In 1994, Drs. Allen S. Lefohn and Paul
J. Lioy, who have published extensively in the peer-review literature
on exposure-response, described in a New
Directions Column, in the distinguished journal Atmospheric
Environment, their concerns about the form of the 8-hour
ozone standard. One of the several concerns measured was the
use of averaging to develop a standard to protect vegetation.
The concerns are real and important. In addition, the "piston"
effect, as described elsewhere on the web pages, may make
it difficult to attain the 8-hour ozone standard at specific
levels of protection.
The figures below show the effect of using
averages to describe ozone exposure.
Both figures summarize the ozone data that
were collected on August 24, 1998. The figure on the right identifies
many more areas of concern than the figure on the left. The hourly
average ozone concentrations are the same in the two figures.
The difference is that the figure on the right averaged the hourly
concentrations over an 8-hour period, while the figure on the
left shows the maximum hourly values for the day. By applying
averages, the data are smoothed and provide the appearence of
greater areas of concern. Laboratory studies show that the peak
concentrations are more important than average concentrations.
Thus, the figure on the right may not be as relevant as the figure
on the left. Yet, the figure on the right uses the average concentrations
similar in the manner that the 8-hour ozone standard is determined.
Scientists and engineers around the world
are becoming aware that the United States 8-hour ozone standard
may present a problem that is called "unattainability."
We discussed this in our peer-review paper
published in 1997. In November 1998, the topic was discussed
at an international meeting in Beijing, China. The unattainability
issue has been raised by A.S.L. & Associates and others.
In the coming years, policymakers will find that the 8-hour ozone
standard will become more and more difficult to attain as the
8-hour standard level is lowered and control strategies will
not work as planned. As the highest hourly average concentrations
are reduced as a result of emissions controls, the remaining
portions of the highest end of the distribution of hourly average
concentrations decline slower than the previously highest values
and make it more difficult to attain the 8-hour standards. Independent
analyses have confirmed the "piston effect". EPA reports
and papers published in 1985, 1995, and 1996 confirm the effect.
Lefohn et al. (2017) illustrate the potential for the "piston
effect" for some of the ozone monitoring sites in the EU.
A slide presentation summarizing
the "piston effect" is available. More detailed information
about the effect can be found by clicking
EPA's web site (https://www.epa.gov/air-trends/ozone-trends),
the Agency in July 2019 summarized trends for the ozone periods
1980-2018, 1990-2018, 2000-2018, and 2010-2018. Note that the
national average for trends for the four time periods were 31%,
21%, 16%, and 4%, respectively. Clearly, the trend is slowing
The "piston effect" makes it difficult to attain
the 8-hour standard for some sites. The
"piston effect" as described in the peer-review literature
and on this web site affects the ability
of the nation to atttain the 8-hour ozone standard at many sites.
The peak hourly average concentrations (i.e., hourly average
concentrations greater than or equal to 0.10 ppm) are reduced
much faster than the mid-level concentrations (i.e., 0.06-0.099
What is the cause
of the "piston effect"? Research appears to point to
the possibility that natural causes are partly responsibile for
it. Possible reasons for it have been discussed in the literature
(Reynolds et al., 2003; Reynolds et al., 2004).
The authors commented on possible chemical explanations for the
observation that more prominent trends in peak 1-h O3 levels
than in peak 8-h O3 concentrations or in occurrences of mid-level
(i.e., 0.06 to 0.09 ppm) concentrations have been reported. The
authors noted that when anthropogenic VOC and NOx emissions are
reduced significantly, the primary sources of O3 precursors are
biogenic emissions and CO from anthropogenic sources. Chemical
process analysis results indicated that a slowly reacting pollutant
such as CO could be contributing on the order of 10 to 20% of
the O3 produced. There are other reasons for the "piston
effect" and our research continues on this very important
Is there a way to
get around the "piston effect". Probably not. We must
realize its existence and deal with it in implementing our national
ozone standards. If we do not, then it is possible that lower
8-hour ozone standards may become a "goal" instead
of an attainable regulation. Thus, as the 8-hour ozone standard
continues to be lowered, there will be a level at which the result
of further emissions reductions may not yield attainability because
of the "piston effect" with the result that political
resistance will begin to increase from a segment of our society.
To learn more about the "piston" effect, please click here.
Lefohn, A.S., Shadwick,
D., Oltmans, S.J. (2010). Characterizing Changes of Surface Ozone
Levels in Metropolitan and Rural Areas in the United States for
1980-2008 and 1994-2008. Atmospheric Environment. 44:5199-5210.
Lefohn, A.S., Malley, C.S.,
Simon, H., Wells. B., Xu, X., Zhang, L., Wang, T. (2017). Responses
of human health and vegetation exposure metrics to changes in
ozone concentration distributions in the European Union, United
States, and China. Atmospheric Environment 152: 123-145. doi:10.1016/j.atmosenv.2016.12.025.
Reynolds, S. D.; Blanchard, C. L.; Ziman,
S. D. (2003) Understanding the effectiveness of precursor reductions
in lowering 8-hr ozone concentrations. J. Air & Waste Manage.
Assoc. 53: 195-205.
Reynolds, S. D.; Blanchard, C. L.; Ziman,
S. D. (2004) Understanding the effectiveness of precursor reductions
in lowering 8-hr ozone concentrations - Part II. The Eastern
United States. J. Air & Waste Manage. Assoc. 54: 1452-1470.