesting good agreement between the two monitors located in different locations within the bus. No systematic differences in CO2 concentrations were observed between different monitoring locations, although small differences in concentrations were evident. For example, see Figure 13.  16  CO2 Concentration (ppm)  Bus #2720 (December 1)  1800 1600 1400 1200 1000 800 600 400 200 0 2:24:00 PM  2:38:24 PM  2:52:48 PM  3:07:12 PM  Time  Set 1  3:21:36 PM  3:36:00 PM  3:50:24 PM  Set 2  Figure 13. CO2 concentrations as measured by two different monitors located on the same bus. Set #1 was located in row 4 of the 12-row bus and Set #2 was in row 12. Repeat measurements of the same bus Due to the random selection of buses for sampling, repeat measurements were made inside three buses, with approximately 1 week in between the sampling periods. There was no standardization of the number of passengers on the buses or the routes on which they traveled for the two sets of measurements. There was relatively poor agreement between the two sets of measurements, suggesting that variables associated with the bus itself (fuel, size, etc.) were relatively unimportant in determining indoor air quality levels, whereas the use of the bus (number of passengers, time spent on bus, etc.) were more influential. The average difference between mean and maximum CO2 measurements for the two sets was 214 ppm and 1675 ppm, respectively. For CO, the average difference was 0.6 and 10.8 ppm for mean and maximum CO, respectively.  Conclusions  Results were generally in agreement with other reports of school bus air quality11. Measurements of air contaminants (NO2, hydrocarbons and CO) were generally low or near outdoor background levels in most instances. Occasional short-term elevated CO concentrations reaching approximately 30 ppm for 1-2 minutes were observed inside buses. These elevated CO concentrations were highly transient appeared to be associated with infiltration of CO from other vehicles and therefore do not indicate any malfunction of the bus exhaust or air intake system itself. However, there was some indication that exhaust gases emitted from nearby idling buses 11  Environmental Health Assessment of Indoor Air Quality in School Buses. Part 1, Part 2 - Student Passengers. Wisdom Consultants. 1999  17  may have entered into the passenger cabin and care should be taken to separate idling buses by distance and/or time to minimize the likelihood of such events. Alternatively, redesign of bus air intakes to a roof top intake may reduce the likelihood of exhaust gases from nearby vehicles entering the bus. Both of these possible remedies should be validated with further testing. CO2 concentrations were elevated above the 1000 ppm (the indoor air quality guideline value used by ASHRAE) in most instances, with the highest concentrations reaching 5000 ppm. Elevated CO2 concentrations were found to be associated with the number of passengers on the bus, but not with any of the other potential predictor variables that were measured. For these measurements, the amount of time spent on the bus was not a significant predictor of indoor CO2 concentrations. Measurements of air exchange rates indicated levels of ventilation on the order of one complete air exchange every 4-6 minutes. Taken together, these observations suggest that elevated CO2 concentrations are mainly a result of high passenger levels and that concentrations decrease quickly once occupant levels decrease. For significantly longer bus runs it is likely that CO2 concentrations above 1000 ppm will be reached and sustained, therefore measures to increase ventilation need to be implemented in these situations.  Acknowledgments This study would not have been possible without the cooperation of School District #34 (SD#34). Specifically we would like to thank Paul Clarke, Dave Boynton and Elaine Trickett of SD#34 Facilities and Transportation Services for their assistance in the organization and implementation of the measurements. We would also like to than all of the bus drivers who agreed to have measurements made during their bus runs. Finally, we thank the SD#34 students.  18  19  Bus Bus Bus Bus Fuel Adult Sampling Sampling # of person*min Sampling Window * min Exhaust Number Model Year Age Type Cap Date Time Passengers Duration Pipe 3025 1 1993 7 1 1 17-Nov-99 1 17 3864 110 0 2 9342 1 1999 1 2 2 17-Nov-99 2 58 6117 101 0 2 7340 1 1998 2 2 2 19-Nov-99 1 45 5867 109 0 2 7341 1 1998 2 2 2 19-Nov-99 2 60 23325 105 0 2 3126 2 1995 5 2 1 22-Nov-99 1 16 4275 104 0 2 2632 1 1990 10 1 1 22-Nov-99 2 13 1764 144 0 1 3203 1 1995 5 3 1 24-Nov-99 1 16 5620 115 65 3 3202 1 1995 5 3 1 24-Nov-99 2 10 2084 103 0 3 2373 1 1987 13 1 2 26-Nov-99 1 4 271 57 0 1 2631 1 1990 10 1 2 26-Nov-99 2 24 1113 46 17 1 7342 1 1998 2 2 2 29-Nov-99 1 67 13223 150 0 2 2446 1 1989 11 1 2 29-Nov-99 2 54 8297 130 0 1 2633 3 1990 10 1 1 1-Dec-99 1 18 5329 134 0 2 2720 2 1991 9 1 2 1-Dec-99 2 45 5468 89 0 1 2830 1 1992 8 2 2 2-Dec-99 1 60 3302 125 0 2 3124 1 1995 5 2 2 2-Dec-99 2 65 12440 230 12 2 9341 1 1999 1 2 2 3-Dec-99 1 34 6961 79 0 2 7342R 1 1998 2 2 2 3-Dec-99 2 64 10318 196 0 2 2721 2 1991 9 1 2 6-Dec-99 2 63 13343 89 14 1 2959 1 1993 7 2 2 7-Dec-99 3 151 12246 346 1105 2 2722 2 1991 9 1 1 8-Dec-99 1 22 1166 124 0 1 3220 2 1996 4 2 2 8-Dec-99 2 59 13310 236 1135 2 2373R 1 1987 13 1 2 9-Dec-99 1 55 4224 129 0 1 2960 2 1993 7 2 1 9-Dec-99 2 8 1939 99 60 2 2721R 2 1991 9 1 2 13-Dec-99 1 46 5465 136 0 1 8340 1 1998 2 2 1 13-Dec-99 2 16 2659 138 0 2 2371 1 1987 13 1 2 15-Dec-99 1 180 4197 221 0 1 2718 2 1991 9 1 2 15-Dec-99 2 26 5121 125 0 1 Table 2. Summary of bus characteristics during sampling sessions. R denotes repeat measurement of same bus. Bus Model: 1=Bluebird, 2=Thomas, 3=Wayne. Fuel: 1=propane, 2=diesel, 3=gas. Bus Size. 1=small (adult capacity 8-24 persons), 2=large (adult capacity 36 or 48 persons). Sampling time: 1 = morning, 2 = afternoon, 3=entire day. Exhaust pipe location: Back corner = 1, Back = 2, Side = 3. Number of passengers who were on the bus during sampling duration. Each sample may include 1-4 separate trips, therefore Number of passengers can exceed bus capacity. Person*min=number of passengers multiplied by number of minutes each spent on bus. Window*min=# of open windows multiplied by the time each window was open.  Bus Sampling CO (ppm) CO2 (ppm) CMS (ppm) NO2 (ppm) Number Date Mean Max. Mean Max. Min. SD Mean Max. CO NO2 HC 3025 17-Nov 1.8 22.0 895 1567 433 274 0.18 0.30 <5 < 0.5 < 20 9342 17-Nov 0.7 1.0 1217 2038 685 386 0.18 0.18 <5 < 0.5 < 20 7340 19-Nov 0.7 1.0 715 1488 449 280 0.18 0.18 <5 < 0.5 < 20 7341 19-Nov 0.7 4.0 1082 2441 529 527 0.18 0.30 <5 < 0.5 < 20 3126 22-Nov 0.7 2.0 1030 1863 549 371 0.18 0.40 <5 < 0.5 < 20 2632 22-Nov 0.8 7.0 928 1504 323 271 0.18 0.30 5.2 < 0.5 < 20 3203 24-Nov 1.7 8.0 1208 1913 590 413 0.18 0.18 <5 < 0.5 < 20 3202 24-Nov 1.6 13.0 0.18 0.30 16.1 < 0.5 < 20 2373 26-Nov 1.0 1.0 1271 1510 980 176 0.18 0.18 <5 < 20 2631 26-Nov 0.7 0.7 1115 1451 647 250 0.18 0.18 <5 < 0.5 < 20 7342 29-Nov 1.8 29.0 1001 1647 275 421 8.8 < 0.5 < 20 2446 29-Nov 0.7 1.0 1088 2000 471 426 0.18 0.18 <5 < 0.5 < 20 2633 1-Dec 2.7 4.0 949 1314 667 205 0.18 0.30 <5 < 0.5 < 20 2720 1-Dec 2.0 4.0 987 1451 745 193 0.18 0.30 <5 < 0.5 < 20 2830 2-Dec 0.7 1.0 1356 1863 627 373 0.18 0.30 <5 < 0.5 < 20 3124 2-Dec 0.7 2.0 806 2000 529 325 0.18 0.18 <5 < 0.5 < 20 9341 3-Dec 0.7 2.0 1293 3618 588 779 0.18 0.18 <5 < 0.5 < 20 7342R 3-Dec 0.7 1.0 1135 4426 608 935 0.18 0.18 <5 < 0.5 < 20 2721 6-Dec 0.7 1.0 1023 2515 490 524 0.18 0.18 <5 < 0.5 < 20 2959 7-Dec 1.4 4.0 2279 5309 392 1383 0.18 0.60 <5 < 0.5 < 20 2722 8-Dec 0.7 2.0 662 1039 490 139 0.18 0.18 3220 8-Dec 0.7 2.0 653 1608 451 240 0.18 0.18 <5 < 0.5 < 20 2373R 9-Dec 1.2 2.0 1136 2515 588 396 0.18 0.18 2960 9-Dec 0.7 1.0 835 1176 490 200 0.18 0.18 11.5 < 0.5 < 20 2721R 13-Dec 1.1 3.0 653 1275 490 200 0.18 0.30 8340 13-Dec 1.4 3.0 813 1196 373 194 0.18 0.30 <5 < 0.5 < 20 2371 15-Dec 1.7 5.0 1264 4647 490 966 0.18 0.40 5.1 < 0.5 < 20 2718 15-Dec 0.7 1.0 1261 1745 471 355 0.18 0.30 <5 < 0.5 < 20 Table 3. Summary statistics of air quality measurements during sampling sessions. R denotes repeat measurement of same bus. 0.707*LOD substituted for continuous monitoring values below LOD. Temp (oC) 17.32 18.17 21.78 20.28 16.83 17.09 18.59 19.60 17.55 21.15 21.32 20.58 19.83 20.38 20.80 20.99 22.13 19.58 18.58 17.72 19.58 22.30 21.92 17.95 18.35 18.64 18.74  RH (%) 62.87 66.85 37.50 50.94 47.62 68.68 59.08 52.02 43.57 43.29 41.81 42.00 58.24 49.74 36.28 55.17 35.43 44.43 44.65 40.17 30.98 44.71 45.15 37.63 37.32 57.14 61.99  20  APPENDICES 1. Information letter 2. Data forms  21  

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