Similar to vegetation experiments performed
in the 1980s (e.g., Musselman et al., 1983; Hogsett et
al., 1985), controlled human health laboratory study results
have shown a difference in response to square-wave (i.e., constant
concentration) O3 exposures and triangular (i.e., variable) exposures.
For example, triangular exposures used by Hazucha et al.
(1992) and Adams (2003; 2006a; 2006b), 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 (Hazucha and Lefohn,
2007; Lefohn et al., 2010). The importance of these experiments
is that long-term average concentrations are not appropriate
for assessing possible human health effects for FEV1 decrements.
In addition, the importance of the higher hourly average concentrations
leads to the conclusion that an alternative form of the current
form of the 8-h health standard may be required. Two 8-h ozone
exposure regimes experiencing different hourly average concentration
distributions may elicit different FEV1 decrements. In other
words, the same 8-h average ozone concentration may lead to different
health effects endpoints. As discussed by Lefohn et al. (2010),
a possible alternative to the 8-h average form of the current
U.S. ozone standard may be a cumulative exposure standard similar
to the W126 vegetation exposure index currently applied by the
US EPA for assessing vegetation effects.
It has become apparent that controlled human health laboratory
simulations of air-pollution risk-assessment need to employ O3
concentration profiles that more accurately mimic those encountered
during summer daylight ambient air pollution episodes (Adams
and Ollison, 1997; Lefohn and Foley, 1993; Rombout et al.,
1986). For many years, vegetation and some human health researchers
have designed exposure regimes that has resulted in the application
of triangular-type exposure regimes. Based on the published literature
combined with findings from the EPA (2006), summarized ambient
air quality data provide important evidence for those vegetation
and human health clinical researchers who are applying varying
hour-by-hour O3 concentrations (i.e., triangular exposure regimes)
in their experiments. Findings reported by Lefohn (2006) indicated
that square-wave O3 patterns of exposure occur fairly rarely
under ambient conditions. The investigator found that if one
defined a "square wave" profile as experiencing a variable
range of 4 ppb or less over an 8-hour period, only 1.51% of all
sequences (28,148) would be classified as representing a "square
wave".
Over the past several years, A.S.L. & Associates has collaborated
with health researchers (Adams, 2003, 2006a; Schelegle et al.,
2009) in designing realistic ambient-type regimes used in experiments
involving controlled laboratory exposures of human volunteers.
These realistic ambient-type O3 exposures are applied to assess
the influence of the exposure patterns of hourly average concentrations
on specific biological endpoints. Using hourly average O3 values,
A.S.L. & Associates has developed a statistical methodology
for identifying "typical" sequences of 8 hourly values
that are associated with actual 8-hour patterns. As part of its
ongoing research to identify viable alternative forms of air
pollution standards to protect human health (Lefohn et al., 2010),
A.S.L. & Associates continues its collaborative research
program with health researchers in assessing the effects of realistic
patterns of O3 exposure on human health endpoints.
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.
Adams, W. C., Ollison, W. M. (1997) Effects
of prolonged simulated ambient ozone dosing patterns on human
pulmonary function and symptomatology. Presented at: 90th annual
meeting of the Air & Waste Management Association; June;
Toronto, Ontario, Canada. Pittsburgh, PA: Air & Waste Management
Association; paper no. 97-MP9.02.
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.
Hazucha, M. J.; Lefohn, A. S. (2007) Nonlinearity
in Human Health Response to Ozone: Experimental Laboratory Considerations.
Atmospheric Environment. 41:4559-4570.
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.
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Lefohn, A. S.; Foley, J. K. (1993). Establishing
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Considerations. J. Air Waste Manag. Assoc. 43(2):106-112.
Lefohn, A.S. (2006). Personal Communication.
A.S.L. & Associates, Helena, Montana.
Lefohn, A.S., Hazucha, M.J., Shadwick,
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Human Health Ozone Standard. Inhalation Toxicology. 22: 999-1011.
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.
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Rombout, P. J. A., Lioy, P. J.; Goldstein,
B. D. (1986). Rationale for an eight-hour ozone standard. J.
Air Pollut. Control Assoc. Vol. 36, no. 8, 913-917.
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.
U.S. Environmental
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Photochemical Oxidants. Research Triangle Park, NC: Office of
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