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How the W126 Ozone

Exposure Index was Developed

A fundamental principle is that higher hourly average ozone concentrations should be weighted more than middle and lower values when assessing human and environmental effects. This principle is based on the published results that were developed independently of any specific exposure and dose metric (see Section 2 of Lefohn et al., 2018 for further discussion). In 1985, based on important biological results published in the peer-reviewed literature, A.S. Lefohn proposed the use of the W126 ozone exposure index for predicting vegetation effects. The W126 index is a cumulative exposure index that focuses on the higher hourly average concentrations, while retaining the mid- and lower-level values. By applying a continuous weighting, the W126 index has the advantage of not utilizing an artificial "threshold." Any index (either exposure or dose) that attempts to link ozone with vegetation injury and or damage must at a minimum agree with the underlying experimental and empirical extensive results published in the peer-reviewed literature that the higher hourly average ozone concentrations should be provided greater weighting than the mid and lower values. If an index fails to confirm these underlying experimental and empirical published results, then it fails to adhere to the fundamental principle described above.

The cumulative W126 exposure index uses a sigmoidally weighted function (i.e., "S" shaped curve) as described by Lefohn and Runeckles (1987) and Lefohn et al. (1988). The W126 index is a cumulative exposure index and not an "average" value. As indicated above, it is a biologically based index, which is supported by research results (i.e., under both experimental and ambient conditions) that show that the higher hourly average ozone concentrations should be weighted greater than the mid- and lower-level values. The W126 index is accumulated over a specified time period.

The W126 cumulative exposure was described and used in the EPA's Ozone Criteria Documents (EPA, 1996, 2006a), as well as EPA's Integrated Assessment Document (EPA, 2013). risk documents (EPA, 2014a), and policy documents (EPA, 2014b) for both characterizing ozone trends and relating vegetation yield reduction losses with ozone exposure. To determine the W126 index, the sigmoidal weighting value at a specific concentration is multiplied by the concentration and then summed over all concentrations. The sigmoidal weighting function is of the form:

where: M and A are arbitrary positive constants

wi = weighting factor for concentration ci

ci = concentration i (in ppm)

The positive constants M and A are 4403 and 126 ppm-1, respectively. Their values were subjectively determined to develop a weighting function that (1) focused on hourly average concentrations as low as 0.04 ppm, (2) had an inflection point near 0.065 ppm, and (3) had an equal weighting of 1 for hourly average concentrations at approximately 0.10 ppm and above.

The name for the W126 exposure index was derived from the following:

"W" was associated with the word "weighted" and;

The number "126" was associated with the 126 value of the constant "A" in the W126 equation (see above).

For more information on the uses of the index, please visit our publications web page for specific citations to the peer-reviewed literature. Reading the critical review paper by Musselman et al. (2006) is a good place to start. In addition, the important publication by Heath et al. (2009) is an excellent contribution that focuses on explaining why the higher hourly average ozone concentrations (i.e., peaks) should be provided greater weight than the mid- and lower-level concentrations for assessing vegetation effects. The publication discusses the linkage of the temporal variability of apoplastic ascorbate with the diurnal variability of defense mechanisms in plants and compares this variability with daily maximum ozone concentration and diurnal uptake and entry of ozone into the plant through stomata. The paper integrates the three processes (i.e., uptake, ozone exposure, and defense) and provides evidence that supports the application of nonlinearity in vegetation responses to ozone exposures and dose. One of the keys to nonlinearity is the out-of-phase relationship among the three processes. The W126 ozone index focuses on this nonlinearity for assessing vegetation impacts. Again, the publications web page is a good place in which to start your reading.

In 2006, the EPA's Ozone Staff Paper (EPA, 2006b) recommended that the W126 exposure index be considered as a possible secondary ozone standard. Following EPA's recommendation, in August 2006, EPA's Clean Air Scientific Advisory Committee (CASAC) recommended that the W126 be adopted as a standard to protect vegetation from ozone exposure. The scientific consensus was that the cumulative W126 exposure index was a more relevant metric to use to protect vegetation than the 8-hour average health-related exposure index. In June 2007, the EPA Administrator recommended the W126 exposure index as a secondary standard to protect vegetation from ozone exposure. 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). The previous standard, set in 1997, was 0.08 ppm. EPA decided not to adopt the W126 exposure index. Although the EPA Administrator had originally 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 (i.e., vegetation) ozone standard the same as the primary (human health) 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, 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 are 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 web site its proposal to strengthen the national ambient air quality standards for ground-level ozone. The EPA's proposal decreases 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 is also proposing to establish a distinct cumulative, seasonal “secondary” standard, referred to as the W126 index, which is 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, 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 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 reconsideration of the ozone standards.

The EPA continues to support the use of the W126 exposure metric as an indication of vegetation effects caused by ozone. 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. EPA 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. On October 1, 2015, the Administrator established the 8-h standard at 0.070 ppm. The 8-h standard of 0.070 ppm is used to prevent the W126 exposure index from exceeding 17 ppm-h.

On August 23, 2019, the D.C. Court of Appeals ruled on the case Murray Energy Corporation v. EPA. The Court found that EPA's explanation for using the 3-year average of the 4th highest 8-hr vegetation standard of 0.070 ppm as a surrogate for the W126 index was insufficient. The Court remanded this issue for EPA to either lower the 8-h vegetation standard to protect against unusually damaging cumulative seasonal exposures that will be obscured in its three-year average, or explain its conclusion that the unadjusted average is an appropriate benchmark notwithstanding CASAC’s contrary advice. Alternatively, the Court suggested that EPA could adopt the single-year W126 exposure index as the form and averaging time, which would presumably moot any problems with the way it translated that index to use as a benchmark. In addition the Court questioned why the EPA did not use vegetation injury as the sensitive indicator versus growth loss. The entire August 23, 2019 decision is available at

The W126 exposure index has been proposed as the ozone secondary standard by either CASAC or EPA for the US since 2006 three separate times (2006, 2010, and 2014). The exposure index has played an important role over the past several years in assessing vegetation effects. The Forest County Potawatomi Community (FCPC) adopted the W126 exposure index as most suitable for its Class I area. Based on the review of the available scientific literature, the FCPC selected to use the W126 exposure index accumulated over a 24-h period for a 3-month period as one of two indices to protect vegetation. The FCPC determined that using the 24-hour W126 index, rather than the 12-hour W126, as suggested by the EPA, provided greater protection of FCPC’s vegetation resources ( For up-to-date information on the W126 and other aspects of air pollution environmental and human health effects information, please visit our News and Views section. Should you wish to learn more about the science associated with assessing the importance of peak ozone concentrations and how the peaks relate to vegetation uptake and detoxification, please click here. Our research, as well as those of others, is indicating that some of the findings associated with our vegetation research observations are applicable to the human health FEV1 lung function responses associated with ozone exposure (see Hazucha and Lefohn, 2007; Lefohn et al., 2010; Lefohn et al., 2018).



Hazucha M. J. and Lefohn A. S. (2007) Nonlinearity in human health response to ozone: Experimental laboratory considerations. Atmospheric Environment. 41:4559-4570.

Heath, R. L., Lefohn, A. S., and Musselman R. C. (2009). Temporal processes that contribute to nonlinearity in vegetation responses to ozone exposure and dose. Atmospheric Environment. 43:2919-2928.

Lefohn A.S. and Runeckles V.C. (1987) Establishing a standard to protect vegetation - ozone exposure/dose considerations. Atmos. Environ. 21:561-568.

Lefohn A.S., Lawrence J.A. and Kohut R.J. (1988) A comparison of indices that describe the relationship between exposure to ozone and reduction in the yield of agricultural crops. Atmospheric Environment. 22:1229-1240.

Lefohn, A.S., Hazucha, M.J., Shadwick, D., Adams, W.C. (2010). An Alternative Form and Level of the Human Health Ozone Standard. Inhalation Toxicology. Inhalation Toxicology. 22:999-1011.

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:

Musselman R.C., Lefohn A.S., Massman W.J., and Heath, R.L. (2006) A critical review and analysis of the use of exposure- and flux-based ozone indices for predicting vegetation effects. Atmospheric Environment. 40:1869-1888.

U.S. Environmental Protection Agency (1996) Air Quality Criteria for Ozone and Related Photochemical Oxidants. Environmental Protection Agency, Office of Research and Development, Research Triangle Park, NC. U.S. EPA report no. EPA/600/P-93/004bF.

U.S. Environmental Protection Agency (2006a) Air Quality Criteria for Ozone and Related Photochemical Oxidants. Research Triangle Park, NC: Office of Research and Development; report no. EPA/600/R-05/004af.

U.S. Environmental Protection Agency (2006b) Review of National Ambient Air Quality Standards for Ozone-Assessment of Scientific and Technical Information. Office of Air Quality Planning and Standards, Research Triangle Park, NC. EPA/600/R-05/004af.

US EPA. 2013. Integrated Science Assessment of Ozone and Related Photochemical Oxidants (Final Report). EPA/600/R-10/076F. Research Triangle Park, NC: Environmental Protection Agency. Available at: (accessed on 18 October 2017).

US EPA. 2014a. Health Risk and Exposure Assessment for Ozone. Final Report. EPA/452/R-14-004a. Research Triangle Park, NC: Environmental Protection Agency. Available at: (accessed on 18 October 2017).

US EPA. 2014b. Policy Assessment for the Review of the Ozone National Ambient Air Quality Standards. Final Report. EPA-452/R-14-006. Research Triangle Park, NC: Environmental Protection Agency. Available at: (accessed on 18 October 2017).

Washington Post (2008) It's Not a Backroom Deal If the Call Is Made in the Oval Office by Cindy Skrzycki. Tuesday, April 8, 2008; Page D02.

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