where: M and A are arbitrary
wi = weighting factor for
ci = concentration i (in
The arbitrary 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 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 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. EPA 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. 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
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).
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.
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.
U.S. Environmental Protection
Agency (1996) Air Quality Criteria for Ozone and Related Photochemical
Oxidants. Environmental Protection Agency, Office of Research
and Development, Research Triangle Park, NC. U.S. EPA report
U.S. Environmental Protection
Agency (2006a) Air Quality Criteria for Ozone and Related Photochemical
Oxidants. Research Triangle Park, NC: Office of Research and
Development; report no. EPA/600/R-05/004af.
U.S. Environmental Protection
Agency (2006b) 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.