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 (i.e., 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, 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 peak 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 latest 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 most recent 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. An important observation from these three experiments
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
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.
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.