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The W126 Ozone Exposure Index

A fundamental principle, based on experimental results, is that higher hourly average ozone concentrations should be weighted more than middle and lower values when assessing human and environmental effects. Independent of the selection of a specific ozone exposure and dose metric, the principle is based on results from laboratory and field experiments performed in the 1980s and 1990s (see Section 2 of Lefohn et al., 2018 for further discussion). In 1985, based on key biological results published in the peer-reviewed literature, A.S. Lefohn created 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 had the advantage of utilizing all hourly average ozone concentrations and not imposing 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 results published in the peer-reviewed literature that clearly show that the higher hourly average ozone concentrations should be provided greater weighting than the mid and lower values. If an ozone index fails to confirm the underlying experimental and empirical published results, then it fails to adhere to the fundamental principle described above. Simply exercising a model that does not agree with the fundamental principle does not disprove the fundamental principle. Rather, the disagreement illustrates the weakness of the model itself.

The cumulative W126 exposure index uses a sigmoidally weighted function (i.e., "S" shaped curve). Lefohn and Runeckles (1987) discussed the use of a sigmoidal weighting scheme and Lefohn et al. (1988) applied the W126 exposure index for assessing growth response from ozone exposures using winter wheat experimental data. 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 natural ambient conditions), that show the higher hourly average ozone concentrations should be weighted more than the mid- and lower-level values. The W126 index is accumulated over a specified time period defined by the user.

The W126 cumulative exposure was described and used in the EPA's Ozone Criteria documents (EPA, 1996, 2006a), as well as EPA's Integrated Scientific Assessment documents (EPA, 2013, 2020), Health Risk and Exposure Assessment document (EPA, 2014a), and Policy Assessment document (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), as well as the Lefohn et al. (2018) internationally authored paper (Section 2), are good places to start. In addition, the important publication by Heath et al. (2009) is an excellent contribution that focuses on the biological rationale for weighting the higher hourly average ozone concentrations more 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 (detoxification) 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 (i.e., uptake, ozone exposure, and defense). 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 on the subject.

The use of the W126 to characterize ozone exposure concentrations regarding potential vegetation effects, particularly growth, has received strong support from the EPA's Clean Air Scientific Advisory Committee (CASAC) in previous reviews (Henderson, 2006; Samet, 2010; Frey, 2014; Cox, 2020). 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 CASAC recommended 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 as the secondary ozone standard. 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, under a new administration, 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 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.

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 reconsideration of the ozone standards. The EPA soon after the President's decision initiated the full 5-year review cycle of the ozone NAAQS.

Following the 2011 decision by the EPA, the Agency continued to support the use of the W126 exposure metric in risk analyses associated with ozone and vegetation 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 the form of the health standard in this range would also protect vegetation from ozone exposures of W126 values within the range of 13-17 ppm-h. The Agency believed that there existed a reasonable mathematical correlation between the 8-h human health standard and the W126 exposure metric. EPA also took comment on setting a W126 value in the range of 7-13 ppm-h, which implied that she still considered 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 for both the human health and welfare ozone standards. The 8-h standard of 0.070 ppm was applied to prevent the W126 exposure index from exceeding 17 ppm-h. Follwing a review of the 2015 ozone standards, the Administrator on December 23, 2020 made the decision that both the human health and vegetation ozone standards would remain at the current levels established in 2015. The current 70 ppb 8-h O3 standard as per the US EPA's 2015 decision serves as a surrogate to achieve O3 levels at or below a W126 value of 17 ppm-hrs.

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 human health form of the standard of the 3-year average of the 4th highest 8-hr value of 0.070 ppm as a substitute 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 EPA Administrator attempted to respond to the Court's concerns in its December 2020 ozone rulemaking decision. The entire August 23, 2019 decision is available at https://bit.ly/2MzY2Hg.

The EPA told the U.S. Court of Appeals for the D.C. Circuit in a court filing in late October 2021 that it would initiate a rulemaking process to reassess by the end of 2023 the Agency's December 2020 decision to retain the 2015 ozone human health and vegetation standards. As a result of the reconsideration, CASAC recommended on June 9, 2023 the W126 index as the secondary ozone standard and that the EPA Administrator should consider that the level of the W126 metric be in the range of 7 to 9 ppm-hrs. The June 9, 2023 CASAC letter to the EPA Administrator can be read by clicking here. Upon reviewing the CASAC recommendations for the human health and vegetation ozone standards, the EPA made the decision to restart the entire ozone rulemaking process and to initiate a new review of the ozone NAAQS. This review process will take many years to complete.

Since 2006, on four separate occasions (i.e., 2006, 2010, 2014, and 2023), the W126 exposure index has been proposed as the ozone secondary standard by either CASAC or EPA for the US. 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 made the decision to use the W126 exposure index (accumulated over a 24-h period for a fixed 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 (https://lnr.fcpotawatomi.com/current-vegetation-w126-n100/). The Forest County Potawatomi Community has coupled the W126 exposure index with the N100 metric (the number of hourly average O3 concentrations greater than or equal to 100 ppb) because of the relevance of both exposure metrics for assessing vegetation effects. For additional information about the N100 exposure metric, please click here.

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 (please see Hazucha and Lefohn, 2007; Lefohn et al., 2010; Lefohn et al., 2018).

 

References

Cox, LA. (2020). Letter from Louis Anthony Cox, Jr., Chair, Clean Air Scientific Advisory Committee, to Administrator Andrew R. Wheeler. Re:CASAC Review of the EPA's Policy Assessment for the Review of the Ozone National Ambient Air Quality Standards (External Review Draft - October 2019). February 19, 2020. EPA-CASAC-20-003. Office of the Adminstrator, Science Advisory Board Washington, DC. Available at: https://bit.ly/3lgmgni.

Frey, H.C. (2014). Letter from Dr. H. Christopher Frey, Chair, Clean Air Scientific Advisory Committee to Honorable Gina McCarthy, Administrator, US EPA. Re: CASAC Review of the EPA's Second Draft Policy Assessment for the Review of the Ozone National Ambient Air Quality Standards. June 26, 2014. EPA-CASAC-14-004. Office of the Administrator, Science Advisory Board Washington, DC. Available at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100JR6F.txt.

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.

Henderson, R. 2006. Letter from Dr. Rogene Henderson, Chair, Clean Air Scientific Advisory Committee to Honorable Stephen L. Johnson, Administrator, US EPA. Re: CASAC Peer Review of the Agency's 2nd Draft Ozone Staff Paper October 24, 2006. EPA-CASAC-07-001. Office of the Administrator, Science Advisory Board U.S. EPA HQ, Washington DC. Available at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1000WO7.txt.

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: http://doi.org/10.1525/elementa.279.

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.

Samet, JM. 2010. Letter from Jonathan Samet, Chair, Clean Air Scientific Advisory Committee, to Administrator Lisa Jackson. Re: CASAC Review of EPA's Proposed Ozone National Ambient Air Quality Standard (Federal Register, Vol. 75, Nov. 11, January 19, 2010). February 19, 2010. EPA-CASAC-10-007. Office of the Administrator, Science Advisory Board U.S. EPA HQ, Washington DC. Available at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P10072T1.txt.

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: https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=247492 (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: https://www.epa.gov/naaqs/ozone-o3-standards-risk-and-exposure-assessments-current-review (accessed on 18 October 2017).

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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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