The Policy Relevant Background (PRB) concentration
or range of concentrations represent what EPA believes would
be experienced if the United States and other countries in North
America were to initiate a zero anthropogenic emissions strategy,
which includes eliminating emissions associated with fertilizer.
The PRB concentrations define the level below which O3 standards
cannot be practicably set. In the 1996 ozone review, the EPA
used 0.04 ppm in its health risk assessment evaluations as the
level it expects as background for an 8-hr daily maximum concentration
for clean sites. In the most current review of the ozone standard,
the EPA is using a model with 2 degree by 2.5 degree spatial
resolution (i.e., great uncertainty) to define ranges of concentrations
for policy relevant background that are much lower than the 0.04
ppm level. At a monitoring site at Trinidad Head, California,
which EPA acknowledges is a Policy Relevant Background site,
numerous occurrences of hourly average concentrations greater
than or equal to 0.05 ppm are measured. In a study, A.S.L.
& Associates characterized the daily maximum 8-hr ozone
concentrations for 3 clean sites in North America. A summary
figure shows that Custer National Forest in Montana, Theodore
Roosevelt National Park in North Dakota, and Yellowstone National
Park in Wyoming all experienced numerous occurrences of 8-hr
daily maximum concentrations greater than or equal to 0.04 ppm
during each of the years monitored. The U.S. EPA, by selecting
0.04 ppm or lower as its Policy Relevant Background level, will
overstate human health risk by a large amount.
