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

Even though research results were published in the 1980s about the importance of higher hourly average ozone concentrations in comparison to the mid and low values in affecting human health and vegetation, these results appear to be overlooked by some scientists. Some scientists continue to use average values (e.g., annual/seasonal averages, M7, and M12 exposure indices) to represent the potential for pollutant exposures to affect human health and/or vegetation. Long-term average concentrations obscure the data and treat all concentrations as if they have the same biological importance. 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 in the past 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 dealing with concentration-based and flux-based exposure/dose indices). The interaction between ozone and plant tissues is driven mainly by three distinct processes: changes in external ozone concentration, ozone uptake, and ozone detoxification (Heath et al., 2009). As noted by the EPA (2020), those species having high amounts of detoxification potential may, in fact, show little relationship between ozone stomatal uptake and plant response. For example, Goumenaki et al. (2021) reported that plants exposed to equivalent ozone fluxes administered during daytime versus nighttime exhibited a significant decline in biomass in both cases, and the losses were greater at night in plants subjected to equivalent ozone flux, implying that diurnal variability in detoxification plays an important role in protecting vegetation. 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. In 1983, Musselman et al. (1983) were the first to provide experimental evidence of the importance of peak hourly average ozone concentrations in affecting vegetation growth and provided important support for the hypothesis associated with the peak values. Hogsett et al. (1985), applying the exposure regimes designed by Dr. Lefohn, provided additional evidence of the importance of the higher hourly average ozone concentrations in affecting vegetation.

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) emphasized 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 2015 0.070 ppm ozone standard (the current level of the 8-hour 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. Liu et al. (2022) conducted PM2.5 respiratory exposure of Wistar rats for 12 weeks. In their study, the authors noted that when the total mass of PM2.5 exposure was the same during the experimental period, high concentration-intermittent exposure operation caused more serious damage to the bronchus than low concentration-continuous exposure operation, which meant according to the authors that the health damage caused by high concentrations PM2.5 were greater. The authors noted that previous toxicological studies on other air pollutants had shown similar results, including formaldehyde and ozone. Liu et al. (2022) noted that one possible explanation for these results was that the relationship between exposure concentrations of these pollutants and health damage did not follow a linear relationship, but was more like an exponential one. For additional information about realistic variable concentrations, please click here.

The EPA has focused on the importance of the higher concentrations for assessing the human health effects associated with air pollution. The EPA (2010a) established a 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 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 has been concerned in the past that background ozone concentrations could cause exceedances of the lower range of proposed ozone standards (Federal Register, 2015). However, the EPA notes that the Agency's exceptional events rule allows it to not count those exceedances of the ozone standard associated with background ozone and therefore, elevated levels of background are not a consideration, as far as EPA is concerned, in the attainment of the federal ozone standard. Background ozone is important when focusing on the margin of safety consideration (i.e., uncertainty in the human effects database) when the EPA Administrator makes the final decision on which level is most appropriate for the protection of the public's health. Therefore, background ozone contributes to the uncertainty in the results associated with the human health risk assessments used in the setting of the human health ozone standard.


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.

Federal Register. Vol. 80, No. 206 / Monday, October 26, 2015. National Ambient Air Quality Standards for Ozone, 40 CFR Part 50, 51, 52, 53, and 58, pp 65292-65468.

Goumenaki, E., González-Fernández, I., Barnes, J. (2021). Ozone uptake at night is more damaging to plants than equivalent day-time flux. Planta 253(3):75. doi: 10.1007/s00425-021-03580-w.

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.

Heath, R.L., Lefohn, A.S., Musselman R.C. (2009) Temporal processes that contribute to nonlinearity in vegetation responses to ozone exposure and dose. Atmos Environ 43: 2919-2928.

Hogsett, W.E.; Tingey, D.T.; Holman, S.R. (1985). A programmable exposure control system for determination of the effects of pollutant exposure regimes on plant growth. Atmos. Environ. 19:1135-1145.

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.

Liu, H., Nie, H., Lai, W., Shi, Y., Liu, X., Li, K., Tian, L., Xi, Z., Lin, B. (2022). Different exposure modes of PM2.5 induces bronchial asthma and fibrosis in male rats through macrophage activation and immune imbalance induced by TIPE2 methylation. Ecotoxicology and Environmental Safety 247 (2022)

Musselman, R.C.; Oshima, R.J.; Gallavan, R.E. (1983). Significance of pollutant concentration distribution in the response of 'red kidney' beans to ozone. J. Am. Soc. Hortic. Sci. 108:347-351.

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.

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

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

U.S. EPA. 2020. Integrated Science Assessment of Ozone and Related Photochemical Oxidants. EPA/600/R-20/012. April. Research Triangle Park, NC: Environmental Protection Agency.

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