On July 19,
1997 EPA made its final decision on the promulgation of new National
Ambient Air Quality Standards (NAAQS) for ozone (O3) and particulate
matter. As a result of an accelerated review schedule caused
by an American Lung Association lawsuit, in December 1996, EPA
formally announced revisions to ozone and particulate matter
standards. New forms of the primary (human health) and secondary
(vegetation) standards for O3 were proposed.
During its
review, the EPA found that several of the key experiments, published
in the late 1980s and early 1990s, showed that individuals exposed
to extended daily periods of O3 at levels below the current 1-hour
standard exhibited health effects. The EPA believed it was necessary
to change the form of the primary standard because additional
protection, not offered by the existing 1-hour standard, was
needed for children and other at-risk populations. Thus, the
EPA proposed that the form of the primary standard be changed
from the current 1-hour daily maximum to a daily maximum 8-hour
average concentration at a level of 0.08 ppm. The EPA proposed
taking the third highest 8-hour daily maximum concentration for
each of three years and then taking the average of these three
concentrations.
Because O3
is phytotoxic to plant species and can produce acute foliar injury
responses, reduced crop yield and biomass production, and shifts
in competitive advantages of vegetation species in mixed populations,
the EPA wanted to establish a secondary standard to protect vegetation.
The EPA believed that the previous 1-hour O3 standard was not
sufficiently protective of crops and forests because O3 effects
are cumulative and not necessarily related to the one-time maximum
peak, on which the current 1-hour standard focuses. Two alternative
forms of a secondary standard were proposed. One would be based
on a level identical to the primary standard; the other on a
new seasonal accumulating-type standard. In the end, EPA adapted
only one standard: the 3-year average of the 4th highest 8-hour
daily maximum ozone concentration.
Although the
EPA devoted considerable time and effort to reviewing and summarizing
the relevant science concerning human health effects and vegetation
in the peer-reviewed literature, there are still areas of uncertainty
associated with the data that form the scientific basis of the
recommendations for both standards. These uncertainties have
ramifications for human health and vegetation, and may influence
whether geographic areas reach attainment.
The paper
published in the June 1997 issue of ES&T addresses the following:
- How closely
did the controlled human health experiments performed in the
laboratory mimic exposures experienced in the real world?
- Can the human
health standard be attained?
- What is the
realistic range of natural background O3 concentrations that
occur under ambient conditions?
- Did the EPA
overestimate its human health risk assessment by using too low
a value for natural background?
- How closely
did the controlled vegetation experiments mimic exposures experienced
in the real world?
- Is the form
of the proposed secondary standard adequate?
- Is there
an alternative form of the secondary standard that would be more
appropriate?
- In which
directions should future human health and vegetation research
be focused?
While scientific
research has played an important role in the discussions concerning
both the primary and secondary standards, uncertainty does exist.
At the very least, the ramifications of these uncertainties need
to be both understood and considered when ultimately defining
and selecting new standards. There has been considerable debate
in the public arena, as well as within the Executive Branch and
Congress, concerning the uncertainty associated with the scientific
database and the attainability of the proposed standards. The
experiments that form the basis for the primary and secondary
O3 standards are not perfect. Only time will tell how adequately
science was able to address the public's desire for clean air.
