A workshop was held in Raleigh, North Carolina on January
12-13, 1996, sponsored by the Emission and Effects Task Group
of the Southern Oxidant Study Program. The meeting was held in
Raleigh, NC to 1) assemble a group of ecological, agricultural
and forest scientists to discuss the strengths and uncertainties
of the current ecological knowledge base available for use in
setting a revised federal secondary ozone standard; 2) discuss
the value of a longer vs the current short-term secondary standard;
3) suggest possible standards (to include daily duration, seasonal
duration, form, and specific ozone concentration values [in relation
to a selected set of ecological end-points]) that would protect
vegetation from ozone injury or damage; and 4) discuss and suggest
research needed to remove some of the current uncertainties relative
to the available ecological data base for ozone. Sixteen vegetation
experts attended the Workshop to discuss the form and level of
a possible O3 secondary standard. Dr. Lefohn, President of A.S.L.
& Associates, was one of the vegetation experts invited and
he attended the meeting. The meeting represented the "best
guess" of the participants for the form and level of a possible
O3 secondary standard.
The participants reached the following consensus:
- There is a need for a secondary standard that was different
from any of the health-based primary standards presently being
considered by the U.S. EPA.
- There is a need to protect the vegetative components of
natural ecosystems and, at this time, foliar (leaf) injury is
the only endpoint that could be used in these systems.
- There is a need to protect some aspect of the integrity
of the forest ecosystem but the data are not sufficiently strong
to select an indicator of forest structure or function. Two indicators
that are pertinent to forest ecosystems, where data were sufficient
to be used, are
Growth effects on seedlings from tree species of natural
forest stands (a one to two percent per year reduction). (This
level was selected to avoid the cumulative effects of yearly
reductions of 2%.)
Growth effects on seedlings and saplings from tree plantations
(a one to two percent per year reduction).
- There is a need to protect against crop yield reductions
of 5% while at the same time acknowledging that the uncertainties
(in terms of confidence limits) were high at the 5% level. Thus
for the purposes of a secondary standard, given the uncertainty
of determining 5% crop loss, the participants felt the crop yield
end-point should be the 10% yield reduction level.
- The 12-h time period from 0800 to 1959 ST (i.e., basically
from 8am until 8pm standard time) is an appropriate time window
for a secondary standard.
- In terms of doing the best job of protecting ecological
systems, the running 90-day maximum is the time period of interest.
The following six forms for the secondary standard were
discussed at some length for the daily and seasonal time periods
agreed to above:
- A mean value (in ppm) - which would give the same results
as a cumulative index (in ppm-hrs) of the sum of 0.000 ppm (SUM00).
- The sum of all hourly average concentrations (in ppm-hrs)
once the ozone value reaches a given concentration or above -
four of these cumulative indices were discussed:
- Values of 0.025ppm or above (SUM025)
- Values of 0.040ppm or above (SUM04)
- Values of 0.050ppm or above (SUM05)
- Values of 0.060ppm or above (SUM06)
- A sigmoidally weighted index (W126 - also in ppm-hrs),
which provided increasing weight to values from 0 to 0.100 ppm
(with little weight to values below 0.040 ppm) and a unit weight
to all values of 0.100 ppm and above.
Consensus was reached on the SUM06 and W126 exposure indices.
The participants acknowledged that these two forms would give
very similar protection against ozone effects on vegetation at
most locations in the United States. The majority of the participants
preferred the W126 (by a 9 to 6 vote), but consensus could be
reached only on the SUM06. The participants felt that the selection
of the SUM06 should not be interpreted to mean acceptance of
a threshold of 0.060 ppm for effects on ecological systems, but
rather that concentrations above 0.060 ppm were well correlated
with plant response to ozone. There was some discussion about
the possibility of one or more exceedances equal to or greater
than 0.10 ppm to be combined with the form of the standard because
the NCLAN fumigation protocol in the experiments resulted in
numerous hourly average concentrations greater than or equal
to 0.10 ppm. The participants could not develop a consensus on
this issue and the recommendation was to forward the concern
to the EPA and let the Agency deal with the issue.
The participants recommended the following range of SUM06
values associated with each of the biological endpoints mentioned
- Foliar injury of the vegetation component of natural ecosystems
- 8 to 12 ppm-hrs
- Growth effects on tree seedlings from natural forest stands
(10 to 15 ppm-hrs) and from plantation tree seedlings and saplings
(12 to 16 ppm-hrs)
- Crop yield reductions of 10% - 15 to 20 ppm-hrs
The recommendations were forwarded to the U.S. EPA for
further consideration. In February 1996, the Agency made the
decision to recommend the SUM06 exposure index. Although the
Agency did not completely follow the recommendations of the workshop
participants, no doubt the recommendations that came forward
from the workshop affected the Agency's thinking concerning whether
to recommend a secondary standard different in form and level
from the primary, health-based standard. In 1997, the Agency
made the decision to not implement a separate secondary standard
different from the health-based primary standard. The Agency
is presently using the 8-hour ozone standard for both the primary
and secondary standard.
In 2006, the EPA's Ozone Staff Paper
(EPA, 2006a) 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 range of suggested
values for the W126 was mainly based on the recommendations that
were made at the Workshop that took place in Raleigh, North Carolina
in 1996. In June 2007, the EPA Administrator proposed a range
of 7 - 21 ppm-hours for a 3-month, 12-hour seasonal W126 exposure
index for public comment on the secondary ozone standard.
It is important to stress that the
recommendations were based on the judgment of the various participants
and very little scientific evidence was presented at the meeting
to support the participants "feelings" about specific
levels to protect vegetation. The results from the statistical
analyses performed by Dr. Henry Lee of the EPA using NCLAN crop
loss data were available from the 1996 EPA Criteria Document
and the 1996 EPA Staff Paper. However, consensus on protective
levels for foliar injury to natural ecosystems, growth effects
to tree seedlings in natural forest stands, and growth effects
to tree seedlings and saplings in plantations was reached based
on undocumented values presented by the Workshop participants.
Since the 1996 Workshop that recommended
a 12-hour cumulative period, evidence has continued to mount
that nocturnal uptake is important, as noted in the Ozone Criteria
Document (EPA, 2006b). On page AX9-186 of the Ozone CD, the authors
emphasize that the potential for nocturnal uptake should be addressed
by modifying the form of currently used exposure indices. Unfortunately,
this recommendation has not been seriously addressed in the final
version of the Ozone Staff Paper (EPA, 2007).
In June 2007, the EPA Administrator
proposed the W126 exposure index as a 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). 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).
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 were 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 proposed to
decrease 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 proposed 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 proposal for the revision of ozone standards.
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. She 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. 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. On October 1, 2015, the Administrator
set the ozone standard at 0.070 ppm for both the human human
and vegetation standards. The Administrator felt that attaining
an 8-hour standard at 0.070 ppm would control the W126 exposure
index so that the W126 would not exceed 17 ppm-h. In December
2020, the Administrator again believed that attaining an 8-hour
standard at 0.070 ppm would control for W126 exposures at 17
ppm-h and below. In late October 2021, the EPA informed the U.S.
Court of Appeals for the D.C. Circuit in a court filing that
it will 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. In 2022 and 2023,
the EPA and the EPA's CASAC ozone panel participated in the reassessment
process. 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.
For more information about the W126
exposure index, please click here.
U.S. Environmental Protection Agency
(2006a) Review of the National Ambient Air Quality Standards
for Ozone: Policy Assessment of Scientific and Technical Information.
OAQPS Staff paper. Research Triangle Park, NC: Office of Air
Quality Planning and Standards, EPA-452/D-05-002. July.
U.S. Environmental Protection Agency
(2006b) 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. February.
U.S. Environmental Protection Agency
(2007) Review of the National Ambient Air Quality Standards for
Ozone: Policy Assessment of Scientific and Technical Information
OAQPS Staff Paper. Research Triangle Park, NC: Office of Air
Quality and Planning and Standards, EPA-452/R-07-007. July.
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.