Residential Indoor Air Quality Guideline: Ozone

In human controlled exposure studies, acute exposure (up to 4 hours) of healthy young adults to ozone resulted in lung function decrements, indicated by decreased forced vital capacity (FVC), forced expiratory volume in one second (FEV₁), and forced inspiratory volume, reduced tidal volume and increased breathing frequency. Subjective symptoms such as pain upon inhalation were also reported (McDonnell et al. 1983; Seal et al. 1993; McDonnell et al. 1993; Seal et al. 1996; Krishna et al. 1997; Liu et al. 1997). Statistically significant effects were seen at 120 ppb O₃, which is considered the Lowest Observed Adverse Effect Level (LOAEL) for this exposure duration (Health Canada, 2010).

The primary effects of prolonged exposure (for durations of 4 to 8 hours) to ozone in human controlled exposure studies were decreases in lung function (FVC and FEV₁) and increases in subjective respiratory symptoms (pain upon deep inhalation and total symptoms score).

Three recent studies assessed the effects of exposure to O₃ for 6.6 hours in healthy adults while performing intermittent exercise (Adams, 2002; 2003a; 2003b), and one of them (Adams, 2002) included a range of exposure levels (40, 80, 120 ppb O₃) enabling the assessment of a dose-response relationship. In this study, no statistically significant effects occurred at the lowest exposure level of 40 ppb O₃ when compared to subjects exposed to filtered air; this level is therefore considered the No Observed Adverse Effect Level (NOAEL). Statistically significant effects were seen at 80 and 120 ppb O₃, with decreases in pulmonary function and increases in respiratory symptoms being more important at 120 ppb than at 80 ppb O₃. The Lowest Observed Adverse Effect Level (LOAEL) is therefore considered to be 80 ppb O₃ for this exposure duration (Health Canada, 2010).

Generally, results from toxicological studies seem to be consistent with data obtained in human controlled exposure studies. However, ozone levels investigated in animal studies were usually higher than those found in Canadian residences and therefore are less useful for risk assessment.

Epidemiological studies of the population health impacts of ozone have been a key component to setting outdoor air quality standards and guidelines for more than a decade. In general, they reflect the temporal relationship between outdoor (ambient) ozone concentrations and various health endpoints, integrating the complex air quality processes that drive the formation and destruction of ozone in outdoor air, as well as factors such as individual health and behaviour that modify susceptibility and exposure. Across the range of observed ambient ozone concentrations, these studies have failed to identify a threshold for mortality as an endpoint. However, in developing indoor air quality guidelines that can be used to formulate health protective measures that modify the indoor environment, studies that relate health outcomes to individual exposure at a given point in time are of greatest importance. Therefore, controlled exposure studies of human subjects were selected as the most appropriate basis for deriving indoor reference concentrations.

For each exposure period (acute or prolonged) the studies reviewed were used to establish a reference concentration, which constitutes a value below which adverse health effects are unlikely to be seen. The reference concentration for acute exposure was calculated by taking the experimentally derived LOAEL and dividing by an uncertainty factor of 3 to extrapolate to a NOAEL, and then dividing by a further uncertainty factor of 10 to account for intraspecies variation in susceptibility to ozone. The reference concentration for chronic exposure was calculated by taking the experimentally derived NOAEL and dividing by the uncertainty factor of 10 for intraspecies variation. Reference concentrations of 8 µg/m³ (4 ppb) O₃ for both acute and prolonged exposure to ozone were derived in this manner based on key controlled exposure studies (Health Canada, 2010).

Assessment under the Canadian Environmental Protection Act, 1999

In 2003, ozone was declared "toxic" under the Canadian Environmental Protection Act (CEPA), 1999 as the Science Assessment Document (SAD) for Ground-Level Ozone (Environment Canada and Health Canada, 1999) concluded that there is a significant association between ambient (outdoor) ozone and adverse health effects and that significant adverse effects to human health (mortality and morbidity) and vegetation (reduced growth and crop yield) are occurring at ozone levels currently experienced across Canada (Environment Canada and Health Canada, 2003).

Residential Indoor Air Quality Guideline for Ozone

The key human controlled exposure studies were used to determine a reference concentration, based on the observed NOAELs or LOAELs and incorporating appropriate uncertainty factors. This reference concentration serves as the basis for setting a recommended maximum exposure limit for residential indoor environments, taking into account the feasibility of achieving such a limit.

The original exposure guideline for residential indoor air quality for ozone was for a 1-hour averaged exposure limit (Health Canada, 1987). This update, however, obtained the same reference concentration for acute (2.5 hour) and prolonged (4-8 hour) exposures (Health Canada, 2010). Given the lack of 1-hour exposure data for comparison and since ozone levels are usually higher during daytime, only an 8-hour residential exposure limit for ozone is recommended at this time. This 8-hour averaged exposure period is expected to be more representative of overall exposure to ozone than a 1-hour averaged exposure.

An exposure limit based on the reference concentration for prolonged ozone exposure may not, however, be achievable in many Canadian homes. An indoor ozone exposure study conducted in Toronto, Ontario, for example, reported average summer indoor ozone concentrations that were higher than the reference concentration (4 ppb), with 12-hour daytime concentrations of 7.1 ± 12.6 ppb (mean ± SD) and 12-hour night time concentrations of 6.2 ± 9.5 ppb (mean ± SD) (Liu et al., 1995).

Health Canada therefore recommends a residential maximum exposure limit of 40 µg/m³ (20 ppb) ozone, based on an averaging time of 8-hours. This exposure limit would still be half of the NOAEL derived from a controlled human exposure study (Adams, 2002), while being more realistically achievable in Canadian homes. In the Toronto indoor ozone exposure study, for example, 95% of homes were below 22.6 ppb and 22.4 ppb respectively for daytime and night time 12-hour summer indoor ozone levels (Liu et al., 1995).

References

• Adams, W.C. (2002) Comparison of chamber and face-mask 6.6-hour exposures to ozone on pulmonary function and symptoms responses. Inhal Toxicol 14, 745-64.
• Adams, W.C. (2003a) Comparison of chamber and face mask 6.6-hour exposure to 0.08 ppm ozone via square-wave and triangular profiles on pulmonary responses. Inhal Toxicol 15, 265-81.
• Adams, W.C. (2003b) Relation of pulmonary responses induced by 6.6-h exposures to 0.08 ppm ozone and 2-h exposures to 0.30 ppm ozone via chamber and face-mask inhalation. Inhal Toxicol 15, 745-59.
• Environment Canada and Health Canada (1999) National Ambient Air Quality Objectives for Ground-Level Ozone: Science Assessment Document, Ottawa, Ontario: Her Majesty the Queen in Right of Canada.
• Environment Canada and Health Canada (2003) Order adding toxic substances to Schedule 1 to the Canadian Environmental Protection Act, 1999. Canada Gazette, Part II 137 (14), 1864-77
• Health Canada (1987) Exposure guidelines for residential indoor air quality. A report of the Federal-Provincial Advisory Committee on Environmental and Occupational Health. Ottawa: Minister of Supply and Services Canada.
• Health Canada (2010) Residential indoor air quality guideline - Science Assessment Document, Ozone. Ottawa: Her Majesty the Queen in Right of Canada.
• Krishna, M.T., Blomberg, A., Biscione, G.L., Kelly, F., Sandstrom, T., Frew, A. and Holgate, S. (1997) Short-term ozone exposure upregulates P-selectin in normal human airways. Am J Respir Crit Care Med 155, 1798-803.
• Liu, L., Leech, J.A., Urch, R.B. and Silverman, F.S. (1997) In vivo salicylate hydroxylation: a potential biomarker for assessing acute ozone exposure and effects in humans. Am J Respir Crit Care Med 156, 1405-12.
• Liu, L.J., Koutrakis, P., Leech, J. and Broder, I. (1995) Assessment of ozone exposures in the greater metropolitan Toronto area. J Air Waste Manag Assoc 45, 223-34.
• McDonnell, W.F., Horstman, D.H., Hazucha, M.J., Seal, E. Jr, Haak, E.D., Salaam, S.A. and House, D.E. (1983) Pulmonary effects of ozone exposure during exercise: dose-response characteristics. J Appl Physiol 54, 1345-52.
• McDonnell, W.F., Muller, K.E., Bromberg, P.A. and Shy, C.M. (1993) Predictors of individual differences in acute response to ozone exposure. Am Rev Respir Dis 147, 818-25.
• Seal, E. Jr, McDonnell, W.F. and House, D.E. (1996) Effects of age, socioeconomic status, and menstrual cycle on pulmonary response to ozone. Arch Environ Health 51, 132-7.
• Seal, E. Jr, McDonnell, W.F., House, D.E., Salaam, S.A., Dewitt, P.J., Butler, S.O., Green, J. and Raggio, L. (1993) The pulmonary response of white and black adults to six concentrations of ozone. Am Rev Respir Dis 147, 804-10.