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.
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 proposed
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.
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. Five years later, following a review of the
2015 ozone standards, the Administrator on December 23, 2020
made the decision that both the human
health and vegetation ozone standards would remain at the current
levels established in 2015. 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 that the level of
the W126 metric be in the range of 7 to 9 ppm-hrs. 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.
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.
