In July 1997, the U.S. EPA decided that the 8-hour primary ozone standard would protect vegetation. Based on sound scientific research, the EPA earlier had discussed the possibility that a new standard, specifically designed to protect crops and trees, might be appropriate. Two exposure indices, the SUM06 and W126, were discussed in detail. The SUM06 cumulative index, using a threshold value, added all of the hourly average concentrations greater than or equal to 0.06 ppm. The W126 exposure index, while focusing on all hourly average concentrations, provided greater weight to the higher hourly average concentrations than the mid- and lower- range values. In January 1996, a group of key vegetation effects scientists met in North Carolina at a workshop to address the form and level of a possible secondary standard. At that meeting, 9 of the 14 experts endorsed the W126 exposure index as the index of choice. The scientific background for the exposure index can be found in the EPA's release of the 1996 Ozone Staff Paper. A.S.L. & Associates' reviewed the document and identified key points. In October 1999, the U.S. Department of Interior and the U.S. Department of Agriculture proposed using the W126 exposure index accumulated over a 24-hour period, combined with the number of hourly average concentrations equal to and greater than 0.10 ppm (i.e., N100), to protect vegetation from injury and damage.

The N100 index was included with the W126 exposure index because the exposure-response equations used to determine the level of the W126 were based on the National Crop Loss Assessment Network (NCLAN) experimental protocol. This protocol included the fumigation of vegetation with frequent numbers of hourly average concentrations greater than or equal to 0.10 ppm. The N100 exposure metric takes into consideration the large number of hourly average concentrations used in the NCLAN fumigation protocol. The use of the N100 exposure index is not a minor adjustment but a major adjustment required for use with the W126 exposure index if NCLAN-type fumigation protocols are used to define the protective number for the W126 index. If the N100 exposure index is not used in combination with the W126 exposure index, the W126 will provide inconsistent predictions for vegetation injury (e.g., spots on plants) and damage (e.g., growth loss).

In 2007, the EPA's Ozone Staff Paper (EPA, 2007) recommended that the W126 exposure index be considered as a possible secondary ozone standard. In March 2007, EPA's Clean Air Scientific Advisory Committee (CASAC) recommended that the W126 be adopted as a standard to protect vegetation from ozone exposure. 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). The previous standard, set in 1997, was 0.08 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). For more information about the W126 exposure index, please click here.

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References

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-07-003. January.

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.