The U.S. EPA originally proposed a separate ozone standard (i.e., SUM06) to protect vegetation. In July 1997, the Agency decided not to adopt such a standard. The EPA believed that the 8-hour ozone standard adopted to protect human health would be adequate to protect vegetation. The important question to answer is whether achieving the 8-hour standard will protect adequately crops and trees. Using 1993-1995 data, we estimated that 55 out of 186 areas (approximately 30%) would have violated the SUM06 vegetation standard but not violate the 8-hour ozone standard. A map describing the violating areas for the SUM06 exposure index can be viewed. Thus, approximately 70% of the areas that would violate the SUM06 proposed vegetation standard would have violated the 8-hour standard. Further analysis is showing that the relationship between the form of the 8-hour standard and biologically relevant exposure indices (e.g., W126) used to predict possible vegetation effects is not statistically strong.

Do we need a separate standard to protect vegetation? Is the 8-hour ozone standard protective enough for vegetation? Our research is continuing in this important area. Several federal agencies are using the W126 exposure index on a national level to protect vegetation from injury and damage. In 2000, the Federal Land Managers Air Quality Related Values Workgroup (FLAG) adopted the use of the W126 exposure index. Many of the federal air quality experts in the U.S. Forest Service are applying the index for assessing the possible deleterious effects associated with ozone exposures. In June 2007, the EPA Administrator proposed the W126 exposure index as the 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).

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 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, 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 two revised 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 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 is also taking comment on setting a W126 value in the range of 7-13 ppm-h, which implied that she was still considering at the time 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 set the human health and vegetation standards at an 8-h level of 0.070 ppm. The 8-h standard is used to control W126 values at 17 ppm-h and below.

Five years later, in December 2020, the EPA Administrator again decided to use the 8-h standard as a surrogate for the W126 exposure metric. Following this decision, on October 29, 2021, the Agency announced it would reconsider the 2020 O3 NAAQS final action. During the reconsideration process, CASAC recommended to the Administrator that the form of the secondary standard should be changed to the cumulative W126 exposure metric, an index recommended by several previous CASAC ozone panels, as well as at times by the EPA, to protect vegetation. CASAC recommended that the Administrator consider 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 considering the CASAC recommendations for the human health and vegetation ozone standards as part of the reconsideration process, in August 2023 the EPA decided to initiate a new review of the ozone NAAQS, which meant that the entire ozone rulemaking process would begin once again and last for several years. The current 70 ppb 8-h O3 standard promulgated in the US EPA's 2015 decision (Federal Register, 2015) serves as a surrogate to achieve O3 levels at or below a W126 value of 17 ppm-hrs, which is above the range of W126 values of 7 to 9 ppm-hrs recommended by CASAC.

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