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 (2003a; 2006), 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).
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. An important question recently
was raised in the human health scientific community as to the
use of the triangular exposure regime versus a constant concentration
exposure regime. Our own recent analysis found that only 1.51%
of all sequences could be classified as representing a "square
wave" or a constant concentration regime. It is important
to investigate why the "square-wave" patterns occur
so infrequently under ambient conditions.
By definition, diurnal variations are those that occur during
a 24-h period. Diurnal patterns of O3 may be expected to vary
with location, depending on the balance among the many factors
affecting O3 formation, transport, and destruction. Although
they vary with locality, diurnal patterns for O3 typically show
a rise in concentration from low (or levels near minimum detectable
amounts) to an early afternoon peak at lower elevation monitoring
sites. The diurnal pattern of concentrations can be ascribed
to three simultaneous processes: (1) downward transport of O3
from layers aloft; (2) destruction of O3 through contact with
surfaces and through reaction with nitric oxide (NO) at ground
level; and (3) in situ photochemical production of O3 (Coffey
et al., 1977; Mohnen et al., 1977; Reiter, 1977a).
The form of an average diurnal pattern may provide information
on sources, transport, and chemical formation and destruction
effects at various sites. Non-transport conditions will produce
early afternoon peaks. However, long-range transport processes
will influence the actual timing of a peak from afternoon to
evening or early morning hours. A flat diurnal pattern is usually
interpreted as indicating a lack of efficient scavenging of O3
or a lack of photochemical precursors, whereas a varying diurnal
pattern is taken to indicate the opposite.
The U.S. EPA (2006) recently described diurnal patterns for urban
locations in the United States. Composite urban, diurnal variations
in hourly averaged O3 for April through October 2000 to 2004
are shown in Figure 3-8. The figure was created by averaging
the hourly average concentrations over the time period at each
hour (i.e., all the 0100 hour values over the desired period
of time, then the 0200 hour values, etc.) As can be seen from
Figure 3-8, at many locations daily 1-hour maxima tend to occur
in mid-afternoon and daily 1-h minima tend to occur during the
early morning, but exceptions do occur.
Source: U.S. Environmental
Protection Agency (2006).
Thus, based on the published literature
combined with the most recent 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. Recent findings from Lefohn (2006)
continue to indicate 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, then only 1.51% of all sequences (28,148) would be classified
as representing a "square wave". Thus, the results
published by Hazucha et al. (1992) and Adams (2003a; 2006)
suggest that the higher hourly average concentrations elicit
a greater effect than the lower hourly average values in a non-linear
manner and that identical 8-hour average concentrations with
different combinations of hourly values will result in different
FEV1 responses. This implies that the current 8-hour ozone standard
is not a stable metric.
References
Adams, W. C. (2003a) 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. (2006). 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.; 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.
Coffey, P.; Stasiuk, W.; Mohnen, V. (1977)
Ozone in rural and urban areas of New York State: part I. In:
Dimitriades, B., ed. International conference on photochemical
oxidant pollution and its control - proceedings: volume I; September
1976; Raleigh, NC. Research Triangle Park, NC: U.S. Environmental
Protection Agency, Environmental Sciences Research Laboratory;
pp. 89-96; report no. EPA-600/3-77-001a. Available from: NTIS,
Springfield, VA; PB-264232
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.
Atmospheric Environment 19: 1135-1145.
Lefohn, A. S.; Foley, J. K. (1993). Establishing
Ozone Standards to Protect Human Health and Vegetation: Exposure/Dose-Response
Considerations. J. Air Waste Manag. Assoc. 43(2):106-112.
Lefohn, A.S. (2006). Personal Communication.
A.S.L. & Associates, Helena, Montana.
Mohnen, V. A.; Hogan, A.; Coffey, P. (1977)
Ozone measurements in rural areas. J. Geophys. Res. 82: 5889-5895.
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.
Reiter, E. R. (1977a) Review and analysis.
In: Mohnen, V. A.; Reiter, E. R., eds. International conference
on oxidants, 1976 - analysis of evidence and viewpoints; part
III. the issue of stratospheric ozone intrusion. Research Triangle
Park, NC: U.S. Environmental Protection Agency, Environmental
Sciences Research Laboratory; pp. 67-117; report no. EPA-600/3-77-115.
Available from: NTIS, Springfield, VA; PB-279010.
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, pp. 913-917.
U.S. Environmental
Protection Agency (2006) Air Quality Criteria for Ozone and Related
Photochemical Oxidants. Research Triangle Park, NC: Office of
Research and Development; report no. EPA/600/R-05/004af.
