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
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.
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.
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.