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For over 50 years, members of our staff have performed scientific research on the effects of air pollutants on the environment. There are hundreds of pages on this website that focus on the various aspects associated with air pollution research. In order to assist you in identifying which pages might be of interest to you, this introduction provides insight into some of the more important scientific issues. At anytime, please feel free to visit our Table of Contents.

Using the most current research information, we continuously assess the scientific rationale for air pollution standards promulgated nationally and internationally. Our research sponsors are government, industry, and environmental groups. Both national and international institutions have supported our research efforts. Thousands of individuals around the world visit our web pages daily. Some of the most popular web pages visited are those associated with (1) natural background of surface ozone, (2) the biological importance of the higher hourly average air pollution concentrations more than the mid- and lower-level values, (3) our exposure- and dose-response research on vegetation and human health, (4) the "piston effect" and how it affects the reduction of hourly average surface ozone concentrations as air pollutant emissions are reduced, (5) our global sulfur emissions database from 1850-1990, (6) spatial interpolation of surface ozone (i.e., kriging), (7) concerns about assumptions associated with epidemiological modeling, (8) our peer-review publications list, and (9) our Albert Einstein quotations with references.

For ozone, the following two key fundamental principles are important in the standards rulemaking activity:

  • Fundamental Principle One: Higher Hourly Average Ozone Concentrations Should be Weighted More than Middle and Lower Values when Assessing Human Health and Environmental Effects. This principle is supported by human clinical laboratory studies and vegetation chamber experiments performed in the 1980s and 1990s. The implication of this principle is that the use of long-term average concentrations for assessing human health and vegetation risk is not appropriate. In addition, based on ozone human clinical laboratory studies, Haber's Rule (i.e., the product of concentration multiplied by time is a constant) is not appropriate for use in an ozone exposure or dose mathematical relationship. Concentration is more important than time of exposure and should be weighted more than duration. For assessing human health and vegetation effects, Fundamental Principle One plays an important role in the selection of exposure or dose metrics used in biological effects models.


  • Fundamental Principle Two: Daily Maximum Hourly Averaged Ozone Concentrations Will Remain Well above 0 Parts per Billion (ppb) Even if all Anthropogenic Emissions Were Eliminated Worldwide. In other words, there are natural sources of ozone (e.g., stratospheric) that contribute to surface ozone concentrations measured daily around the world.

An important observation reported in the peer-review literature is that as a result of emission reductions, at many ozone monitoring sites, the highest hourly average concentrations are reduced and the lowest concentrations are increased due to the reduction of NOx scavenging (Lefohn et al., 2017; Lefohn et al., 2018). As a result, annual average or median concentrations consisting of hourly average concentrations increase as emissions are reduced. The increase in annual or seasonal average concentrations is also associated with the reduction of NOx scavenging on the lower concentrations. During the COVID-19 lockdown that occurred worldwide in 2020, many peer-viewed papers were written that described the increase of weekly average ozone concentrations during the lockdown period in comparison to previous years. Many of the authors of these papers attributed the ozone increase to the reduction of NOx scavenging resulting from decreases in NOx emissions (Lefohn, 2020).

For ozone, an important phenomenon is the "piston effect". The EPA has confirmed the existence of the effect that was first described by A.S.L. & Associates in November 1996 and published in the peer-review literature in 1997 and 1998. As emissions are reduced, larger reductions occur in the higher ozone concentrations than the middle values. To attain the 8-hour ozone standard of 0.070 ppm standard, the high and upper middle concentration values must be reduced. As emissions are reduced to attain the 8-hour standard, at many monitoring sites the lower hourly average concentrations shift upward toward the middle concentration values (see Lefohn et al., 2017; Lefohn et al., 2018). The result of emission reductions to attain the ozone standard is a compression of the high and low hourly average concentrations toward the middle values. Emission reduction modeling performed by the EPA in 2014 illustrated the importance of the "piston effect" in attaining the 8-hour standard. In the EPA's published trends report for ozone (, the Agency indicated that while O3 levels are still decreasing nationwide, the rate of decrease for both the 1-hour and 8-hour levels has continued to slow down over time. If the limitations associated with the"piston effect" are ignored, the EPA and others who recommend lower and lower ozone standards may find that there is a limit to how low the standard can be set. Fundamental Principle Two mentioned above limits how low the ozone standard can be set. For more information on this important subject, please click here.

Many years ago at a meeting that took place on August 24-25, 2006 in Durham, NC, the EPA's Clean Air Scientific Advisory Committee (CASAC) recommended a more stringent human health standard than the 8-hour 0.08 ppm ozone standard. For the secondary ozone standard which protects vegetation, CASAC recommended the W126 cumulative exposure index integrated over a 3-month growing season period measured daily from 0800 to 1959 hr. In June 2007, the EPA Administrator proposed the W126 index as the secondary ozone standard. Both the primary and secondary standard recommendations by CASAC were reflected in the final version of the EPA Ozone Staff Paper. On March 12, 2008, the EPA Administrator made the final decision on the human health and vegetation ozone standards. EPA revised the 8-hour "primary" ozone standard, designed to protect public health, to a level of 0.075 parts per million (ppm). EPA decided not to adopt the W126 exposure index. Although the EPA Administrator recommended the W126 as the secondary ozone standard, based on advice from the White House (Washington Post, April 8, 2008; Page D02), the EPA Administrator made the secondary ozone standard the same as the primary 8-hour average standard (0.075 ppm). In May 27, 2008, health and environmental organizations filed a lawsuit arguing that the EPA failed to protect public health and the environment when it issued in March 2008 new ozone standards. On March 10, 2009 as the new Administration began to review the ozone standard situation, the US EPA requested that the Court vacate the existing briefing schedule and hold the consolidated cases in abeyance. EPA requested the extension to allow time for appropriate EPA officials that were appointed by the new Administration to review the Ozone NAAQS Rule to determine whether the standards established in the Ozone NAAQS Rule should be maintained, modified, or otherwise reconsidered. EPA further requested that it be directed to notify the Court and the Parties within 180 days of the Court's order vacating the briefing schedule of the actions the Agency has taken or intends to take, if any, with regard to the Ozone NAAQS Rule, and the anticipated time frame for any such actions.

On September 16, 2009, the EPA announced it would reconsider the 2008 national ambient air quality standards (NAAQS) for ground-level ozone for both human health and environmental effects. The Agency planned to propose any needed revisions to the ozone standards by December 2009 and issue a final decision by August 2010. On January 7, 2010, the EPA announced on its website its proposal to strengthen the national ambient air quality standards for ground-level ozone. The EPA's proposal decreased the 8-hour “primary” ozone standard level, designed to protect public health, to a level within the range of 0.060-0.070 parts per million (ppm). EPA also proposed to establish a distinct cumulative, seasonal “secondary” standard, referred to as the W126 index, which was designed to protect sensitive vegetation and ecosystems, including forests, parks, wildlife refuges, and wilderness areas. EPA proposed to set the level of the W126 secondary standard within the range of 7-15 ppm-hours. The proposed revisions resulted from a reconsideration of the identical primary and secondary ozone standards set at 0.075 ppm in March 2008. On August 20, 2010, the Agency announced that it would delay its final announcement to on or around the end of October. In early November, the EPA announced that it would reach a final decision on the ozone standards by December 31, 2010. On December 8, the EPA announced that it would delay its final decision on the ozone standards until July 2011. EPA announced on July 26, 2011 that it would not make a decision on the ozone standards by its previously announced deadline of July 29. On September 2, 2011, President Obama requested that the EPA withdraw its proposal to revise the ozone standards and deferred to the normal cycle of evaluating the current state of science associated with ozone and its effects on human health and vegetation.

After performing its evaluation of the science associated with ozone and its effects, on November 26, 2014, the EPA Administrator announced that she was proposing an ozone human health (primary) standard in the range of 65 to 70 ppb and would take comment on a standard as low as 60 ppb. For the welfare (secondary) ozone standard, she proposed that the standard be the same as the health standard IF the final health standard were set in the range of 65 to 70 ppb. The Administrator believed that a health standard in this range would protect vegetation from ozone exposures of W126 values within the range of 13-17 ppm-h, which was a different range (7-15 ppm-h) proposed in 2010. She also took comment on setting a W126 value in the range of 7-13 ppm-h, which implied that she was still considering establishing a secondary standard separate in form from the human health 8-h standard. Prior to the November 2014 recommendation, in August 2014, the EPA Staff recommended to the Administrator that she select the ozone primary standard at a specific level between 60-to-70-parts-per-billion. For the secondary standard, the EPA Staff recommended that the Administrator establish a 3-month, 12-h W126 secondary standard, which would have a specific value within the range of 7 to 17 ppm-h. On October 1, 2015, the EPA Administrator announced that both the human health and vegetation ozone standards were to be 70 ppb. The Administrator concluded that protection of vegetation from adverse effects could be provided by an 8-h ozone standard of 70 ppb that limits cumulative 3-month, 12-h W126 exposures to 17 ppm-hrs or lower. The 70 ppb 8-h ozone standard as per the US EPA's 2015 decision and more recent 2020 decision currently serves as a surrogate to achieve ozone levels at or below a 3-month, 12-h W126 value of 17 ppm-hrs.

There are still many uncertainties associated with the science that was used to support the development of the 8-hour ozone standard. For over 40 years, environmental groups, industry, and government officials have worked with A.S.L. & Associates to provide them with the latest scientific information on ozone and particulate matter. We have carefully noted the limitations of the science involved in the decision-making process. The June 1997 Feature Article in Environmental Science & Technology, the September 1997 New Directions Column in Atmospheric Environment, the June 1998 Policy Analysis article in Environmental Science & Technology, the May 2001 peer-reviewed paper in the Journal of Geophysical Research, and the comments submitted by Lefohn (2020) on the most current ozone rulemaking proposal discuss several of the important issues described on the various web pages associated with this site and the uncertainties associated with the underlying science. Four important issues that we address in these web pages are

Based on a review of empirical ozone concentration information and our own research results, actual background ozone levels are more of a problem than researchers and policymakers believe. As the ozone standards are lowered, background ozone will play a very important role in attaining the lower 8-h ozone standard across the U.S. Background ozone plays an important role in the margin of safety determinations that establish the ozone standards proposed by the EPA Administrator.

Is there a way to get around the "piston effect"? Probably not. The "piston effect" apparently controls the ability of a specific violating area reaching attainment in a reasonable time frame. The June 1998 policy analysis article in the peer-reviewed journal, Environmental Science & Technology, discusses this effect in detail. Additional articles dealing with the difficulty in achieving the ozone standard have been published in peer-reviewed journals. For a review of some of these articles, please visit our publications page. What causes the "piston effect"? Our most current research results are providing the answers to what causes the effect.

What is the result of EPA using chemistry transport models to generate highly uncertain estimates for background? Inadequate estimates of background ozone concentrations may result in an overestimate by the EPA of the human health risks associated with 8-hour ozone levels, as well as the ability to attain lower ozone standard levels that may be proposed. In our research, we have combined modeling results with estimates of the importance of the contributions from the natural sources, such as stratospheric ozone. In its most recent modeling efforts (2020), the EPA noted that it did not include stratospheric considerations in its modeled background estimates. Our results on background modeling can be found in the list of papers provided in our publications page.

The EPA has indicated a pattern of inconsistent results in epidemiological time-series studies. This raises concern about the utility of these types of studies in the current NAAQS-setting process. The EPA Administrator in October 2015 agreed that the epidemiological risk results did not provide strong evidence for reducing the current level of the 8-hour ozone standard below 0.070 ppm. Similar conclusions were reached by the EPA Administrator in his December 2020 decision. We have commented on the limitations associated with the use of epidemiological results in the NAAQS-setting process.

A.S.L. & Associates hopes you will take the time to carefully read the scientific material provided here and learn more about the science that affects our ability to meet proposed lower and lower ozone standards. Much of our attention in the last few years has been focused on background, the frequency distribution (i.e., rollback) scenarios that result from emission reductions in NOx and VOCs, human health and vegetation dose-response modeling, and the shortcomings associated with epidemiological modeling. We appreciate having the opportunity to provide you with our research information. If you desire further information, please contact A.S.L. & Associates.

By introducing the 8-hour ozone standard in 1997, the U.S. EPA began an unintentional experiment to quantify how important anthropogenic emissions are in comparison with natural emissions and processes. The "piston effect", a natural controller of ozone concentrations, will probably dominate the ability of states to attain the 8-hour standard as ozone standards are proposed to be reduced. We continue to perform our research and report, both in the peer-review literature and on this website, our results. We hope you will return to our website again and again to continue to learn more about this most fascinating topic.

Science is both interesting and fun. It is when policymakers attempt to work with the current state of science that things really get fascinating. Science is the search for truth. Sometimes we have answers and sometimes we do not. However, scientists have an obligation to provide guidance to policymakers, whether requested or not, to assure that the scientific facts presented are accurate and are not obfuscated for political purposes. The purpose of this website is to provide to the general public, scientists, policymakers, and decision makers the opportunity to review the science that helps make the decisions that influence the direction in which our environmental policies are directed. As indicated above, each day thousands of visitors around the world read the materials contained within our web pages. To start your visit on our web pages, please visit our Table of Contents or News sections. Welcome aboard!


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:

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