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An Important Issue: Relating Vegetation Experiments
with Real-World Exposures

 

It is important to relate experimental conditions with those experienced under real-world conditions. The exposure regimes experienced under actual ambient conditions did not match those that elicited growth reduction in the controlled vegetation experiments on which the 1996 proposed secondary SUM06 standard was based. The data on which the 1996 proposed secondary standard was based were derived from experiments associated with the National Crop Loss Assessment Network (NCLAN). It was pointed out in the peer-review literature, and acknowledged in the EPA Criteria Document (U.S. EPA, 1996; U.S. EPA, 2006), Staff Paper, December 13, 1996, and July 18, 1997 Federal Notices that these experiments contained numerous hourly average concentrations greater than or equal to 0.10 ppm near the 10% yield reduction level, the level at which the standard was proposed. Many of the locations across the United States that would violate the 1996 proposed vegetation standard did not experience numerous occurrences of hourly average concentrations greater than or equal to 0.10 ppm. This observation has been carefully integrated into the vegetation effects projections made by A.S.L. & Associates for the Southern Appalachian Mountain area. The Phase I report describing the projections is available from the Southern Appalachian Mountain Initiative (SAMI) program. Please call A.S.L. & Associates for details on how to receive a copy of the report.

In 2006, the EPA's Ozone Staff Paper (EPA, 2006) 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. 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 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 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 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, 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 proposed revisions of the 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 will take comment on a standard as low as 60 ppb. For the welfare (secondary) ozone standard, she was proposing that the standard be the same as the health standard if the final health standard is set in the range of 65 to 70 ppb. The rationale for the EPA proposal can be found at the EPA website. The Administrator believes 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 implies 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. In October 2015, the Administrator made the decision to set both the human health and vegetation ozone standards at 70 ppb. The W126 exposure index is the exposure metric used to assess vegetation effects. The W126 exposure of 17 ppm-h it was decided by the Administrator could be controlled through attainment of the 8-h standard of 70 ppb.

References

U.S. Environmental Protection Agency (1996) Air quality criteria for ozone and related photochemical oxidants. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC. U.S. EPA report no. EPA/600/P-93/004bF.

U.S. Environmental Protection Agency (2006) 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.

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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