where: M and A are arbitrary
wi = weighting factor for
ci = concentration i (in
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) is a good place to start. In addition, the important
publication by Heath et al. (2009) is an excellent contribution
that focuses on explaining why the higher hourly average ozone
concentrations (i.e., peaks) should be provided greater weight
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 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. 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.
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 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. EPA decided not
to adopt the W126 exposure index. Although the EPA Administrator
had originally 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 (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,
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 decreases 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
is also proposing to establish a distinct cumulative, seasonal
secondary standard, referred to as the W126 index,
which is 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 reconsideration of the ozone standards.
EPA continues to support the use of the W126 exposure metric
as an indication of vegetation effects caused by ozone. 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 a health standard in this range would protect vegetation
from ozone exposures of W126 values within the range of 13-17
ppm-h. EPA 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. On October 1, 2015, the Administrator established the
8-h standard at 0.070 ppm. The 8-h standard of 0.070 ppm is used
to prevent the W126 exposure index from exceeding 17 ppm-h.
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
3-year average of the 4th highest 8-hr vegetation standard of
0.070 ppm as a surrogate 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 CASACs 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 entire August 23, 2019 decision is available
The W126 exposure index
has been proposed as the ozone secondary standard by either CASAC
or EPA for the US since 2006 three separate times (2006, 2010,
and 2014). 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 selected to
use the W126 exposure index accumulated over a 24-h period for
a 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 FCPCs vegetation resources (https://lnr.fcpotawatomi.com/current-vegetation-w126-n100/). 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 (see Hazucha and Lefohn, 2007; Lefohn et al.,
2010; Lefohn et al., 2018).
Hazucha M. J. and Lefohn
A. S. (2007) Nonlinearity in human health response to ozone:
Experimental laboratory considerations. Atmospheric Environment.
Heath, R. L., Lefohn, A.
S., and Musselman R. C. (2009). Temporal processes that contribute
to nonlinearity in vegetation responses to ozone exposure and
dose. Atmospheric Environment. 43:2919-2928.
Lefohn A.S. and Runeckles
V.C. (1987) Establishing a standard to protect vegetation - ozone
exposure/dose considerations. Atmos. Environ. 21:561-568.
Lefohn A.S., Lawrence J.A.
and Kohut R.J. (1988) A comparison of indices that describe the
relationship between exposure to ozone and reduction in the yield
of agricultural crops. Atmospheric Environment. 22:1229-1240.
Lefohn, A.S., Hazucha,
M.J., Shadwick, D., Adams, W.C. (2010). An Alternative Form and
Level of the Human Health Ozone Standard. Inhalation Toxicology.
Inhalation Toxicology. 22:999-1011.
Lefohn, A.S., Malley, C.S.,
Smith, L., Wells, B., Hazucha, M., Simon, H., Naik, V., Mills,
G., Schultz, M.G., Paoletti, E., De Marco, A., Xu, X., Zhang,
L., Wang, T., Neufeld, H.S., Musselman, R.C., Tarasick, T., Brauer,
M., Feng, Z., Tang, T., Kobayashi, K., Sicard, P., Solberg, S.,
and Gerosa. G. 2018. Tropospheric ozone assessment report: global
ozone metrics for climate change, human health, and crop/ecosystem
research. Elem Sci Anth. 2018;6(1):28. DOI:
Musselman R.C., Lefohn
A.S., Massman W.J., and Heath, R.L. (2006) A critical review
and analysis of the use of exposure- and flux-based ozone indices
for predicting vegetation effects. Atmospheric Environment. 40:1869-1888.
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Oxidants. Environmental Protection Agency, Office of Research
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U.S. Environmental Protection
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Oxidants. Research Triangle Park, NC: Office of Research and
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It's Not a Backroom Deal If the Call Is Made in the Oval Office
by Cindy Skrzycki. Tuesday, April 8, 2008; Page D02.