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 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. 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 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 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. However, background ozone is important
when focusing on margin of safety consideration when the EPA
Administrator makes the decision on which level is most appropriate
for the protection of the public's health. Background ozone creates
uncertainty in human health risk assessments associated with
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
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.
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.
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.
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.
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) https://doi.org/10.1016/j.ecoenv.2022.114200
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.
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.