The Act was amended in 1977 primarily to set new goals (dates) for achieving attainment of NAAQS since many areas of the country had failed to meet the deadlines.

The 1990 amendments to the Clean Air Act in large part were intended to meet unaddressed or insufficiently addressed problems such as

• acid rain,
• ground-level ozone,
• stratospheric ozone depletion, and
• air toxics.