As part of their efforts to protect human health and the environment, governments assume the responsibility for setting pollution standards. Authorities consider the effects of ozone exposure on children and other people with sensitive respiratory systems, and also decide on acceptable concentrations of ozone (and other toxic chemicals) on the basis of acceptable risk. A given risk is acceptable if severe ill effects remain at a reasonable level. For example an acceptable risk might be one chance in 500,000 of one person contracting lung cancer from breathing polluted air of a specific composition for one year. However determining a specific health effect of any given atmospheric condition on any particular population is impossible. Individual people are too different physiologically from one another, environmental conditions vary too widely, and the atmosphere is too dynamic and complex. Factors such as high and low temperature, humidity, altitude, concentrations of other pollutants, and nutritional status can influence peoples’ health outcomes due to overexposure to air pollution.

Since 1971, the EPA has established national air quality standards for ozone. Set in 1997, the current national air quality standard for ozone is 0.08 parts per million (ppm), or 80 parts per billion (ppb), averaged over 8 hours. For a given geographic area to be in compliance, its fourth highest 8-hour concentration in a year, averaged over three years, must be equal to or less than that amount. However implementation of the 8-hour standard is taking years to get underway because of legal challenges by industry leaders who feel that compliance will prove too expensive. In February 2001, the U.S. Supreme Court unanimously upheld the principle that national air quality standards must be based on health considerations alone, and need not take into account the cost to businesses of meeting the standards. That major question is resolved, but minor legal issues remain unresolved. In the meantime, a less healthful standard of 0.12 ppm (120 ppb) for one hour, with an average of exceeding that standard only once over three consecutive years, still guides state governments in the U.S.

The World Health Organization (WHO) recommends stricter guidelines than the EPA standards, 60 ppb over 8 hours not to be exceeded more than 20 days in one year. Such guidelines do not carry the legal “teeth” that national standards do, but many countries choose to follow them or even more stringent ones when setting standards. In Canada, the Air Quality Standard for ozone is 50 ppb over 8 hours.

In addition to establishing national standards for ozone pollution, governments enact clean air legislation to control the emissions of ozone’s precursor chemicals. In the absence of government controls, the United States experienced an increase of 690 percent NO_x emissions and 260 percent volatile organic compound emissions between 1900 and 1970. The Clean Air Act became law in 1970, with amendments in 1977 and 1990, and has subsequently helped improve air quality. Of the six principal air pollutants—carbon monoxide, lead, nitrogen oxides, particulate matter, sulfur dioxide, and volatile organic compounds—all but nitrogen oxides have decreased significantly.

In 1999, the EPA revised the format of its Air Quality Index (AQI) to make it useful to the public. This index depicts ozone levels in five categories of increasing health concern, each with an associated color. If the air is Code Orange, Red, or Purple, an alert is in effect, and the public can take action to avoid overexposure to ozone. Federal, state, and local agency, newspapers, broadcast news, and Internet publications use the index when reporting on air quality information.

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Watching Our Ozone Weather
• Introduction
• Ozone in the Troposphere
• Ozone Pollution Standards
• Monitoring Ozone
• Solving the Problem
• Links and References