Ground-level Ozone

Basic Information

Ozone is found in two regions of the Earth's atmosphere – at ground level and in the upper regions of the atmosphere.  Both types of ozone have the same chemical composition (O3). While upper atmospheric ozone protects the earth from the sun's harmful rays, ground level ozone is the main component of smog. 
Troposheric, or ground level ozone, is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC).  Ozone is likely to reach unhealthy levels on hot sunny days in urban environments. Ozone can also be transported long distances by wind.  For this reason, even rural areas can experience high ozone levels. 
High ozone concentrations have also been observed in cold months, where a few high elevation areas in the Western U.S. with high levels of local VOC and NOx emissions have formed ozone when snow is on the ground and temperatures are near or below freezing. Ozone contributes to what we typically experience as "smog" or haze, which still occurs most frequently in the summertime, but can occur throughout the year in some southern and mountain regions.
Ground level ozone- what we breathe- can harm our health. Even relatively low levels of ozone can cause health effects.  People with lung disease, children, older adults, and people who are active outdoors may be particularly sensitive to ozone. 
Children are at greatest risk from exposure to ozone because their lungs are still developing and they are more likely to be active outdoors when ozone levels are high, which increases their exposure.  Children are also more likely than adults to have asthma.
Ozone also affects sensitive vegetation and ecosystems, including forests, parks, wildlife refuges and wilderness areas.  In particular, ozone harms sensitive vegetation, including trees and plants during the growing season. 
Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC.

National Ambient Air Quality Standards (NAAQS) 

The Clean Air Act, which was last amended in 1990, requires EPA to set National Ambient Air Quality Standards (40 CFR part 50) for pollutants considered harmful to public health and the environment. The Clean Air Act identifies two types of national ambient air quality standards. Primary standards provide public health protection, including protecting the health of "sensitive" populations such as asthmatics, children, and the elderly. Secondary standards provide public welfare protection, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings.
EPA has set National Ambient Air Quality Standards for six principal pollutants, which are called "criteria" pollutants. They are listed below. Units of measure for the standards are parts per million (ppm) by volume, parts per billion (ppb) by volume, and micrograms per cubic meter of air (µg/m³).

Pollutant [final rule cite] Primary/ Secondary Averaging Time Level Form Carbon Monoxide [76 FR 54294, Aug 31, 2011] primary 8-hour 9 ppm Not to be exceeded more than once per year 1-hour 35 ppm Lead [73 FR 66964, Nov 12, 2008] primary and secondary Rolling 3 month average 0.15 μg/m3 (1) Not to be exceeded Nitrogen Dioxide [75 FR 6474, Feb 9, 2010] [61 FR 52852, Oct 8, 1996] primary 1-hour 100 ppb 98th percentile, averaged over 3 years primary and secondary Annual 53 ppb (2) Annual Mean Ozone [73 FR 16436, Mar 27, 2008] primary and secondary 8-hour 0.075 ppm (3) Annual fourth-highest daily maximum 8-hr concentration, averaged over 3 years Particle Pollution [71 FR 61144, Oct 17, 2006] PM2.5 primary and secondary Annual 15 μg/m3 annual mean, averaged over 3 years 24-hour 35 μg/m3 98th percentile, averaged over 3 years PM10 primary and secondary 24-hour 150 μg/m3 Not to be exceeded more than once per year on average over 3 years Sulfur Dioxide [75 FR 35520, Jun 22, 2010] [38 FR 25678, Sept 14, 1973] primary 1-hour 75 ppb (4) 99th percentile of 1-hour daily maximum concentrations, averaged over 3 years secondary 3-hour 0.5 ppm Not to be exceeded more than once per year

as of October 2011

(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.