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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	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.

Learn more about setting and reviewing standards to control CO pollution.

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

Carbon Monoxide (CO) Pollution in Outdoor Air