There is a risk-based component to MACT standards that follows the initial technology-based MACT standard.
Under the 1990 CAAA, EPA was mandated to set technology-based standards (based on Maximum Achievable Control
Technology, MACT) for the source categories specified under 112(d). While these MACT standards typically
apply to major sources (those at facilities with greater than 10 tpy of a single HAP, or greater than 25 tpy
of aggregate HAPs), many MACTs also apply to area sources (sources with less than the 10/25 tpy HAP
thresholds); a few MACTs apply only to area sources. Section 112(f) specifies that EPA determine whether or
not to promulgate additional standards beyond the MACT within 8 years after promulgation of the MACT
standard (but within 9 years after promulgation of the 2-year MACT source categories).
Thus, EPA is required to evaluate the NESHAPs developed according to the MACT standards for any "residual
risk" with 8 years of promulgation. If the "residual risk" for a source category does not protect public
health with "an ample margin of safety", then EPA must promulgate health-based standards for that source
category to further reduce HAP emissions. EPA is required to set more stringent standards if necessary to
prevent adverse environmental effects (considering energy, costs, and other relevant factors).
The chart, "National Emissions Standards for Hazardous Air Pollutants (NESHAPs) Applicability
Determination", contains information which may be used as an aid in determining applicability with NESHAPs
(contained in 45CSR15 or 45CSR34) and associated regulations.
EPA has also began to establish air toxic standards based on generally achievable control technology (GACT)
for certain area (nonmajor) source categories.