The Clean Air Act requires the EPA to regulate toxic air pollutants, also known as air toxics, from large industrial facilities in two phases.

Phase 1

The first phase is “technology‑based,” where EPA develops standards for controlling the emissions of air toxics from sources in an industry group (or “source category”). These maximum achievable control technology (MACT) standards are based on emissions levels that are already being achieved by the controlled and low‑emitting sources in an industry.

Phase 2

The second phase is a “risk-based” approach called residual risk.  Here, EPA must determine whether more health-protective standards are necessary.  Within 8 years of setting the MACT standards, the Clean Air Act requires EPA to assess the remaining health risks from each source category to determine whether the MACT standards protect public health with an ample margin of safety, and protect against adverse environmental effects.

Every 8 years after setting the MACT standards, the Clean Air Act also requires EPA to review and revise the standards, if necessary, to account for improvements in air pollution controls and/or prevention.  The first eight year review, when combined with the residual risk review, is called the risk and technology review - RTR.