The W126 index is a cumulative
exposure index that is biologically based. The W126 index focuses
on the higher hourly average concentrations, while retaining
the mid- and lower-level values.
In 1985, A.S. Lefohn proposed
the use of the W126 ozone exposure index for predicting vegetation
effects. The cumulative W126 exposure index uses a sigmoidally
weighted function (i.e., "S" shaped curve) as described
by Lefohn and Runeckles (1987) and Lefohn et al. (1988).
The W126 index is a cumulative exposure index and not
an "average" value. It is a biologically based index,
which is supported by research results (i.e., under both experimental
and ambient conditions) that show that the higher hourly average
ozone concentrations should be weighted greater than the mid-
and lower-level values. The W126 index is accumulated over a
specified time period.
The W126 cumulative exposure
was described and used in the EPA's Ozone Criteria Documents
(EPA, 1996, 2006a) for both characterizing ozone trends and relating
vegetation yield reduction losses with ozone exposure. To determine
the W126 index, the sigmoidal
weighting value at
a specific concentration is multiplied by the concentration and
then summed over all concentrations. The sigmoidal weighting
function is of the form:
where: M and A are arbitrary
positive constants
wi = weighting factor for
concentration ci
ci = concentration i (in
ppm)
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 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
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).
References
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.
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
no. EPA/600/P-93/004bF.
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.
