Similar to key 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; 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;
Lefohn et al., 2010; Lefohn et al., 2018). The result of this
is that Haber's Rule (i.e., concentration multiplied by time)
is not applicable and its application would be problematic when
used in ozone exposure or dose equations. The importance of concentration
relative to other factors in the dose concept was mentioned earlier
by other researchers. Silverman et al. (1976) noted that there
was some suggestion in their study that for a given effective
dose, exposure to a high concentration for a short period had
more effect than a longer exposure to a lower concentration.
Similarly, Drechsler-Parks (1990) noted that their results suggested
that O3 concentration was the most important factor within the
effective dose concept, followed by exercise Ve, followed by
exposure duration.
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; Lefohn et al., 2010). For many years, vegetation and some
human health researchers have designed triangular-type exposure
regimes. An important question was raised in the human health
scientific community as to the use of the triangular exposure
regime versus constant concentration exposure regimes. Our own
analysis found that only 1.51% of all sequences could be classified
as representing a "square wave" or a constant concentration
regime. Clearly, the "square-wave" patterns occur so
infrequently under ambient conditions that it may not be relevant
to use these artificial exposure regimes in assessing response
considering the timing of response following exposure to O3 (Hazucha
and Lefohn, 2007; Lefohn et al., 2010).
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 values in the early morning
hours (or levels near minimum detectable amounts) to an early
afternoon peak to lower values in the late evening hours 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) 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 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 et al. (2010)
continue to indicate that square-wave O3 patterns of exposure
are fairly rare under ambient conditions. The investigators 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 (2003; 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 necessary
the most optimum metric for assessing human health effects associated
with ozone exposure (Hazucha and Lefohn, 2007; Lefohn et al.,
2010). Lefohn et al. (2010) discuss a possible alternative form
of the current standard that uses the 8-hour daily maximum concentration
metric.
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