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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
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

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