Table 2.

Evolution of SO₂ primary National Ambient Air Quality Standards (modified).¹³⁷

YEAR	AVERAGING TIME	LEVEL
1971	24-hr	140 ppb
	Annual	30 ppb
1996	(Existing 1971 standards retained)
2010	1 -hr	75 ppb
	24-hr	(revoked)
	Annual	(revoked)

Notes: Averaging time is defined as the “time period established for specific national ambient air quality standards, which must be used when interpreting air quality data.” The 1971 standards were revoked in 2010 because they “would not provide additional public health protection given a 1-hour standard at 75 ppb.” The SO₂ regulatory standard is the 99th percentile of 1-hour daily maximum concentrations, averaged over 3 years.