Carbon Monoxide (CO) Standards -
Table of Historical CO NAAQS

History of the National Ambient Air Quality Standards for Carbon Monoxide
During the Period 1971-2011

Final Rule/Decision	Primary/ Secondary	Indicator	Averaging Time	Level ⁽²⁾	Form
1971 36 FR 8186 Apr 30, 1971	Primary and Secondary	CO ⁽¹⁾	1-hour period	35 ppm	Maximum, not to be exceeded more than once in a year
1971 36 FR 8186 Apr 30, 1971	Primary and Secondary	CO ⁽¹⁾	8-hour period	9 ppm	Maximum, not to be exceeded more than once in a year ⁽³⁾
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	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 of 9 ppm