Timeline of Carbon Monoxide (CO) National Ambient Air Quality Standards (NAAQS)

History of the National Ambient Air Quality Standards for CO
Final Rule/Decision	Primary/ Secondary	Indicator ⁽¹⁾	Averaging Time	Level ⁽²⁾	Form
1971 36 FR 8186 Apr 30, 1971	Primary and Secondary	CO	1-Hour period	35 ppm	Maximum, not to be exceeded more than once in a year ⁽³⁾
1971 36 FR 8186 Apr 30, 1971	Primary and Secondary	CO	8-hour period	9 ppm	Maximum, not to be exceeded more than once in a year
1985 50 FR 37484 Sept 13, 1985	Primary standards retained, without revision; secondary standards revoked.
1994 59 FR 38906 Aug 1, 1994	Primary standards retained, without revision.
2011 76 FR 54294 Aug 31, 2011 (pdf) (2.3 MB)	Primary standards retained, without revision.

⁽¹⁾CO=Carbon Monoxide

⁽²⁾ Units of measure are in parts per million (ppm).

⁽³⁾ Second highest, non-overlapping 8-hour average concentration.

Carbon Monoxide (CO) Pollution in Outdoor Air