US: Air Quality Standards | Transport Policy

Technical Standards

Carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone (O3), particulate pollution (PM10 and PM2.5), and sulfur dioxide (SO2) are known as “criteria pollutants.” The EPA sets both primary standards (designed to protect public health) and secondary standards (designed to protect public welfare including visibility, crops, buildings, etc.)

EPA’s latest scheduled 5-year review of the NAAQS is to be completed in 2013. The most recent updates are as follows:

• CO: last reviewed August 2011; no changes made to the limit value but monitoring requirements were revised
• Lead: limit revised in November 2008; monitoring requirements revised in December 2010
• NOx: primary standard strengthened Jan 22, 2010
• PM10: currently being reviewed; EPA has indicated they will propose the retention of the previous standard set in 1997
• PM2.5: Annual primary limit strengthened in 2013; 24-hour limit retained from 2006 levels
• Ozone: revised limit value (between 60 and 70 ppb) proposed in 2011 but implementation delayed at request of executive office; revision expected in 2013
• SO2: strengthened in 2010

Notes:

(1) Final rule signed October 15, 2008.  The 1978 lead standard (1.5 µg/m3 as a quarterly average) remains in effect until one year after an area is designated for the 2008 standard, except that in areas designated nonattainment for the 1978, the 1978 standard remains in effect until implementation plans to attain or maintain the 2008 standard are approved.

(2) The official level of the annual NO2 standard is 0.053 ppm, equal to 53 ppb, which is shown here for the purpose of clearer comparison to the 1-hour standard.

(3) Final rule signed March 12, 2008.  The 1997 ozone standard (0.08 ppm, annual fourth-highest daily maximum 8-hour concentration, averaged over 3 years) and related implementation rules remain in place.  In 1997, EPA revoked the 1-hour ozone standard (0.12 ppm, not to be exceeded more than once per year) in all areas, although some areas have continued obligations under that standard (“anti-backsliding”).  The 1-hour ozone standard is attained when the expected number of days per calendar year with maximum hourly average concentrations above 0.12 ppm is less than or equal to 1.

(4) Final rule signed June 2, 2010.  The 1971 annual and 24-hour SO2 standards were revoked in that same rulemaking.  However, these standards remain in effect until one year after an area is designated for the 2010 standard, except in areas designated nonattainment for the 1971 standards, where the 1971 standards remain in effect until implementation plans to attain or maintain the 2010 standard are approved.