where: M and A are arbitrary
positive constants
wi = weighting factor for
concentration ci
ci = concentration i (in
ppm)
The positive constants
M and A are 4403 and 126 ppm-1, respectively. Their values were
subjectively determined to develop a weighting function that
(1) focused on hourly average concentrations as low as 0.04
ppm, (2) had an inflection point near 0.065 ppm, and (3) had
an equal weighting of 1 for hourly average concentrations at
approximately 0.10 ppm and above.
The name for the W126 exposure
index was derived from the following:
"W" was associated
with the word "weighted" and;
The number "126"
was associated with the 126 value of the constant "A"
in the W126 equation (see above).
For more information on
the uses of the index, please visit our publications web page for specific citations to the peer-reviewed
literature. Reading the critical review paper by Musselman et
al. (2006), as well as the Lefohn et al. (2018) internationally
authored paper (Section 2), are good places to start. In addition,
the important publication by Heath et al. (2009) is an excellent
contribution that focuses on the biological rationale for weighting
the higher hourly average ozone concentrations more than the
mid- and lower-level concentrations for assessing vegetation
effects. The publication discusses the linkage of the temporal
variability of apoplastic ascorbate with the diurnal variability
of defense (detoxification) mechanisms in plants and compares
this variability with daily maximum ozone concentration and diurnal
uptake and entry of ozone into the plant through stomata. The
paper integrates the three processes (i.e., uptake, ozone exposure,
and defense) and provides evidence that supports the application
of nonlinearity in vegetation responses to ozone exposures and
dose. One of the keys to nonlinearity is the out-of-phase relationship
among the three processes (i.e., uptake, ozone exposure, and
defense). The W126 ozone index focuses on this nonlinearity for
assessing vegetation impacts. Again, the publications web page is a good place in which to start your reading
on the subject.
The use of the W126 to
characterize ozone exposure concentrations regarding potential
vegetation effects, particularly growth, has received strong
support from the EPA's Clean Air Scientific Advisory Committee
(CASAC) in previous reviews (Henderson, 2006; Samet, 2010; Frey,
2014; Cox, 2020). In 2006, the EPA's Ozone Staff Paper (EPA,
2006b) recommended that the W126 exposure index be considered
as a possible secondary ozone standard. Following EPA's recommendation,
in August 2006, EPA's CASAC recommended 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. EPA decided not
to adopt the W126 exposure index as the secondary ozone standard.
Although the EPA Administrator had originally 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 (i.e., vegetation) ozone standard the same as the primary
(human health) 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,
under a new administration, 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 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.
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 website 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, 2010, 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, 2011 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 reconsideration of the ozone standards.
The EPA soon after the President's decision initiated the full
5-year review cycle of the ozone NAAQS.
Following
the 2011 decision by the EPA, the Agency continued to support
the use of the W126 exposure metric in risk analyses associated
with ozone and vegetation effects. 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
were set in the range of 65 to 70 ppb. The Administrator believed
that the form of the health standard in this range would also
protect vegetation from ozone exposures of W126 values within
the range of 13-17 ppm-h. The Agency believed that there existed
a reasonable mathematical correlation between the 8-h human health
standard and the W126 exposure metric. EPA also took comment
on setting a W126 value in the range of 7-13 ppm-h, which implied
that she still considered establishing a secondary standard separate
in form from the human health 8-h standard. On October 1, 2015,
the Administrator established the 8-h standard at 0.070 ppm for
both the human health and welfare ozone standards. The 8-h standard
of 0.070 ppm was applied to prevent the W126 exposure index from
exceeding 17 ppm-h. Follwing 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. The current 70 ppb
8-h O3 standard as per the US EPA's 2015 decision serves as a
surrogate to achieve O3 levels at or below a W126 value of 17
ppm-hrs.
On August 23, 2019, the
D.C. Court of Appeals ruled on the case Murray Energy Corporation
v. EPA. The Court found that EPA's explanation for using the
human health form of the standard of the 3-year average of the
4th highest 8-hr value of 0.070 ppm as a substitute for the W126
index was insufficient. The Court remanded this issue for EPA
to either lower the 8-h vegetation standard to protect against
unusually damaging cumulative seasonal exposures that will be
obscured in its three-year average, or explain its conclusion
that the unadjusted average is an appropriate benchmark notwithstanding
CASAC’s contrary advice. Alternatively, the Court suggested
that EPA could adopt the single-year W126 exposure index as the
form and averaging time, which would presumably moot any problems
with the way it translated that index to use as a benchmark.
In addition the Court questioned why the EPA did not use vegetation
injury as the sensitive indicator versus growth loss. The EPA
Administrator attempted to respond to the Court's concerns in
its December 2020 ozone rulemaking decision. The entire August 23, 2019 decision
is available at https://bit.ly/2MzY2Hg.
The EPA told the U.S. Court
of Appeals for the D.C. Circuit in a court filing in late October
2021 that it would initiate a rulemaking process to reassess
by the end of 2023 the Agency's December 2020 decision to retain
the 2015 ozone human health and vegetation standards. As a result
of the reconsideration, CASAC recommended on June 9, 2023 the
W126 index as the secondary ozone standard and that the EPA Administrator
should consider that 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 reviewing the CASAC recommendations for the human health
and vegetation ozone standards, the EPA made the decision to
restart the entire ozone rulemaking process and to initiate a
new review of the ozone NAAQS. This review process will take
many years to complete.
Since 2006, on four separate
occasions (i.e., 2006, 2010, 2014, and 2023), the W126 exposure
index has been proposed as the ozone secondary standard by either
CASAC or EPA for the US. The exposure index has played an important
role over the past several years in assessing vegetation effects.
The Forest County Potawatomi Community (FCPC) adopted the W126
exposure index as most suitable for its Class I area. Based on
the review of the available scientific literature, the FCPC made
the decision to use the W126 exposure index (accumulated over
a 24-h period for a fixed 3-month period) as one of two indices
to protect vegetation. The FCPC determined that using the 24-hour
W126 index, rather than the 12-hour W126, as suggested by the
EPA, provided greater protection of FCPC’s vegetation resources
(https://lnr.fcpotawatomi.com/current-vegetation-w126-n100/). The Forest County Potawatomi Community
has coupled the W126 exposure index with the N100 metric (the
number of hourly average O3 concentrations greater than or equal
to 100 ppb) because of the relevance of both exposure metrics
for assessing vegetation effects. For additional information
about the N100 exposure metric, please click here.
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.
Our research, as well as those of others, is indicating that
some of the findings associated with our vegetation research
observations are applicable to the human health FEV1 lung function
responses associated with ozone exposure (please see Hazucha
and Lefohn, 2007; Lefohn et al., 2010; Lefohn et al., 2018).
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It's Not a Backroom Deal If the Call Is Made in the Oval Office
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