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Carbon Dioxide in Indoor Air

Topics: Air, Contaminants and Hazards, Indoor Air, Chemical Agents

A Public Health Inspector calls about an issue flagged at an older elementary school.  As part of a teacher complaint process, the provincial health and safety agency has been conducting a workplace investigation.  Carbon dioxide (CO2) levels were measured and ranged from 412 ppm in an unoccupied classroom to 1,130 ppm in the school library and 1,660 ppm in an occupied classroom that had closed windows.  Parents are now concerned and the school board has asked you, as Medical Officer of Health, about the potential health risks to students from CO2 levels that exceed formal workplace standards.

CO2  and air quality concerns
Where does CO2 come from?
What physiological responses occur in response to elevated CO2 levels?
What is the significance of slightly elevated CO2 in school classrooms?
What is the relationship between ventilation (air exchange rate) and respiratory health?
What are commonly cited workplace-applicable standards for CO2?  What rationale is given?
What was found by the health and safety agency? 
The same agency ordered a health survey...  Do you expect this to be useful? What might be more useful? How would you respond to the School Board’s query about possible health risks to students?
Acknowledgements
References


CO2  and air quality concerns

CO2 is frequently an issue in the context of indoor air quality concerns.1  It is relatively easy to measure and is commonly included in indoor air quality monitoring.  CO2 at levels found in classroom settings do not appear to pose any direct health effects.  High CO2 levels, however, are an indication that there is not sufficient ventilation for the number of occupants in the room.  Results of COmeasurements are often incorrectly interpreted and in some cases may be confused with the more serious carbon monoxide (CO).  

Where does CO2 come from?

CO2 is a natural constituent of the air we breathe; it is a colourless, odourless and non-flammable gas produced by metabolic processes (such as respiration) and by combustion of fossil fuels.  The average outdoor air concentration of CO2 is in the order of 300 to 400 ppm.  Indoor levels are usually higher, due to the CO2 exhaled by building occupants.  Human metabolism alone can lead to CO2 levels in excess of 3,000ppm, especially in poorly ventilated rooms. Indoor combustion appliances, in particular gas stoves, can also increase CO2 levels. 

What physiological responses occur in response to elevated CO2 levels?

Human health effects have been observed at very high levels (> 7,000 ppm) of CO2, but it is unlikely that you would ever find levels this high in homes or classrooms. 

Increased ambient CO2 levels cause acidification of the blood with compensatory increase in rate and depth of breathing. After prolonged exposure (days), acid-base regulation can occur via renal mechanisms which can affect calcium metabolism in bone. 

The lowest level at which a human health effect (i.e. acidosis) has been observed in humans is 7,000 ppm, and that only after several weeks of continuous exposure in a submarine environment.  In its 1987 Exposure Guidelines for Residential Indoor Air Quality,2  Health Canada set an exposure limit of 3,500 ppm to protect against such undesirable adaptive changes to acidosis, in particular calcium release from bones.
The occupational limits for CO2 recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) are 5000 ppm (TLV-TWA) and 30,000 ppm (TLV-STEL),3 based on the direct effects on acidification of the blood.

Of note, no major pediatric studies have been identified to address the possibility of a significant differential in response between adults and children other than realizing that infants and children breathe more air than adults relative to their body size and thus tend to be more susceptible to respiratory exposures.4

What is the significance of slightly elevated CO2 in school classrooms?

CO2 at the levels in this school do not pose a direct risk to health.  The elevated CO2 levels (e.g. > 1100 ppm), however, may suggest the need for more ventilation. 

Indoor CO2 levels are generally higher than outside, as building occupants produce CO2 when they exhale.  Ventilation exchanges indoor for outdoor air and reduces indoor CO2 levels.  High indoor CO2 levels therefore may indicate that the air exchange rate is too low for the number of people in the room.   School officials should look to increase ventilation by turning up mechanical ventilation systems or by opening windows. 

What is the relationship between ventilation (air exchange rate) and respiratory health?

Poor ventilation can make classrooms uncomfortable and can reduce productivity.  Poor ventilation can also lead to increased humidity, as moisture produced indoors is not vented to the outside.  High humidity can encourage the growth of mould and dust mites; both of which are allergens and asthma triggers.   

In addition, ventilation also helps reduce the levels of other indoor air pollutants released from furnishings, building products or chemical cleaners such as formaldehyde or volatile organic compounds (VOCs).  Since some of these chemicals have known or suspected health effects, keeping levels as low as possible is always advisable. 

What are commonly cited workplace-applicable standards for CO2?  What rationale is given?

Workplace standards applicable to the school environment, if they exist, will be set by the provincial government.

The occupational limits for CO2 recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) are 5000 ppm (TLV-TWA) and 30,000 ppm (TLV-STEL),3 based on the direct effects on acidification of the blood.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 62-2007 Ventilation for Acceptable Indoor Air Quality provides standards for ventilation based on surface area and occupancy.  Typically, in an occupied classroom situation, the recommended level of ventilation would correspond to a CO2 level of approximately 1000-1100 ppm.5   A CO2 level of 1,000 to 1100 ppm is therefore offered as “a surrogate for human comfort (odour)” but “not considered a health risk.”

What was found by the health and safety agency? 

The investigation followed complaints of persistent respiratory irritation by teachers. CO2  measurements were taken in classrooms over the course of schooldays between 9am and 3pm on three separate occasions during the past year.  The highest level was 1,660 ppm.  Records showed that there had been historical issues with water intrusion into the school. CO was not tested. Two indoor air samples for mould spores were found to be comparable to one outdoor air sample.

What are your thoughts? Would you discount mould as a possible source of the problem based on these test results?

The same agency ordered a health survey...  Do you expect this to be useful? What might be more useful? How would you respond to the School Board’s query about possible health risks to students?

It will be difficult to link health data with the ventilation issue given the lack of before and after data for comparison.  As high CO2 levels can simply be an indicator of poor ventilation, there is a possibility that other contaminants are accumulating. It is better to focus on taking steps to improve ventilation and reduce CO2.  Illness rates are more likely to be related to community-wide seasonal trends than to the ventilation rates in a school.  

As the MOH, you could discuss with parents and teachers about the meaning of the findings, highlighting the low direct risk to health.  You could recommend that school officials look at increasing ventilation in the areas with high CO2.  Simple measures for improving ventilation may include opening windows and increasing play time outdoors, but the possibility of a source of contaminants may need to be considered and possibly investigated. 

A good resource is the US EPA Indoor Air Quality Tools for Schools Program6 that provides checklists and tools to deal with indoor air quality issues. Also, for mould-related problems, a good resource is the US Environmental Protection Agency’s guide on Mold Remediation in Schools and Commercial Buildings.7

Also, possible information sources that can help determine if there is a possible problem at this school are records of student, teacher and staff absenteeism rates, records of nurse visits, and comparisons with other schools.

Acknowledgements

We would like to thank the following individuals for their valuable input and review of this document: Charmaine Enns, Charlene MacKinnon, Josh Moran, and Claudette Erdman for the question and context;  Tim Foggin for research and write-up;  Catherine Donovan, Brian Giles, and Deborah Schoen for review and comments.

References

  1. Miller J, Semple S, Turner S. High carbon dioxide concentrations in the classroom: the need for research on the effects of children's exposure to poor indoor air quality at school. Aberdeen (UK): Occ & Env Med 67(11):799.
  2. Health Canada. Exposure guidelines for residential indoor air quality. A report of the Federal-Provincial Advisory Committee on Environmental and Occupational Health. Ottawa (ON): Environmental Health Directorate, Health Protection Branch; 1987 Apr.
  3. American Conference of Governmental Industrial Hygienists (ACGIH). 2010 TLVs and BEIs. Based on the documentation of the threshold limit values for chemical substances and physical agents & biological exposure indices. Cincinnati (OH): ACGIH; 2010 Mar. 272 p.
  4. Snodgrass WR. Physiological and biochemical differences between children and adults as determinants of toxic exposure to environmental pollutants. In: PS Guzelain, CJ Henry, SS Olin, eds. Similarities and differences between children and adults: Implications for risk assessment. Washington (DC): ILSI Press 1992:35-42.
  5. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 62-2007 Ventilation for Acceptable Indoor Air Quality. Atlanta (GA): American Society of Heating, Refrigerating and Air-Conditioning Engineers; 2002. 6 p.
  6. Environmental Protection Agency (US). Indoor air quality (IAQ) tools for schools program. Washington (DC): Office of Air and Radiation, Indoor Environments Division, 1995 [updated 2010 Jun 8; cited 2010 Jun 14].
  7. Environmental Protection Agency (US). Mold remediation in schools and commercial buildings. Washington (DC): Office of Air and Radiation, Indoor Environments Division, 2001 Mar. Contract No.: EPA 402-K-01-001 [updated 2008 Sept 18; cited 2010 Jun 14].

June 2010

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