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  • The EPA Administrator on Friday, August 14, 2020 published in the Federal Register the Agency's draft ozone rulemaking proposal. The Federal Register notice can be found here. The final ozone rulemaking decision was made on December 23, 2020. Both the human health and vegetation ozone standards will remain at the current levels established in 2015. The EPA's decision on the ozone standards can be reviewed here. In regard to the secondary ozone standard used to protect vegetation, the EPA Administrator focused his comments on Lefohn et al. (1997) and Lefohn and Foley (1992). Both papers appear to play an important role in the Administrator's December 2020 decision for the secondary ozone standard. In the months ahead, more will be discussed about the use of the current 8-h ozone standard as a surrogate for the W126 exposure metric to protect vegetation.

Lefohn, AS, Jackson, W, Shadwick, DS and Knudsen, HP (1997). Effect of surface ozone exposures on vegetation grown in the southern Appalachian Mountains: Identification of possible areas of concern. Atmospheric Environment 31(11): 1695-1708.

Lefohn, AS and Foley, JK (1992). NCLAN Results and their Application to the Standard-Setting Process: Protecting Vegetation from Surface Ozone Exposures, J Air Waste Manag Assoc 42(8): 1046-1052.

On October 1, 2020, Dr. Lefohn, President of A.S.L. & Associates, LLC, responded to EPA's request for comments on the draft ozone rulemaking proposal. His 222 page document can be downloaded here. In December 2019, his comments on the drafts of the EPA's Integrated Science Assessment for Ozone (ISA) and Policy Assessment (PA) documents were filed in the U.S. government's official Docket. Dr. Lefohn's comments on the Integrated Science Assessment for Ozone (ISA) report can be downloaded here. His comments on the Policy Assessment (PA) document can be downloaded here. Some of the points made in the three submissions are

  • There are two key fundamental principles important in the ozone 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. The use of long-term average concentrations is not supported by human health and vegetation laboratory experiments. Based on ozone laboratory experiments, Haber's Rule (i.e., concentration multiplied by time) is not appropriate.


  • 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 that contribute substantially to surface ozone concentrations that are measured daily around the world.


  • As a result of emission reductions, the highest ozone concentrations are reduced and the lowest concentrations are increased due to the reduction of NOx scavenging.


  • Annual average or median concentrations increase as emissions are reduced at some monitoring sites in the US. This is a result of the reduction of NOx scavenging on the lower concentrations.


  • In 2015, EPA noted in its ozone rulemaking process that both acute and chronic ozone health effects could be reduced by reducing the higher hourly average concentrations. This is an important statement by the Agency because it indicates that ozone exposure metrics, used in models for estimating long-term risk to humans, might better be focused on the reduction of the higher ozone values rather than attempting to reduce the entire distribution of hourly average concentrations. The use of ozone exposure metrics based on annual average concentrations, which increase at many sites because of the reduction of NOx scavenging associated with emission reductions, could result in problematic human health ozone risk estimates.

  • In a recent published article, Go slow to go fast: A plea for sustained scientific rigor in air pollution research during the COVID-19 pandemic, the authors (Heederik, Smit, and Vermeulen), all associated with the Division of Environmental Epidemiology, Institute for Risk Assessment Sciences, Utrecht University, Utrecht, The Netherlands, note that over a ten-day period, three papers involving original research associating COVID-19 mortality and air pollution were published. These publications attracted considerable attention from international news outlets and on social media. According to the authors, all three ecological studies relied on aggregate data, which can suffer from the well-known problem of ecological fallacy, where a misjudgment in interpretation occurs as inferences about individuals are reasoned from the group to which the individual belongs. The authors believe that a a major issue, mostly ignored in these studies resides in the complexity of a potential association between air pollution and COVID-19 morbidity and mortality. The authors' article was published in the European Respiratory Journal. The Journal is the flagship journal of the European Respiratory Society. The entire article can be downloaded at

  • The DC Court of Appeals on August 23, 2019 rendered its decision on the EPA’s 2015 ozone (smog) standards. The Court has remanded back to the EPA for reconsideration the ozone standard to protect vegetation. The W126 metric, created by A.S. Lefohn, that CASAC recommended to protect vegetation is involved in this decision. The DC Court of Appeal's decision may be appealed and headed for the Supreme Court or the EPA may simply respond to the Court as part of its review of the current human health and vegetation ozone standards. For more information, please read the entire decision by the Court.

  • Design values published by the EPA provide an opportunity to quantitatively evaluate for the period 2015-2017 the status of ozone (smog) exposures in the national parks in the United States. Ozone data from 43 monitors in the US national park system were evaluated for potential human health risk. Sixty-one percent of the monitoring sites in the park system received a grade of either "A" or "B", 12% received a grade of "C", and 28% received a grade of either "D" or "F". More information available at

  • Over the years in the United States, we have made considerable progress in reducing ozone (i.e., smog) exposures. The 3-year average of the annual 4th highest daily 8-h concentration is the form of the US ozone standard to protect human health and welfare (i.e., vegetation). In 2015, the US EPA lowered the standard to 70 parts-per-billion (ppb). More information available at

  • The use of different air quality markers (metrics) for surface ozone calculated from the same time series can result in different trend patterns. This outcome is important to researchers, as well as policymakers and regulators, who use exposure metrics to assess how changes in ozone levels affect human health, vegetation, and climate.

That's one conclusion from a new metrics assessment based on the Tropospheric Ozone Assessment Report or TOAR, an effort by the International Global Atmospheric Chemistry Project to create the world's largest database of surface ozone observations from all available ozone monitoring stations around the globe. The TOAR paper, Global surface ozone metrics identified for climate change, human health, and crop/ecosystem research, was published in the journal Elementa: Science of the Anthropocene. The list of metrics used in the TOAR program can be downloaded here. The paper is available at the Elementa website at:

The 24 international researchers who worked on the paper anticipate that their effort will provide scientists, regulators, and policymakers with better insight about spatial and temporal variation that relate to climate change, human health, and crop/ecosystem around the world.

The paper provides the following:

A description of 25 metrics, which are used for assessing spatial and temporal trends by environmental agencies and researchers around the world (4 for model-measurement comparison, 5 for characterization of ozone in the free troposphere, 11 for human health impacts, and 5 for vegetation impacts).

• The scientific rationale for the selection of each of the 25 metrics.

• A detailed description of the statistical methods based on stringent scientific principles used in the Tropospheric Ozone Assessment Report (TOAR) program.

• A comparison of the trend behavior for each of the ozone impact metrics when using the same surface ozone concentration time series.

Key components of the Tropospheric Ozone Assessment Report (TOAR) ( are the use of metrics that are biologically defensible, as well as the use of statistical methods that adhere to stringent scientific principles. This paper provides the background for the selection of the metrics and the statistical methods used in the international TOAR program.

  • Background ozone is an important part of the challenge to attaining the new 0.070 ppm ozone standard. Although the EPA is continuing the ozone area designation process, the Agency is still concerned about the effect that background has on attainment of the 2015 ozone standard. Specifically, the key reasons that the EPA proposed a delay in 2017 to implementing the new ozone standard were
  • Fully understanding the role of background ozone levels;
  • Appropriately accounting for international transport; and
  • Timely consideration of exceptional events demonstrations.

There is much controversy on what the range of background ozone is in the United States. Our research is indicating that frequent occurrences greater than or equal to 50 ppb that occur at both high- and low-elevation monitoring sites across the US are influenced by transport from the stratosphere to the lower troposphere. The enhanced ozone concentrations that appear to be related to stratospheric transport occur during the springtime and sometimes during the summertime. In addition, long-range transport of Eurasian biomass burning, as well as wildfires in the US, contribute to background ozone concentrations. Estimating the range of background ozone properly is important because the range of background concentrations is used in the EPA's risk assessment for human health and vegetation, as well as assessing the amount of emission reductions required to attain a specific ozone level (i.e., standard). If the actual background level of ozone is higher than EPA estimates with models, then the Agency may overestimate human health, as well as vegetation risks and present inaccurate information to the public and policymakers. Our published material on background ozone can be found here. Our current research continues to address how to integrate background ozone with the attainment process.

  • Since A.S.L. & Associates' founder, Dr. Allen Lefohn, participated like others in the first Earth Day on April 22, 1970, we have seen much progress in controlling environmental pollution and improving the Nation's health and welfare (i.e., vegetation). For example, the US EPA began to regulate ozone with the promulgation of a ground-level National Ambient Air Quality Standard (NAAQS) in 1971, with subsequent revisions in 1979, 1997, 2008, and 2015. Following promulgation of the 1997 ozone standard, the US EPA issued a NOx State Implementation Plan (SIP) Call, which reduced regional summertime NOx emissions from power plants and other large stationary sources by 57% in 22 Eastern US states. In addition, the US EPA established national rules that substantially reduced NOx and VOC emissions from on-road mobile sources by 53% and 77% between 1990 and 2014, respectively. Overall, NOx and VOC have decreased in the US by 52% and 39% from all sources since 1990.

  • Changes in the magnitude of national and regional emissions, as well as any long-term changes in international emissions, climate, and inter-annual meteorological variability, can drive shifts in the distributions of hourly surface ozone (O3) concentrations. Changes in the distributions of hourly average O3 concentrations can result in changes in the magnitude of exposure metrics used for assessing human health and vegetation effects. Surprisingly, trend patterns in O3 exposure metrics may be in a similar direction as emissions changes (e.g., metrics increase as emissions increase) or may not (Lefohn et al., 2017 - see publications list). This is a very important observation because if a biologically irrelevant O3 metric is selected for assessing trends, an incorrect conclusion may be drawn concerning the relationship between emissions reductions and the protection of the public's health and welfare. Over the past 20-30 years, substantial changes in O3 concentrations have been observed at many sites across the world, likely driven by a combination of the large emissions changes and potentially by shifts in various meteorological conditions. A recent paper by Lefohn et al. (2017) investigated the relationship between exposure metrics, hourly O3 concentration distributions, and emission changes. To achieve this, we analyzed the response of 14 human health and vegetation O3 metrics to long-term changes in the hourly O3 concentration distribution, as measured at 481 monitoring sites in the EU, US, and China. The study provides insight into the utility of using specific exposure metrics for assessing emission control strategies. One important aspect of the study was that trends in mean or median concentrations did not appear to be well associated with some of the exposure metrics applicable for assessing human health or vegetation effects. Additional insights concerning the relationships between emissions reductions, hourly average concentration distributions, and human health and vegetation exposure metrics are discussed in Lefohn et al. (2017) (see publications list).

  • In October 2015, the EPA announced that both the human health and vegetation ozone standards will now be 70 ppb. Prior to that, on November 26, 2014, the EPA Administrator proposed an ozone human health (primary) standard in the range of 65 to 70 ppb and indicated that she would take comment on a standard as low as 60 ppb. The EPA Administrator noted that she placed the greatest weight on controlled human exposure studies, citing significant uncertainties with epidemiologic studies. Reasons for placing less weight on epidemiologic-based risk estimates are key uncertainties about (1) which co-pollutants are responsible for health effects observed, (2) the heterogeneity in effect estimates between locations, (3) the potential for exposure measurement errors, and (4) uncertainty in the interpretation of the shape of concentration-response functions for ozone concentrations in the lower portions of ambient distributions. The health standard is mainly based on the controlled human exposure study of Schelegle et al. (2009) that reported clinical effects at 72 ppb. Dr. Milan Hazucha of UNC Chapel Hill and I (Allen S. Lefohn) designed the ozone hourly exposure regimes used in the Schelegle et al. (2009) study. To the 72 ppb threshold of effects resulting from the Schelegle et al. (2009) study, the Administrator applied a Margin of Safety that helped her establish the ozone health standard below the 72 ppb level. Although the EPA initially recommended a separate exposure metric for the secondary standard (the W126 vegetation metric), the EPA adopted the 8-hour standard of 0.070 ppm to protect vegetation. The Agency felt that the 3-month, 12-h W126 exposure index used for assessing vegetation effects could be controlled to a level of 17 ppm-h or less by using the 8-hour standard. Industry and environmental organizations are back in court contesting the decision of the 8-hour ozone standard set at the 0.070 ppm level. On August 23, 2019, the D.C. Court of Appeals rendered its decision on the various challenges to the Environmental Protection Agency's 2015 revisions to the primary and secondary national ambient air quality standards for ozone. The Court denied the petitions, except with respect to the secondary ozone standard, which it remanded for reconsideration, and grandfathering provision, which the Court vacated.The Court of Appeal's decision makes interesting reading and is available by clicking here.

Click here to find out more information about W126Some historical perspective is important in understanding the background concerning the events that led to the EPA's Administrator's decision on revising the 8-hour ozone standard in October 2015. On March 12, 2008, the EPA Administrator announced a decision on the human health and vegetation ozone standards. At that time, EPA revised the 8-hour "primary" ozone standard, designed to protect public health, to a level of 0.075 parts per million (ppm). The previous standard, set in 1997, was 0.08 ppm. EPA decided not to adopt the cumulative exposure index as the vegetation standard (i.e., secondary ozone standard). Although the EPA Administrator recommended the W126 index 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). On 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, 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. After an extensive review process, the Obama Administration decided to not revise the 0.075 ppm standard that was set during the Bush Administration. The reason provided was that the EPA would soon begin a new review cycle of the science associated with surface ozone and recommend whether the 0.075 ppm standard needed to be revised.

  • For several years, A.S.L. & Associates has had an on-going effort to better understand the range and frequency of occurrence of background ozone levels that may not be affected by emission reduction strategies. In a paper published in May 2001, the research team consisting of Allen Lefohn, Samuel Oltmans, Tom Dann, and Hanwant Singh discussed that background ozone levels were higher and that the natural short-term variability was more frequent and greater than previously believed. The authors are associated with the following institutions: A.S.L. & Associates, NOAA, Environment Canada, and NASA, respectively. In our 2001 paper, we concluded that hourly levels greater than or equal to 50 ppb occur more frequently as a result from natural sources than previously believed. In 2006, the US EPA defined Policy-Relevant Background (PRB) for ozone as those concentrations that would occur in the United States in the absence of anthropogenic emissions in continental North America (i.e., the United States, Canada, and Mexico). PRB concentrations (renamed by the EPA as North American Background (NAB)) include contributions from (1) natural sources everywhere in the world and (2) anthropogenic sources outside the United States, Canada, and Mexico. In 2008, we published results, using empirical data, confirming that at some locations in the US, PRB ozone concentrations are greater than or equal to 50 ppb. In late September 2009, the National Research Council released the report, Global Sources of Local Pollution. In the report, the Committee states that modeling and analysis supports the finding that Policy-Relevant Background (PRB) is 20-40 ppb for the United States. Unfortunately, the NRC conclusion does not agree with the peer-review literature using empirical data that hourly averaged PRB ozone concentrations are greater than or equal to 50 ppb. Although spatially low-resolution models have been exercised and indicated that conclusions reached by Lefohn et al. (2001) were incorrect, our current research and the results published by other research groups support the conclusions reached by Lefohn et al. (2001) that PRB ozone concentrations are greater than or equal to 50 ppb at both high- and low-elevation monitoring sites. Clearly, low-resolution models are unable to adequately capture the important processes that are important for characterizing PRB and therefore, underestimate policy relevant background concentrations. The latest results using GEOS-Chem models continue to underestimate PRB ozone concentrations. An Internet-based slide presentation is available for purposes of previewing our paper. Also please be sure to check out the answer to our quiz that identifies the month in which the highest 8-hour daily maximum concentration occurred for the 4 remote ozone monitoring sites. Additional information on background ozone concentrations can be found in the Air Quality Analyses section of our Table of Contents. In-depth discussions are provided on this very important topic.

  • The range of suggested values for the W126 ozone vegetation standard is in part historically based on the recommendations that were made at a Workshop that took place in Raleigh, North Carolina in 1996. To better understand what took place at this workshop, please click here. The EPA recommends an accumulation over a 12-hour (8 am – 8 pm) exposure period over a 3-month period giving greater weight to exposures at higher levels of ozone. Our analyses and peer-reviewed published papers indicate that such a secondary ozone standard, in the proposed form, would overestimate vegetation effects. For information about why the use of a 12-hour versus a 24-hour accumulation period would contribute to the inconsistency problem of the W126 index, please click here. You can learn more about the subject of vegetation effects by visiting our Table of Contents web page.

  • Lefohn, Shadwick, and Oltmans (2010) have statistically quantified in a paper published in the peer-reviewed journal, Atmospheric Environment, a site-by-site trending analysis for the period 1980-2008 and 1994-2008. Lefohn et al. (2010) point out that many ozone monitoring sites show no statistical changes over time as well as a small number of sites show increases in trending. Please see the publications list for the citation.

As indicated above, Lefohn et al. (2010) published their trending findings for surface ozone monitoring sites across the United States. Using statistical trending on a site-by-site basis of the (1) health-based annual 2nd highest 1-hour average concentration and annual 4th highest daily maximum 8-hour average concentration and (2) vegetation-based annual seasonally corrected 24-hour W126 cumulative exposure index, they investigated temporal and spatial statistically significant changes that occurred in surface ozone in the United States for the periods 1980-2008 and 1994-2008. For more information about the Lefohn et al. (2010) and Lefohn et al. (2008) (for the period 1980-2005 and 1990-2005) findings, please click here.

Since 1997, we have been discussing the "piston effect" in the peer-reviewed literature (see publications listing). In 1997, we predicted that there would be a leveling off of improvements in O3 concentrations as O3 emission precursors were reduced at some monitoring sites Our prediction apparently has been verified by the most current trends analysis by the EPA. On EPA's web site (, the Agency in July 2016 summarized trends for the ozone periods 1980-2015, 1990-2015, and 2000-2015. Note that the national average for trends for the three time periods were 32%, 22%, and 17%, respectively. Clearly, the trend is slowing down.

The "piston effect", as described in the peer-review literature and on this web site, affects the ability of the nation to attain the 8-hour ozone standard as lower and lower 8-hour standards are established. As we discussed in our original paper, the peak hourly average concentrations are reduced much faster than the mid-level concentrations. This pattern is discussed in our most current publication on trends in the EU, US, and China (Lefohn et al., 2017-see publications list). Clearly the "piston effect" heavily influences the Nation's ability to attain an 8-hour ozone standard as standard levels are reduced. We discuss more about the "piston effect" and how it affects the attainability of the ozone standard in our concerns web area.

  • Over the past years, A.S.L. & Associates and its consultants have commented on the strengths and weaknesses associated with the mathematical and statistical methodologies used in epidemiological studies to link exposure with human health effects. Many of the statistical caveats raised throughout the PM and Ozone Criteria Documents and the PM and Ozone Staff Papers indicate a pattern of inconsistent results that is troubling. Examples of the growing pattern of inconsistent and inconclusive findings include the following:
  • Instability of PM mortality effect estimates resulting from different model specifications of weather effects and time trends.
  • Instability of PM effect estimates resulting from different selections of monitoring sites within cities.
  • Increased heterogeneity of PM effect estimates across cities.
  • Greater diversity of findings among studies and across study areas.
  • Contradictory results from mortality displacement studies.
  • PM effect lags that are inconsistent across cities and across studies.
  • Exposure-response relationships that are inconsistent across cities and across studies.
  • Inconsistencies between short-term and long-term effect studies, such as respiratory effects of fine particles.
  • Contradictory findings among long-term studies.

Additional details about the Team's epidemiological concerns are discussed on our epidemiological concerns web page. The Team's comments on the first draft of the PM Staff Paper were submitted to EPA in October 2003. To read more about our concerns about the first draft, please visit our web page.

  • Sometimes science and politics mixed together become science fiction. Such is the case that occurred, when in September 2002, many newspapers across the United States printed a story summarizing the report, Code Red: America's Five Most Polluted National Parks, which described The Great Smoky Mountains as the nation's most polluted national park, with air quality rivaling that of Los Angeles. For the period 1997-2001, the report claims that the annual ozone exposure was higher at Great Smoky Mountains National Park than at Los Angeles, California. There is a serious technical problem associated with the report and the report's conclusions are flawed. Please read "The Rest of the Story."

  • In 2000, Haywood County, NC experienced its 4th highest 8-hour ozone concentration at 0.085 ppm. On May 1, a daily maximum 8-hour average concentration of 0.089 ppm was experienced. A detailed meteorological analysis suggests that stratospheric ozone played an important role in this ozone episode.

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