Return to Home Page

Exposure Indices Used to Protect Vegetation

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 recommended 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 (Lefohn and Foley, 1992). 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 artificial 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). Similar to the NCLAN fumigation protocol, Lefohn et al. (1997) discussed the frequent occurrences of hourly average ozone concentrations greater than or equal to 0.10 ppm for some of the tree seedling experiments, whose results were used to generate exposure-response equations currently used in the EPA's vegetation risk assessments as a part of the ozone rulemaking activity.

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 changed his mind and 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 and continue with the normal state-of-science evaluation leading up to the recommendation of whether to make the ozone standard more stringent than the current level.

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 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 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 ppb. 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 0.070 ppm.

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. 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 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.


Lefohn, AS and Foley, JK (1992). NCLAN Results and their Application to the Standard-Setting Process: Protecting Vegetation from Surface Ozone Exposures, J Air Waste Manag Assoc 42(8): 1046-1052.

Lefohn, AS, Jackson, W, Shadwick, DS and Knudsen, HP (1997). Effect of surface ozone exposures on vegetation grown in the southern Appalachian Mountains: Identification of possible areas of concern. Atmospheric Environment 31(11): 1695-1708.

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.

Home Page | News | Corporation | Maps | Publications | Table of Contents | Multimedia Center

Copyright © 1995-2024 A.S.L. & Associates. All rights reserved.