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;
Lefohn et al., 2010).
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 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. Clearly, the "square-wave"
patterns occur so infrequently under ambient conditions that
it may not be relevant to use them in assessing response considering
the timing of response following exposure to O3 (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 (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) 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. Findings reported by Lefohn et
al. (2010) continue to indicate that square-wave O3 patterns
of exposure occur fairly rarely 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
(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 (Hazucha and Lefohn, 2007; Lefohn et al.,
2010). Lefohn et al. (2010) discuss a possible alternative to
the 8-hour ozone standard in their recently published paper.
References
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