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 used to predict possible vegetation effects is not statistically
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
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.
November 26, 2014, the EPA Administrator announced that she is
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 is 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 is also taking
comment on setting a W126 value in the range of 7-13 ppm-h, which
implies that she is 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 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. In December 2020,
the EPA Administrator again decided to use the 8-h ozone standard
to control for W126 exposures at 17 ppm-h and below.
For more information about
the W126 exposure index, please click