must promulgate regulations establishing emission standards
for about 175 categories of HAP sources. The standards require
the maximum degree of emission reduction that USEPA determines
to be achievable for each particular source category. The
standards are established for sources of HAPs that emit
10 tons per year of a single HAP or 25 tons per year of
combined HAPs. Different criteria for maximum achievable
control technology (MACT) apply for new and existing sources.
At the Administrators discretion, less stringent standards,
known as generally available control technology (GACT) will
be promulgated. For more information on MACT, see USEPA's
MACT web page.