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Human Health and Vegetation: Weighting the Higher
Concentrations More than the Lower Values

An important aspect of defining ways to protect human health and vegetation that seems to get overlooked is that the higher hourly average concentrations of ozone should be given greater weight than the mid and low values. Some scientists simply use average values (e.g., annual/seasonal averages, M7, and M12 exposure indices) to represent the potential for pollutant exposures to affect an organism. However, average concentrations "smooth" the data and treat all concentrations the same. With the higher hourly average concentrations shown to be more important than the lower values based on experimental studies, calculating an average concentration index, using many hourly average concentrations, is an inappropriate approach for developing exposure metrics for protecting humans and plants.

Vegetation scientists have focused on the important research relating exposure and effects and quantifying the results. Researchers collaborating with A.S.L. & Associates have published numerous peer-reviewed papers on the subject of the importance of the higher hourly average ozone concentrations and are continuing to perform research on this very important and relevant scientific issue (see Musselman et al., 2006 for a critical review of the literature). Lefohn and Benedict (1982) initially proposed that the higher hourly average concentrations should be given greater weight than the mid- and low-level values when assessing crop growth reduction.

Similarly, several researchers collaborating with A.S.L. & Associates, have published peer-reviewed papers describing controlled laboratory exposures of human volunteers indicating that higher ozone hourly average concentrations elicit a greater effect on hour-by-hour physiologic response (i.e., forced expiratory volume in 1 s [FEV1]) than lower hourly average values. The results applied realistic, variable ozone exposures in contrast to the 3 scientific experiments, which utilized constant concentration exposures. These 3 scientific experiments, whose results formed the basis for the 1997 8-h average 0.08 ppm ozone standard, as well as the 0.075 ppm ozone standard, were based on constant ozone exposures, which rarely occur under realistic ambient conditions. Hazucha and Lefohn (2007) emphasize that realistic triangular ozone exposures used by Hazucha et al. (1992) and Adams (2003; 2006a, b), suggest that variable exposures can potentially lead to higher FEV1 responses than square-wave exposures at overall equivalent O3 doses. The current 0.070 ppm ozone standard is based on the work by Schelegle et al. (2009), who applied variable hour-by-hour average concentrations in their 6.6-h human health laboratory experiment. An important observation from the work by Hazucha et al. (1992) and Adams (2003; 2006a) is that the higher hourly average concentrations elicit a greater effect than the lower hourly average values in a non-linear manner. Lefohn et al. (2010) discuss the quantification of these findings in relationship to FEV1 response. For additional information about realistic variable concentrations, please click here.

Recent decisions by the EPA have begun to focus on the importance of the higher concentrations for assessing the health effects associated with air pollution. The EPA (2010a) established a new nitrogen dioxide 1-hour standard at a level of 100 ppb, based on the 3-year average of the 98th percentile of the yearly distribution of 1-hour daily maximum oncentrations, to supplement the existing nitrogen dioxide annual standard. In addition, for sulfur dioxide, EPA (2010b) established a new 1-hour SO2 standard of 75 parts per billion (ppb), based on the 3-year average of the annual 99th percentile (or 4th highest) of 1-hour daily maximum oncentrations. The EPA revoked both the existing 24-hour and annual primary SO2 standards. In its discussions of the proposed revisions to the current ozone standards, the US EPA was concerned that background ozone concentrations would cause exceedances of the lower range of proposed ozone standards (US Federal Register, 2015). Recognizing this possibility, Lefohn et al. (2010) proposed a cumulative exposure index, W90, that accumulates hourly average concentrations above the majority of background ozone concentrations.


Adams, W. C. (2003) Comparison of chamber and face mask 6.6-hour exposure to 0.08 ppm ozone via square-wave and triangular profiles on pulmonary responses. Inhalation Toxicology 15: 265-281.

Adams, W. C. (2006a). Comparison of Chamber 6.6-h Exposures to 0.04 - 0.08 ppm Ozone Via Square-Wave and Triangular Profiles on Pulmonary Responses. Inhal Toxicol. Inhalation Toxicology 18, 127-136.

Adams, W.C. (2006b). Human pulmonary responses with 30-minute time intervals of exercise and rest when exposed for 8 hours to 0.12 ppm ozone via square-wave and acute triangular profiles. Inhalation Toxicology 18, 413-422.

Hazucha, M.J.; Lefohn, A.S. (2007) Nonlinearity in Human Health Response to Ozone: Experimental Laboratory Considerations. Atmospheric Environment. 41:4559-4570.

Hazucha, M.J.; Folinsbee, L.J.; Seal, E., Jr. (1992) Effects of steady-state and variable ozone concentration profiles on pulmonary function. Am. Rev. Respir. Dis. 146: 1487-1493.

Lefohn A.S.; Benedict H.M. (1982) Development of a mathematical index that describes ozone concentration, frequency, and duration. Atmospheric Environment. 16:2529-2532.

Lefohn, A.S., Hazucha, M.J., Shadwick, D., Adams, W.C. (2010). An Alternative Form and Level of the Human Health Ozone Standard. Inhalation Toxicology. 22:999-1011.

Musselman R.C., Lefohn A.S., Massman W.J., and Heath, R.L. (2006) A critical review and analysis of the use of exposure- and flux-based ozone indices for predicting vegetation effects. Atmospheric Environment. 40:1869-1888.

US Environmental Protection Agency, US EPA, 2010a. Primary National Ambient Air Quality Standards for Nitrogen Dioxide. Federal Register, 75, No. 26, 6474-6537.

Schelegle, E.S., Morales, C.A., Walby, W.F., Marion, S., Allen, R.P., 2009. 6.6-hour inhalation of ozone concentrations from 60 to 87 ppb in healthy humans. Am. J. Respir. Crit. Care Med. 180:265-272.

US Environmental Protection Agency, US EPA, 2010b. Primary National Ambient Air Quality Standards for Sulfur Dioxide. Federal Register, 75, No. 119, 35520-35603.

US Federal Register. 2015. National Ambient Air Quality Standards for Ozone, 40 CFR Part 50, 51, 52, 53, and 58, pp 65292-65468.

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