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.
References
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. https://doi.org/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) 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.
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.
