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Some Concerns about Continuing to Lower the 8-Hour Ozone Standard

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 mentioned 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 8-hour 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 value for the day. By applying 8-hour averages, the data are smoothed and provide the appearence of greater areas of concern, which not necessarily be accurate. Clinical health laboratory studies show that the peak hourly concentrations are more important than extended-period average concentrations (see Lefohn et al., 2018-Section 2). 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 8-hour average concentrations similar in the manner that the 8-hour ozone standard is determined.

Some scientists and engineers are 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. 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; control strategies may 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 here.

On EPA's web site (, the Agency in May 2022 summarized trends for the ozone periods 1980-2021, 1990-2021, 2000-2021, and 2010-2021. Note that the national average for trends for the four time periods were 29%, 21%, 16%, and 5%, respectively. Clearly, the trend is slowing down.

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 website 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 ppm).

What is the cause of the "piston effect"? Research appears to point to the possibility that natural processes are partly responsibile for the effect. 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 subject.

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 future 8-hour ozone standards, as they are lowered, 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." 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.

Lefohn, A.S., Malley, C.S., Smith, L., Wells, B., Hazucha, M., Simon, H., Naik, V., Mills, G., Schultz, M.G., Paoletti, E., De Marco, A., Xu, X., Zhang, L., Wang, T., Neufeld, H.S., Musselman, R.C., Tarasick, T., Brauer, M., Feng, Z., Tang, T., Kobayashi, K., Sicard, P., Solberg, S., and Gerosa. G. (2018). Tropospheric ozone assessment report: global ozone metrics for climate change, human health, and crop/ecosystem research. Elem Sci Anth. 2018;6(1):28. DOI:

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.

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