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A cross-sectional study of schools for compliance to ventilation rate requirements
RESOURCE ARTICLE
A cross-sectional study of schools for compliance to ventilation rate requirements The use of continuous data logging instruments recorded levels of carbon dioxide (CO2) exhaled by students in schools. This allowed a check of a large data set in order to determine ventilation rates from the rate of its decay (DCO2). Data collected on CO2 levels through time was a useful tool for determining the effect of mechanical equipment to ventilation rates as part of a larger study on indoor air quality. Calculation of the air exchange rate (AER) used ASTM tracer gas methods applied to a regression analysis where the data showed a logarithmic decay rate. Ventilation measured in this study was total ventilation including mechanical ventilation from heating ventilation and air conditioning (HVAC) units, replacement air from exhaust fans and infiltration through outside walls.
By Roger G. Morse, Paul Haas, Stephen M. Lattanzio, Dean Zehnter, Matthew Divine INTRODUCTION
This paper reports on measurements of carbon dioxide (CO2) levels taken in schools that were part of larger study of indoor air quality. Schools reported on Roger G. Morse, AIA, is affiliated with Morse Zehnter Associates, Rensselaer Technology Park, 165 Jordan Road, Troy, NY 12180, USA. Paul Haas, CSP, CIH, is affiliated with Morse Zehnter Associates, 2240 Palm Beach Lakes Blvd., West Palm Beach, Florida, USA. Stephen M. Lattanzio, PE, is affiliated with Morse Zehnter Associates, Rensselaer Technology Park, 165 Jordan Road, Troy, NY 12180, USA. Dean Zehnter is affiliated with Morse Zehnter Associates, 2240 Palm Beach Lakes Blvd., Ste. 300, West Palm Beach, FL 33409, USA. Matthew Divine is affiliated with Morse Zehnter Associates, 2240 Palm Beach Lakes Blvd., West Palm Beach, Florida, USA.
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in this paper have heating ventilation and air conditioning (HVAC) systems designed to supply varying volumes of outside air per occupant for dilution ventilation. This study attempts to determine if the existing ventilation rate maintains acceptable indoor air quality. The study included schools that represented differing ages of construction, student grade levels and mechanical equipment. The indoor air quality (IAQ) procedure found in American Society of Heating Refrigeration and Air-Conditioning Engineers (ASHRAE) Standard 62.1-2004 Section 6.3 evaluated the ability of mechanical equipment to control contaminants by dilution and thus maintain good IAQ.1 Continuous logging of CO2 was employed and ventilation rates were estimated. Modeling used ASTM International (ASTM) tracer gas decay methodology found in E741-00 and D6245-98 and the ASHRAE primary outdoor air fraction method in 6.2.5.1 of ASHRAE 62.1.2,3 Findings from modeling indicate that several schools in the study had ventilation levels below their original design due to improperly maintained outside air ductwork and blocked dampers. In these classrooms evaluated, CO2 levels were elevated above 2000 ppm during occupancy. Classrooms served by equipment maintaining design ventilation, had CO2 levels that averaged 1100 ppm. Estimating ventilation rates using
ß Division of Chemical Health and Safety of the American Chemical Society Elsevier Inc. All rights reserved.
Equilibrium CO2 found in ASTM D6245 Section 10 and the ASHRAE Constant Emission Equilibrium in Appendix C of 62.1 were found to be largely inapplicable to the estimation of ventilation rate due to intermittent and/or variable occupancy in spaces studied. In evaluating indoor air quality, the larger study not only considered CO2, but also evaluated odors and measured other contaminants including carbon monoxide (CO), total volatile organic compounds (TVOC), and particulate, all of which were typically found at levels too low to be detected or were below established exposure levels. Odor complaints were logged for all spaces studied.
BACKGROUND
A school district desired to assess the indoor air quality provided by existing ventilation rates in a number of the district’s school buildings. A study designed specifically for school buildings in the hot humid climate of South Florida determined by rigorous scientific analysis, ventilation rates required to maintain acceptable indoor air quality in schools operating under specific conditions. This study relies on actual measurements of indoor air quality in occupied and operating schools rather than laboratory studies and measurements in other types of buildings in other parts of the country. As such,
1871-5532/$36.00 doi:10.1016/j.jchas.2009.02.001
this study has been able to determine with scientific rigor, the ventilation rates best tailored to provide optimal indoor air quality in schools in the study. Consensus, governmental and standard setting organizations were consulted for the choice of contaminants to be tested. Selection is based on sound industrial hygiene practice and the limits referenced by American National Standards Institute (ANSI)/ ASHRAE 62.1. Architectural, mechanical and operational confounders encountered and the variations addressed are part of the study data. Substantial quantity of data logging of temperature, relative humidity and CO2 levels as well five other Contaminants of Concern identified in the ASHRAE IAQ procedure allowed a database created to review these conditions on a global basis through campuses within the school district. An evaluation of a cross-section of a group of schools involving design measurements of ventilation rates and indoor air quality parameters such as CO, particulate and volatile organic compounds the data supports an acceptable air exchange rate for constant volume systems with building pressurization and a code compliant ventilation rate at design. A decay rate model used to confirm the ventilation rate based on data collected compared occupant exhaled CO2, temperature and relative humidity. Use of an interview process established occupant perception of indoor quality for classrooms and community spaces. This data showed that when design levels of outside air was used to dilute indoor contaminants an acceptable concentration of contaminants of concern and occupant satisfaction was achieved while relative humidity levels measured had a range of 55–60%. Notable exceptions included plenum space mixing chambers with multiple air handling units (AHUs) and unit ventilator (UV) equipment that had too much variation in operation to confirm the accuracy of data collected. Insufficient ventilation can result in degraded indoor air quality. The amount of adequate outdoor air used for improving air quality using dilution ventilation in schools is extensively
evaluated. An evaluation of references contained in a meta-analysis of studies suggests that ventilation rates at design do not show a direct affect on indoor environmental conditions in schools in attainment regions and that occupant perception of odors is not greatly influenced at ventilation rate modifications when residual confounding is taken into account.4
MATERIALS AND METHODS
To guide the evaluation the indoor environmental quality (IEQ) in schools in this assessment, a limited number of contaminants were selected for sampling. A methodology was developed from criteria for the assessment of a representation of a potentially vast number of chemical and physical agents that could be present in the school environment. Guidelines published by the ANSI/ASHRAE and ASTM formed the basis of the evaluation criteria for which standards are applied against for determination of acceptable dilution ventilation using outdoor air. There are a number of consensus, governmental and standard setting organizations, which are sources referenced by ANSI/ASHRAE and ASTM that developed sample methodology for measuring contaminants in workplaces. These are: The Centers for Disease Control (CDC) – National Institute of Occupational Safety and Health (NIOSH) Sampling and Analytical Methods and Recommended Exposure Limits (RELs), the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLVsTM), and the U.S. Department of Labor, Occupational Safety and Health Administration (OSHA) Permissible Exposure Limits (PELs), Short Term Exposure Limits (STELs) and Ceiling Limits (C).5–7 Indoor contaminants chosen for testing in this study are based on limits expressed in ANSI/ASHRAE 62.12004 Appendix B – ‘Guidance for the Establishment of Air Quality Criteria for the Indoor Environment’ (see ANSI/ASHRAE 62.1-2004 Appendix B, Tables B-1 to B-4). Additional references from the ACGIH, U.S. Environ-
Journal of Chemical Health & Safety, November/December 2009
mental Protection Agency (USEPA), Housing and Urban Development Department (HUD), NIOSH, OSHA and the World Health Organization (WHO) were considered as guidance for which contaminants required testing.8 The recommendations of ANSI/ ASHRAE in publications include design criteria for indoor environments including schools. The criteria are found in ANSI/ASHRAE Standard 62.1-2004: Ventilation for Acceptable Indoor Air Quality. ASHRAE 62.12004 adapts contaminants and maximum concentrations deemed acceptable by the USEPA for air quality from the use of outdoor air. These are termed ‘contaminants of concern’ by ASHRAE. Contaminants of concern measured in this study were CO2, CO, VOCs and particulate. Temperature and relative humidity were also measured in the study. Indoor air quality is achieved in the study schools by dilution of indoor contaminants with outdoor air, called dilution ventilation. This study used the acceptable concentration of contaminants of concern for outdoor air as indoor air criteria thereby setting the most stringent criteria that is attainable by ordinary means. The school district covered by this study accepted four of these contaminants of concern as the basis for criteria for testing to assure acceptable ventilation by outdoor air (OA). OA ventilation rates are typically expressed as liters per second (L/s) or in this study reported as cubic feet per minute of OA per occupant (CFM/person), which are designed to dilute human bioeffluent, odors and other nuisance contaminants to acceptable levels. Mixed air is filtered to remove particulate. A number of spaces such as classrooms and community function rooms in each of the schools were measured as representative for the concentration and type of indoor air contaminants throughout each campus. The levels of contaminants were expected to vary within the spaces due to changes in indoor activities, building materials, furnishings, population, scheduling, and surface finishes as sources. Common contaminants generated by indoor pollutant sources produced 5
TVOC’s and particulate and they were measured following the IAQ procedure outlined in the ANSI/ASHRAE 62 standard. In no way should this sampling data or the evaluation criteria as is presented be construed as a measure of safety for school occupants, particularly children. The primary purpose of the study measured concentrations of contaminants of concern at existing operation and compared the level against recommendations for acceptable IAQ as described in the IAQ procedure of ASHRAE. The results consider mechanical equipment upgrades as a basis for the school district design criteria and applicable mechanical code requirements. The data obtained for schools in this study was analyzed by first calculating airflow values from mechanical equipment with parameters obtained in a prior study for the same schools. HVAC operation was assessed including collecting exhaust, supply, return and outdoor air intake volumes. Measurement of outdoor air during the period of continuous data logging period and populations in classrooms were followed. This approach is summarized in the following discussion of the estimation of outdoor air fraction supplied to air handlers and air exchange rates estimated. The spaces studied were a single zone or classrooms in a portion of a designated air handling zone in a school. A dedicated HVAC device such as a fan coil unit (FCU) or a UV normally served single zone classrooms. The majority of spaces sampled in this study were in a multiple zone with a number of classrooms served typically by a constant volume AHU’s with dehumidification enhancements such as face and bypass. In this study, ventilation design and performance was first assessed by the sum of the measured supply airflow to the space along with the outdoor air fraction delivered from mechanical equipment such as AHU’s, FCU’s and UV’s. Ventilation values included return air recirculation and exhaust airflows to the spaces. Occupancy of spaces is contrasted between design values and contemporary schedules provided by school administrators. Data from the 6
Figure 1. Carbon dioxide CO2 levels vs ventilation rate.
study along with prior airflow measurements includes data calculated using field instruments. For example, data for supply and return airflow of a system using pitot duct traverse measurements that were converted into volumetric airflow. Raw data taken in the field also included flow hood measurements of room supply, return, exhaust and outdoor air intake volumes. The flow hood values were used whenever possible to account for duct leaks. Traverse readings of the outdoor air intake at the unit and the intake size measured outdoor air volumes. In some cases, readings
revealed little or no useful data about outdoor air volumes due to blocked or damaged intakes. The uncertainty of these methods in obtaining ventilation air as well as quality control checks performed by our firm of a particular building by analyzing the prior airflow study should be taken into account in reviewing the data in this study. (Data is available but is not published.)
RESULTS
The schools assessed for indoor air quality where data logging of CO2
Figure 2. Comparison of carbon dioxide levels in three classrooms.
Journal of Chemical Health & Safety, November/December 2009
was performed for the period from April 2005 to November 2006. This interval represents two annual periods for the schools studied. Data from approximately 40 classrooms logged for outdoor air exchange rate (OAER) in terms of number of changes per person due to ventilation based on carbon dioxide decay estimation was analyzed. The following figure describes a trend of the data set for the classrooms logged for rates of ventilation rates in values of outdoor per person versus the range of CO2 in parts per million (ppm) of the gas in a million parts of air as graphs of the geometric mean and confidence limits (Figure 1). Carbon dioxide decay (DCO2) estimation use is an objective to determine if the ventilation was operating in accordance with the original design intent for air exchange. The correlation of the rate of ventilation from the decay of carbon dioxide after occupants left the spaces served by mechanical equipment resulted in an impression of the overall ability of the building ventilation. This included exhaust, infiltration and mixing with re-circulated air to control indoor pollutants producing odors. In this group of schools studied, carbon dioxide levels were found in certain instances to be elevated greater than the recommended 700 ppm above background. The estimation of ventilation from the decay or DCO2 representing an air exchange rate in classrooms designed and determined to be supplying approximately 5 cubic feet per minute (CFM) to 10 CFM outdoor air per occupant produced values to levels from 1000 to 1500 ppm above background during occupancy. These measured concentrations represented a peak above maximum CO2 during certain portions of occupied periods that would decay over time from ventilation once occupants left the space. Averaged concentrations during occupancy ranged from 800 to 1200 ppm above background when ventilation rates approximated 5 CFM/occupant. Rather than being used as an indicator of the overall IAQ in the schools, the rate of decay with DCO2 used to estimate ventilation rates in the study group tended to show a predictable,
Figure 3. Decay period for ES05 Building 4, Classroom 405.
easily measured product. The decay estimation from DCO2 was used to ensure that the objective of the IAQ Method as described by ASHRAE 62.1 was part of and not the entire deliverable showing the existing ventilation levels controlled indoor pollutants and odors or that they should be increased by design. Instead, the data from the schools studied indicated what has been written about the use of DCO2 to estimate ventilation rates and IAQ – ‘‘Carbon
dioxide measurements say little about how healthy an indoor environment is. But they can tell a great deal about how comfortable it is and, with good observation, can give important clues as to where to look for factors that may be affecting health.’’9 ASTM methods use a tracer gas to determine air exchange rates in buildings – ASTM D 6245 & E 741 have standardized the use of CO2 generated from occupants for ventilation estimation. ASHRAE 62.1-2004 indicates
Figure 4. Regression method plot of concentrations and estimate of concentrations for ES05 Building 4, Classroom 405.
Journal of Chemical Health & Safety, November/December 2009
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determination of CO2 levels from occupant exhalation along with odors can be considered as a surrogate of the air quality based on the rate of outdoor air used to dilute indoor pollutants.10 The use of continuous data logging instruments recorded levels of CO2 in the schools in this study and then an estimate of the ventilation rate from the rate of its decay by ventilation (DCO2) was made. This approach has been described in references as affordable, infers as to air exchange in mechanically ventilated single and multi-zone spaces and is comparable to other tracer gases such as sulfur hexafluoride (SF6) use, particularly when air exchange rates are low (<2.5 times per hour).11–13 The data evaluated conditions that would influence a rate of decay from a steady state concentration. These principally architectural, mechanical and occupant factors could confound data interpretation. In some cases, the influence of these factors was observed after the post-data processing was made and the DCO2 graphs analyzed. Modifications to the decay rate time interval were then made or the data was discarded as inapplicable to an estimation of ventilation from the DCO2 or decay concentrations. These factors occurred from such diverse things such as the influence of a wash out effect when doors to the outside were simultaneously opened during and after school dismissal, poor mixing (stratification) of outside air with recirculated air or the presence of occupants in a zone after the school day. In general, curve fitting due to the slope of the decay curve(s) was patterned after school dismissal to some given time and then the rate of decay was estimated based on the intervals of 1, 2 and 4 hour timeframes. In several cases, very low rates of outdoor air exchange existed likely due to infiltration through the building envelope. In cases where decay curves were not representative of the logarithmic regression, the data was eliminated as a measure of a particular room air exchange rate. The following example of decay graphs from one of the study schools emphasizes the type of effects from certain factors on the decay estimation. 8
Figure 5. Decay period for ES05 Building 13, Music 926.
DISCUSSION
The use of CO2 generated from occupants as a tracer gas to determine air exchange rates in buildings are described by ASTM in D6245 Standard Guide for Using Indoor Carbon Dioxide Concentrations to Evaluate Indoor Air Quality and Ventilation. Refer to ASHRAE 62.1-2004, Appendix C as a means to determine CO2 levels from occupant exhalation along with odors that consider the gas as a surrogate of the air quality based on
the rate of outdoor air used to dilute it and other indoor pollutants. CO2 levels in each tested space were logged for a seven-day period with an interval of four (4) to six (6) minutes between measurements. Measurements were taken with a Telaire 7000 dual path infrared monitor for CO2 cabled to a HOBO H8 data logger. CO2 concentrations were plotted against time for the entire seven-day period and evaluated for consistency. A typical day was then selected and plotted for a more detailed evaluation.
Figure 6. Regression method plot of concentrations and estimate of concentrations for ES05 Building 13, Music 926.
Journal of Chemical Health & Safety, November/December 2009
It was clear from inspection of the plots that the classrooms were not occupied for a sufficiently long period for CO2 levels to reach equilibrium. As such, the ASTM D6245 Section 10 method, which bases its calculation on equilibrium conditions, cannot be used to determine ventilation rates. This left methods that were based on the decay rate as CO2 levels were reduced by ventilation. The data plots from a typical day were examined to determine the time window during which CO2 levels decayed in a logarithmic fashion. It was found that these conditions were typically found at the end of the school day when the rooms were vacated. Figure 2 presents data plots for CO2 concentrations typically found in classrooms. This typical pattern shows a rapid rise in concentration upon initial occupancy, followed by fluctuations occurring during changes in occupancy at lunch time, with a decay induced by ventilation after the classroom is vacated for the day. However, several confounders could affect the data. These factors occurred from such diverse things such as the influence of a wash out effect when doors to the outside were simultaneously opened during and after school dismissal, poor mixing (stratification) of outside air with re-circulated air or the presence of occupants in a zone after the school day. Once familiarity had been gained with the data the consequences of these factors could easily be seen in the data plots. Once a subset of the data representing the decay of CO2 levels due to dilution with ventilation air was selected it was found that there was good agreement between air exchange rates (AER) determined by the Averaged Method and the Optional Regression Method of ASTM E741-00. Figures 3 and 4 are an example of a lognormal decay rate in Building 4 with a comparison of the same decay period evaluated using the ASTM Averaged and Optional Regression Method. When confounders existed such as poor mixing or occupant generated CO2 in the zone, there was disagreement between the AERs determined by these two methods. This allowed a check on the data set selected to determine ventilation rates.
Figure 7. Decay period for ES05 Building 9, Classroom 903.
Decay rates used in this manner will have an artificial appearance of precision, thus making it look more accurate since only one of five days of occupancy was chosen. This bears further discussion that is beyond the scope of this paper. This decay rate evaluation could also be used for the theoretical time for an occupied room to reach equilibrium as a means to check the primary air fraction values. Great care was used to select the data from a time interval where there were minimal influence of confounders and where the data showing a logarithmic
decay rate. The AER was then calculated using the two methods as a check on the suitability of the data selection. Figures 5 and 6 point out how occupancy in a zone after school dismissal influences carbon dioxide decay in Building 13 in the study school. Figures 7 and 8 show CO2 levels in a room with excessive infiltration due to openings to the outside above the porous classroom ceiling. As can be seen, the CO2 concentrations are greatly affected by changes in the ventilation rate that resulted from wind driven changes in the infiltration rate. Despite
Figure 8. Regression method plot of concentrations and estimate of concentrations for ES05 Building 9, Classroom 903.
Journal of Chemical Health & Safety, November/December 2009
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the variability introduced into the data, it was still possible to calculate an air change rate for this room
CONCLUSION
Data collected on CO2 levels through time was found to be a useful tool for determining ventilation rates as part of a study on indoor air quality. Data was collected for seven days and then plotted. It was possible to select a typical day from this first plot. A more detailed plot was prepared of this typical day. By inspection, it was possible to select a period for analysis where the pattern of CO2 decay was consistent with a vacated space. The AER was calculated using methods in ASTM E741-00. As a check on the data selection, the AER was calculated using two different methods.
Data collected on CO2 levels through time was found to be a useful tool for determining ventilation rates as part of a study on indoor air quality. Ventilation measured in this study was total ventilation including mechanical ventilation from HVAC units, replacement air from exhaust fans and infiltration through outside walls. If substantial changes are made to door and window installations that could reduce infiltration, this could affect total ventilation rates and IAQ
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measurements would need to be repeated. After completion of renovation work on HVAC, equipment ventilation rates should be verified and continuation of indoor air quality complying with ANSI/ASHRAE 62.1-2004 should be confirmed by measurement of the contaminants of concern. Measurements were performed in mature buildings whose finishes and furniture had out gassed any VOCs prior to the study. Any renovation work involving introduction of new finish materials, furniture or other materials that could release volatile organic compounds such as painting, wall coverings, furniture, carpeting, etc. should incorporate an out gassing period that includes purging of building air. If the criterion for indoor air quality set forth in ANSI/ASHRAE 62.1 changes, the indoor air quality of the school needs to be evaluated against the new standard. REFERENCES
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1. ANSI/ASHRAE. Standard Ventilation for Acceptable Indoor Air Quality 622001. American National Standards Institute/American Society of Heating, Refrigeration and Air Conditioning Engineers: Atlanta, GA, 2001. 2. American Society for Testing and Materials. Standard Test Method for Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution (ASTM E 741-00). Author: West Conshohocken, PA, 2000. 3. American Society for Testing and Materials. Standard Guide for Using Indoor Carbon Dioxide Concentrations to Evaluate Indoor Air Quality and Ventilation (ASTM D 6245-98). Author: West Conshohocken, PA, 1998. 4. Seppanen O.; Fisk W.; Mendell M. Ventilation Rates and Health. ASHRAE J. 2002, Daisey J.; Angell W.; Apte M. Indoor Air Quality, Ventilation and Health Symptoms in Schools: An Ana-
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lysis of Existing Information LBNL 48287; US DOE/EPA DE-AC0376SF0098. NIOSH. Handbook of Analytical Methods; U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH); Cincinnati, OH, 2004. ACGIH. 2004–2005 TLVs1 and BEIs1; Threshold Limit Values for Chemical Substances and Physical Agents. American Conference of Governmental Industrial Hygienists, Cincinnati, OH, 2004. CFR. 29 CFR 1910.1000 to end Code of Federal Regulations; U.S. Government Printing Office, Office of the Federal Register; Washington, DC, 2002. US Environmental Protection Agency, US Public Health Service, & National Environmental Health Association. Introduction to air quality: A reference manual (EPA-400-3-91-003), (87). US Environmental Protection Agency, Washington, DC, 1991 [http://www. epa.gov/]. Woodcock, R. CO2 measurements for IAQ analysis. Occup. Health Saf. 2006, [http://ohsonline.com/stevens/ ohspub.nsf]. Daisey, J.; Angell, W; Apte M. Indoor air quality, ventilation and health symptoms in schools: an analysis of existing information (LBNL 48287; US DOE/EPA DE-AC03-76SF0098). Persily, A. Manual for Ventilation Assessment in Mechanically Ventilated Commercial Buildings, NISTIR 5329, National Institute of Standards and Technology, Gaithersburg, MD, 1994 [Manual for Ventilation Assessment NISTIR 5329]. Roulet, C.; Foradini, F. Simple and cheap air change measurements using CO2 concentration decays. Int. J. Ventil. 2001, 1(1), 39–44. Sherman M. Air Infiltration Measurement Techniques (Revision of LBL10705) Proceedings 9th AIVC Conference. Effect. Ventil. 1(1), 1998, pp. 1–26.
Journal of Chemical Health & Safety, November/December 2009
A cross-sectional study of schools for compliance to ventilation rate requirements The use of continuous data logging instruments recorded levels of carbon dioxide (CO2) exhaled by students in schools. This allowed a check of a large data set in order to determine ventilation rates from the rate of its decay (DCO2). Data collected on CO2 levels through time was a useful tool for determining the effect of mechanical equipment to ventilation rates as part of a larger study on indoor air quality. Calculation of the air exchange rate (AER) used ASTM tracer gas methods applied to a regression analysis where the data showed a logarithmic decay rate. Ventilation measured in this study was total ventilation including mechanical ventilation from heating ventilation and air conditioning (HVAC) units, replacement air from exhaust fans and infiltration through outside walls.
By Roger G. Morse, Paul Haas, Stephen M. Lattanzio, Dean Zehnter, Matthew Divine INTRODUCTION
This paper reports on measurements of carbon dioxide (CO2) levels taken in schools that were part of larger study of indoor air quality. Schools reported on Roger G. Morse, AIA, is affiliated with Morse Zehnter Associates, Rensselaer Technology Park, 165 Jordan Road, Troy, NY 12180, USA. Paul Haas, CSP, CIH, is affiliated with Morse Zehnter Associates, 2240 Palm Beach Lakes Blvd., West Palm Beach, Florida, USA. Stephen M. Lattanzio, PE, is affiliated with Morse Zehnter Associates, Rensselaer Technology Park, 165 Jordan Road, Troy, NY 12180, USA. Dean Zehnter is affiliated with Morse Zehnter Associates, 2240 Palm Beach Lakes Blvd., Ste. 300, West Palm Beach, FL 33409, USA. Matthew Divine is affiliated with Morse Zehnter Associates, 2240 Palm Beach Lakes Blvd., West Palm Beach, Florida, USA.
4
in this paper have heating ventilation and air conditioning (HVAC) systems designed to supply varying volumes of outside air per occupant for dilution ventilation. This study attempts to determine if the existing ventilation rate maintains acceptable indoor air quality. The study included schools that represented differing ages of construction, student grade levels and mechanical equipment. The indoor air quality (IAQ) procedure found in American Society of Heating Refrigeration and Air-Conditioning Engineers (ASHRAE) Standard 62.1-2004 Section 6.3 evaluated the ability of mechanical equipment to control contaminants by dilution and thus maintain good IAQ.1 Continuous logging of CO2 was employed and ventilation rates were estimated. Modeling used ASTM International (ASTM) tracer gas decay methodology found in E741-00 and D6245-98 and the ASHRAE primary outdoor air fraction method in 6.2.5.1 of ASHRAE 62.1.2,3 Findings from modeling indicate that several schools in the study had ventilation levels below their original design due to improperly maintained outside air ductwork and blocked dampers. In these classrooms evaluated, CO2 levels were elevated above 2000 ppm during occupancy. Classrooms served by equipment maintaining design ventilation, had CO2 levels that averaged 1100 ppm. Estimating ventilation rates using
ß Division of Chemical Health and Safety of the American Chemical Society Elsevier Inc. All rights reserved.
Equilibrium CO2 found in ASTM D6245 Section 10 and the ASHRAE Constant Emission Equilibrium in Appendix C of 62.1 were found to be largely inapplicable to the estimation of ventilation rate due to intermittent and/or variable occupancy in spaces studied. In evaluating indoor air quality, the larger study not only considered CO2, but also evaluated odors and measured other contaminants including carbon monoxide (CO), total volatile organic compounds (TVOC), and particulate, all of which were typically found at levels too low to be detected or were below established exposure levels. Odor complaints were logged for all spaces studied.
BACKGROUND
A school district desired to assess the indoor air quality provided by existing ventilation rates in a number of the district’s school buildings. A study designed specifically for school buildings in the hot humid climate of South Florida determined by rigorous scientific analysis, ventilation rates required to maintain acceptable indoor air quality in schools operating under specific conditions. This study relies on actual measurements of indoor air quality in occupied and operating schools rather than laboratory studies and measurements in other types of buildings in other parts of the country. As such,
1871-5532/$36.00 doi:10.1016/j.jchas.2009.02.001
this study has been able to determine with scientific rigor, the ventilation rates best tailored to provide optimal indoor air quality in schools in the study. Consensus, governmental and standard setting organizations were consulted for the choice of contaminants to be tested. Selection is based on sound industrial hygiene practice and the limits referenced by American National Standards Institute (ANSI)/ ASHRAE 62.1. Architectural, mechanical and operational confounders encountered and the variations addressed are part of the study data. Substantial quantity of data logging of temperature, relative humidity and CO2 levels as well five other Contaminants of Concern identified in the ASHRAE IAQ procedure allowed a database created to review these conditions on a global basis through campuses within the school district. An evaluation of a cross-section of a group of schools involving design measurements of ventilation rates and indoor air quality parameters such as CO, particulate and volatile organic compounds the data supports an acceptable air exchange rate for constant volume systems with building pressurization and a code compliant ventilation rate at design. A decay rate model used to confirm the ventilation rate based on data collected compared occupant exhaled CO2, temperature and relative humidity. Use of an interview process established occupant perception of indoor quality for classrooms and community spaces. This data showed that when design levels of outside air was used to dilute indoor contaminants an acceptable concentration of contaminants of concern and occupant satisfaction was achieved while relative humidity levels measured had a range of 55–60%. Notable exceptions included plenum space mixing chambers with multiple air handling units (AHUs) and unit ventilator (UV) equipment that had too much variation in operation to confirm the accuracy of data collected. Insufficient ventilation can result in degraded indoor air quality. The amount of adequate outdoor air used for improving air quality using dilution ventilation in schools is extensively
evaluated. An evaluation of references contained in a meta-analysis of studies suggests that ventilation rates at design do not show a direct affect on indoor environmental conditions in schools in attainment regions and that occupant perception of odors is not greatly influenced at ventilation rate modifications when residual confounding is taken into account.4
MATERIALS AND METHODS
To guide the evaluation the indoor environmental quality (IEQ) in schools in this assessment, a limited number of contaminants were selected for sampling. A methodology was developed from criteria for the assessment of a representation of a potentially vast number of chemical and physical agents that could be present in the school environment. Guidelines published by the ANSI/ASHRAE and ASTM formed the basis of the evaluation criteria for which standards are applied against for determination of acceptable dilution ventilation using outdoor air. There are a number of consensus, governmental and standard setting organizations, which are sources referenced by ANSI/ASHRAE and ASTM that developed sample methodology for measuring contaminants in workplaces. These are: The Centers for Disease Control (CDC) – National Institute of Occupational Safety and Health (NIOSH) Sampling and Analytical Methods and Recommended Exposure Limits (RELs), the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLVsTM), and the U.S. Department of Labor, Occupational Safety and Health Administration (OSHA) Permissible Exposure Limits (PELs), Short Term Exposure Limits (STELs) and Ceiling Limits (C).5–7 Indoor contaminants chosen for testing in this study are based on limits expressed in ANSI/ASHRAE 62.12004 Appendix B – ‘Guidance for the Establishment of Air Quality Criteria for the Indoor Environment’ (see ANSI/ASHRAE 62.1-2004 Appendix B, Tables B-1 to B-4). Additional references from the ACGIH, U.S. Environ-
Journal of Chemical Health & Safety, November/December 2009
mental Protection Agency (USEPA), Housing and Urban Development Department (HUD), NIOSH, OSHA and the World Health Organization (WHO) were considered as guidance for which contaminants required testing.8 The recommendations of ANSI/ ASHRAE in publications include design criteria for indoor environments including schools. The criteria are found in ANSI/ASHRAE Standard 62.1-2004: Ventilation for Acceptable Indoor Air Quality. ASHRAE 62.12004 adapts contaminants and maximum concentrations deemed acceptable by the USEPA for air quality from the use of outdoor air. These are termed ‘contaminants of concern’ by ASHRAE. Contaminants of concern measured in this study were CO2, CO, VOCs and particulate. Temperature and relative humidity were also measured in the study. Indoor air quality is achieved in the study schools by dilution of indoor contaminants with outdoor air, called dilution ventilation. This study used the acceptable concentration of contaminants of concern for outdoor air as indoor air criteria thereby setting the most stringent criteria that is attainable by ordinary means. The school district covered by this study accepted four of these contaminants of concern as the basis for criteria for testing to assure acceptable ventilation by outdoor air (OA). OA ventilation rates are typically expressed as liters per second (L/s) or in this study reported as cubic feet per minute of OA per occupant (CFM/person), which are designed to dilute human bioeffluent, odors and other nuisance contaminants to acceptable levels. Mixed air is filtered to remove particulate. A number of spaces such as classrooms and community function rooms in each of the schools were measured as representative for the concentration and type of indoor air contaminants throughout each campus. The levels of contaminants were expected to vary within the spaces due to changes in indoor activities, building materials, furnishings, population, scheduling, and surface finishes as sources. Common contaminants generated by indoor pollutant sources produced 5
TVOC’s and particulate and they were measured following the IAQ procedure outlined in the ANSI/ASHRAE 62 standard. In no way should this sampling data or the evaluation criteria as is presented be construed as a measure of safety for school occupants, particularly children. The primary purpose of the study measured concentrations of contaminants of concern at existing operation and compared the level against recommendations for acceptable IAQ as described in the IAQ procedure of ASHRAE. The results consider mechanical equipment upgrades as a basis for the school district design criteria and applicable mechanical code requirements. The data obtained for schools in this study was analyzed by first calculating airflow values from mechanical equipment with parameters obtained in a prior study for the same schools. HVAC operation was assessed including collecting exhaust, supply, return and outdoor air intake volumes. Measurement of outdoor air during the period of continuous data logging period and populations in classrooms were followed. This approach is summarized in the following discussion of the estimation of outdoor air fraction supplied to air handlers and air exchange rates estimated. The spaces studied were a single zone or classrooms in a portion of a designated air handling zone in a school. A dedicated HVAC device such as a fan coil unit (FCU) or a UV normally served single zone classrooms. The majority of spaces sampled in this study were in a multiple zone with a number of classrooms served typically by a constant volume AHU’s with dehumidification enhancements such as face and bypass. In this study, ventilation design and performance was first assessed by the sum of the measured supply airflow to the space along with the outdoor air fraction delivered from mechanical equipment such as AHU’s, FCU’s and UV’s. Ventilation values included return air recirculation and exhaust airflows to the spaces. Occupancy of spaces is contrasted between design values and contemporary schedules provided by school administrators. Data from the 6
Figure 1. Carbon dioxide CO2 levels vs ventilation rate.
study along with prior airflow measurements includes data calculated using field instruments. For example, data for supply and return airflow of a system using pitot duct traverse measurements that were converted into volumetric airflow. Raw data taken in the field also included flow hood measurements of room supply, return, exhaust and outdoor air intake volumes. The flow hood values were used whenever possible to account for duct leaks. Traverse readings of the outdoor air intake at the unit and the intake size measured outdoor air volumes. In some cases, readings
revealed little or no useful data about outdoor air volumes due to blocked or damaged intakes. The uncertainty of these methods in obtaining ventilation air as well as quality control checks performed by our firm of a particular building by analyzing the prior airflow study should be taken into account in reviewing the data in this study. (Data is available but is not published.)
RESULTS
The schools assessed for indoor air quality where data logging of CO2
Figure 2. Comparison of carbon dioxide levels in three classrooms.
Journal of Chemical Health & Safety, November/December 2009
was performed for the period from April 2005 to November 2006. This interval represents two annual periods for the schools studied. Data from approximately 40 classrooms logged for outdoor air exchange rate (OAER) in terms of number of changes per person due to ventilation based on carbon dioxide decay estimation was analyzed. The following figure describes a trend of the data set for the classrooms logged for rates of ventilation rates in values of outdoor per person versus the range of CO2 in parts per million (ppm) of the gas in a million parts of air as graphs of the geometric mean and confidence limits (Figure 1). Carbon dioxide decay (DCO2) estimation use is an objective to determine if the ventilation was operating in accordance with the original design intent for air exchange. The correlation of the rate of ventilation from the decay of carbon dioxide after occupants left the spaces served by mechanical equipment resulted in an impression of the overall ability of the building ventilation. This included exhaust, infiltration and mixing with re-circulated air to control indoor pollutants producing odors. In this group of schools studied, carbon dioxide levels were found in certain instances to be elevated greater than the recommended 700 ppm above background. The estimation of ventilation from the decay or DCO2 representing an air exchange rate in classrooms designed and determined to be supplying approximately 5 cubic feet per minute (CFM) to 10 CFM outdoor air per occupant produced values to levels from 1000 to 1500 ppm above background during occupancy. These measured concentrations represented a peak above maximum CO2 during certain portions of occupied periods that would decay over time from ventilation once occupants left the space. Averaged concentrations during occupancy ranged from 800 to 1200 ppm above background when ventilation rates approximated 5 CFM/occupant. Rather than being used as an indicator of the overall IAQ in the schools, the rate of decay with DCO2 used to estimate ventilation rates in the study group tended to show a predictable,
Figure 3. Decay period for ES05 Building 4, Classroom 405.
easily measured product. The decay estimation from DCO2 was used to ensure that the objective of the IAQ Method as described by ASHRAE 62.1 was part of and not the entire deliverable showing the existing ventilation levels controlled indoor pollutants and odors or that they should be increased by design. Instead, the data from the schools studied indicated what has been written about the use of DCO2 to estimate ventilation rates and IAQ – ‘‘Carbon
dioxide measurements say little about how healthy an indoor environment is. But they can tell a great deal about how comfortable it is and, with good observation, can give important clues as to where to look for factors that may be affecting health.’’9 ASTM methods use a tracer gas to determine air exchange rates in buildings – ASTM D 6245 & E 741 have standardized the use of CO2 generated from occupants for ventilation estimation. ASHRAE 62.1-2004 indicates
Figure 4. Regression method plot of concentrations and estimate of concentrations for ES05 Building 4, Classroom 405.
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determination of CO2 levels from occupant exhalation along with odors can be considered as a surrogate of the air quality based on the rate of outdoor air used to dilute indoor pollutants.10 The use of continuous data logging instruments recorded levels of CO2 in the schools in this study and then an estimate of the ventilation rate from the rate of its decay by ventilation (DCO2) was made. This approach has been described in references as affordable, infers as to air exchange in mechanically ventilated single and multi-zone spaces and is comparable to other tracer gases such as sulfur hexafluoride (SF6) use, particularly when air exchange rates are low (<2.5 times per hour).11–13 The data evaluated conditions that would influence a rate of decay from a steady state concentration. These principally architectural, mechanical and occupant factors could confound data interpretation. In some cases, the influence of these factors was observed after the post-data processing was made and the DCO2 graphs analyzed. Modifications to the decay rate time interval were then made or the data was discarded as inapplicable to an estimation of ventilation from the DCO2 or decay concentrations. These factors occurred from such diverse things such as the influence of a wash out effect when doors to the outside were simultaneously opened during and after school dismissal, poor mixing (stratification) of outside air with recirculated air or the presence of occupants in a zone after the school day. In general, curve fitting due to the slope of the decay curve(s) was patterned after school dismissal to some given time and then the rate of decay was estimated based on the intervals of 1, 2 and 4 hour timeframes. In several cases, very low rates of outdoor air exchange existed likely due to infiltration through the building envelope. In cases where decay curves were not representative of the logarithmic regression, the data was eliminated as a measure of a particular room air exchange rate. The following example of decay graphs from one of the study schools emphasizes the type of effects from certain factors on the decay estimation. 8
Figure 5. Decay period for ES05 Building 13, Music 926.
DISCUSSION
The use of CO2 generated from occupants as a tracer gas to determine air exchange rates in buildings are described by ASTM in D6245 Standard Guide for Using Indoor Carbon Dioxide Concentrations to Evaluate Indoor Air Quality and Ventilation. Refer to ASHRAE 62.1-2004, Appendix C as a means to determine CO2 levels from occupant exhalation along with odors that consider the gas as a surrogate of the air quality based on
the rate of outdoor air used to dilute it and other indoor pollutants. CO2 levels in each tested space were logged for a seven-day period with an interval of four (4) to six (6) minutes between measurements. Measurements were taken with a Telaire 7000 dual path infrared monitor for CO2 cabled to a HOBO H8 data logger. CO2 concentrations were plotted against time for the entire seven-day period and evaluated for consistency. A typical day was then selected and plotted for a more detailed evaluation.
Figure 6. Regression method plot of concentrations and estimate of concentrations for ES05 Building 13, Music 926.
Journal of Chemical Health & Safety, November/December 2009
It was clear from inspection of the plots that the classrooms were not occupied for a sufficiently long period for CO2 levels to reach equilibrium. As such, the ASTM D6245 Section 10 method, which bases its calculation on equilibrium conditions, cannot be used to determine ventilation rates. This left methods that were based on the decay rate as CO2 levels were reduced by ventilation. The data plots from a typical day were examined to determine the time window during which CO2 levels decayed in a logarithmic fashion. It was found that these conditions were typically found at the end of the school day when the rooms were vacated. Figure 2 presents data plots for CO2 concentrations typically found in classrooms. This typical pattern shows a rapid rise in concentration upon initial occupancy, followed by fluctuations occurring during changes in occupancy at lunch time, with a decay induced by ventilation after the classroom is vacated for the day. However, several confounders could affect the data. These factors occurred from such diverse things such as the influence of a wash out effect when doors to the outside were simultaneously opened during and after school dismissal, poor mixing (stratification) of outside air with re-circulated air or the presence of occupants in a zone after the school day. Once familiarity had been gained with the data the consequences of these factors could easily be seen in the data plots. Once a subset of the data representing the decay of CO2 levels due to dilution with ventilation air was selected it was found that there was good agreement between air exchange rates (AER) determined by the Averaged Method and the Optional Regression Method of ASTM E741-00. Figures 3 and 4 are an example of a lognormal decay rate in Building 4 with a comparison of the same decay period evaluated using the ASTM Averaged and Optional Regression Method. When confounders existed such as poor mixing or occupant generated CO2 in the zone, there was disagreement between the AERs determined by these two methods. This allowed a check on the data set selected to determine ventilation rates.
Figure 7. Decay period for ES05 Building 9, Classroom 903.
Decay rates used in this manner will have an artificial appearance of precision, thus making it look more accurate since only one of five days of occupancy was chosen. This bears further discussion that is beyond the scope of this paper. This decay rate evaluation could also be used for the theoretical time for an occupied room to reach equilibrium as a means to check the primary air fraction values. Great care was used to select the data from a time interval where there were minimal influence of confounders and where the data showing a logarithmic
decay rate. The AER was then calculated using the two methods as a check on the suitability of the data selection. Figures 5 and 6 point out how occupancy in a zone after school dismissal influences carbon dioxide decay in Building 13 in the study school. Figures 7 and 8 show CO2 levels in a room with excessive infiltration due to openings to the outside above the porous classroom ceiling. As can be seen, the CO2 concentrations are greatly affected by changes in the ventilation rate that resulted from wind driven changes in the infiltration rate. Despite
Figure 8. Regression method plot of concentrations and estimate of concentrations for ES05 Building 9, Classroom 903.
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the variability introduced into the data, it was still possible to calculate an air change rate for this room
CONCLUSION
Data collected on CO2 levels through time was found to be a useful tool for determining ventilation rates as part of a study on indoor air quality. Data was collected for seven days and then plotted. It was possible to select a typical day from this first plot. A more detailed plot was prepared of this typical day. By inspection, it was possible to select a period for analysis where the pattern of CO2 decay was consistent with a vacated space. The AER was calculated using methods in ASTM E741-00. As a check on the data selection, the AER was calculated using two different methods.
Data collected on CO2 levels through time was found to be a useful tool for determining ventilation rates as part of a study on indoor air quality. Ventilation measured in this study was total ventilation including mechanical ventilation from HVAC units, replacement air from exhaust fans and infiltration through outside walls. If substantial changes are made to door and window installations that could reduce infiltration, this could affect total ventilation rates and IAQ
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measurements would need to be repeated. After completion of renovation work on HVAC, equipment ventilation rates should be verified and continuation of indoor air quality complying with ANSI/ASHRAE 62.1-2004 should be confirmed by measurement of the contaminants of concern. Measurements were performed in mature buildings whose finishes and furniture had out gassed any VOCs prior to the study. Any renovation work involving introduction of new finish materials, furniture or other materials that could release volatile organic compounds such as painting, wall coverings, furniture, carpeting, etc. should incorporate an out gassing period that includes purging of building air. If the criterion for indoor air quality set forth in ANSI/ASHRAE 62.1 changes, the indoor air quality of the school needs to be evaluated against the new standard. REFERENCES
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1. ANSI/ASHRAE. Standard Ventilation for Acceptable Indoor Air Quality 622001. American National Standards Institute/American Society of Heating, Refrigeration and Air Conditioning Engineers: Atlanta, GA, 2001. 2. American Society for Testing and Materials. Standard Test Method for Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution (ASTM E 741-00). Author: West Conshohocken, PA, 2000. 3. American Society for Testing and Materials. Standard Guide for Using Indoor Carbon Dioxide Concentrations to Evaluate Indoor Air Quality and Ventilation (ASTM D 6245-98). Author: West Conshohocken, PA, 1998. 4. Seppanen O.; Fisk W.; Mendell M. Ventilation Rates and Health. ASHRAE J. 2002, Daisey J.; Angell W.; Apte M. Indoor Air Quality, Ventilation and Health Symptoms in Schools: An Ana-
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Journal of Chemical Health & Safety, November/December 2009