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Background Information on the Originally Proposed Federal Standard to Protection Vegetation in the United States from Surface Ozone

A workshop was held in Raleigh, North Carolina on January 12-13, 1996, sponsored by the Emission and Effects Task Group of the Southern Oxidant Study Program. The meeting was held in Raleigh, NC to 1) assemble a group of ecological, agricultural and forest scientists to discuss the strengths and uncertainties of the current ecological knowledge base available for use in setting a revised federal secondary ozone standard; 2) discuss the value of a longer vs the current short-term secondary standard; 3) suggest possible standards (to include daily duration, seasonal duration, form, and specific ozone concentration values [in relation to a selected set of ecological end-points]) that would protect vegetation from ozone injury or damage; and 4) discuss and suggest research needed to remove some of the current uncertainties relative to the available ecological data base for ozone. Sixteen vegetation experts attended the Workshop to discuss the form and level of a possible O3 secondary standard. Dr. Lefohn, President of A.S.L. & Associates, was one of the vegetation experts invited and he attended the meeting. The meeting represented the "best guess" of the participants for the form and level of a possible O3 secondary standard.

The participants reached the following consensus:

  • There is a need for a secondary standard that was different from any of the health-based primary standards presently being considered by the U.S. EPA.
  • There is a need to protect the vegetative components of natural ecosystems and, at this time, foliar (leaf) injury is the only endpoint that could be used in these systems.
  • There is a need to protect some aspect of the integrity of the forest ecosystem but the data are not sufficiently strong to select an indicator of forest structure or function. Two indicators that are pertinent to forest ecosystems, where data were sufficient to be used, are

Growth effects on seedlings from tree species of natural forest stands (a one to two percent per year reduction). (This level was selected to avoid the cumulative effects of yearly reductions of 2%.)

Growth effects on seedlings and saplings from tree plantations (a one to two percent per year reduction).

  • There is a need to protect against crop yield reductions of 5% while at the same time acknowledging that the uncertainties (in terms of confidence limits) were high at the 5% level. Thus for the purposes of a secondary standard, given the uncertainty of determining 5% crop loss, the participants felt the crop yield end-point should be the 10% yield reduction level.
  • The 12-h time period from 0800 to 1959 ST (i.e., basically from 8am until 8pm standard time) is an appropriate time window for a secondary standard.
  • In terms of doing the best job of protecting ecological systems, the running 90-day maximum is the time period of interest.

The following six forms for the secondary standard were discussed at some length for the daily and seasonal time periods agreed to above:

  • A mean value (in ppm) - which would give the same results as a cumulative index (in ppm-hrs) of the sum of 0.000 ppm (SUM00).
  • The sum of all hourly average concentrations (in ppm-hrs) once the ozone value reaches a given concentration or above - four of these cumulative indices were discussed:
      • Values of 0.025ppm or above (SUM025)
      • Values of 0.040ppm or above (SUM04)
      • Values of 0.050ppm or above (SUM05)
      • Values of 0.060ppm or above (SUM06)
      • A sigmoidally weighted index (W126 - also in ppm-hrs), which provided increasing weight to values from 0 to 0.100 ppm (with little weight to values below 0.040 ppm) and a unit weight to all values of 0.100 ppm and above.

Consensus was reached on the SUM06 and W126 exposure indices. The participants acknowledged that these two forms would give very similar protection against ozone effects on vegetation at most locations in the United States. The majority of the participants preferred the W126 (by a 9 to 6 vote), but consensus could be reached only on the SUM06. The participants felt that the selection of the SUM06 should not be interpreted to mean acceptance of a threshold of 0.060 ppm for effects on ecological systems, but rather that concentrations above 0.060 ppm were well correlated with plant response to ozone. There was some discussion about the possibility of one or more exceedances equal to or greater than 0.10 ppm to be combined with the form of the standard because the NCLAN fumigation protocol in the experiments resulted in numerous hourly average concentrations greater than or equal to 0.10 ppm. The participants could not develop a consensus on this issue and the recommendation was to forward the concern to the EPA and let the Agency deal with the issue.

The participants recommended the following range of SUM06 values associated with each of the biological endpoints mentioned above:

      • Foliar injury of the vegetation component of natural ecosystems - 8 to 12 ppm-hrs
      • Growth effects on tree seedlings from natural forest stands (10 to 15 ppm-hrs) and from plantation tree seedlings and saplings (12 to 16 ppm-hrs)
      • Crop yield reductions of 10% - 15 to 20 ppm-hrs

    The recommendations were forwarded to the U.S. EPA for further consideration. In February 1996, the Agency made the decision to recommend the SUM06 exposure index. Although the Agency did not completely follow the recommendations of the workshop participants, no doubt the recommendations that came forward from the workshop affected the Agency's thinking concerning whether to recommend a secondary standard different in form and level from the primary, health-based standard. In 1997, the Agency made the decision to not implement a separate secondary standard different from the health-based primary standard. The Agency is presently using the 8-hour ozone standard for both the primary and secondary standard.

    In 2006, the EPA's Ozone Staff Paper (EPA, 2006a) 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 range of suggested values for the W126 was mainly based on the recommendations that were made at the Workshop that took place in Raleigh, North Carolina in 1996. In June 2007, the EPA Administrator proposed a range of 7 - 21 ppm-hours for a 3-month, 12-hour seasonal W126 exposure index for public comment on the secondary ozone standard.

    It is important to stress that the recommendations were based on the judgment of the various participants and very little scientific evidence was presented at the meeting to support the participants "feelings" about specific levels to protect vegetation. The results from the statistical analyses performed by Dr. Henry Lee of the EPA using NCLAN crop loss data were available from the 1996 EPA Criteria Document and the 1996 EPA Staff Paper. However, consensus on protective levels for foliar injury to natural ecosystems, growth effects to tree seedlings in natural forest stands, and growth effects to tree seedlings and saplings in plantations was reached based on undocumented values presented by the Workshop participants.

    Since the 1996 Workshop that recommended a 12-hour cumulative period, evidence has continued to mount that nocturnal uptake is important, as noted in the Ozone Criteria Document (EPA, 2006b). On page AX9-186 of the Ozone CD, the authors emphasize that the potential for nocturnal uptake should be addressed by modifying the form of currently used exposure indices. Unfortunately, this recommendation has not been seriously addressed in the final version of the Ozone Staff Paper (EPA, 2007).

    In June 2007, the EPA Administrator proposed the W126 exposure index as a secondary ozone standard. 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, 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 web site its proposal to strengthen the national ambient air quality standards for ground-level ozone. The EPA proposed to decrease 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 proposed 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 proposal for the revision of ozone standards.

    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. 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. 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 Administrator set the ozone standard at 0.070 ppm for both the human human and vegetation standards. The Administrator felt that attaining an 8-hour standard at 0.070 ppm would control the W126 exposure index so that the W126 would not exceed 17 ppm-h.

    In December 2020, the Administrator reaffirmed that attaining an 8-hour standard at 0.070 ppm would control for W126 exposures at 17 ppm-h and below. With the appointment of a new EPA administrator, the EPA informed in late October 2021 the U.S. Court of Appeals for the D.C. Circuit in a court filing that it was going to 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. In 2022 and 2023, the EPA and the EPA's CASAC ozone panel participated in the reassessment process. As a result of the reconsideration of the December 2020 ozone decision, CASAC recommended to the EPA Administrator on June 9, 2023 that the W126 index be considered 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 in August 2023 to restart the entire ozone rulemaking process and initiate a new review of the ozone NAAQS. This review process will take many years to complete.

    For more information about the W126 exposure index, please click here.


U.S. Environmental Protection Agency (2006a) Review of the National Ambient Air Quality Standards for Ozone: Policy Assessment of Scientific and Technical Information. OAQPS Staff paper. Research Triangle Park, NC: Office of Air Quality Planning and Standards, EPA-452/D-05-002. July.

U.S. Environmental Protection Agency (2006b) 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. February.

U.S. Environmental Protection Agency (2007) Review of the National Ambient Air Quality Standards for Ozone: Policy Assessment of Scientific and Technical Information OAQPS Staff Paper. Research Triangle Park, NC: Office of Air Quality and Planning and Standards, EPA-452/R-07-007. July.

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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