Ozone Concentrations

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There doesn't have to be much ozone to cause problems. The federal government standard is that concentrations of 0.075 parts per million of ozone or more are unhealthful, especially to people with respiratory conditions. The presence of ozone at these levels for long periods of time can cause serious lung irritation. As a result, cities with ozone problems often have programs that alert people if high ozone levels are expected.

The U.S. Environmental Protection Agency (EPA) has developed an Air Quality Index for reporting the levels of ozone and other common air pollutants. To make it easier for the public to quickly understand the air quality in their communities, EPA assigns a specific color to each category. For example, the color orange means that conditions are "unhealthful for sensitive groups," and red means that conditions are "unhealthful" for everyone.

Code Red
Unhealthful

Hazy, hot and humid
Stationary high pressure
Sunny skies
Temperatures 90°F and up

Code Yellow
Moderate

Light to moderate wind
Partly cloudy to sunny skies
High-pressure system
Temperatures in the 70s to mid 80s

Code Orange
Approaching Unhealthful

Sunny skies and light winds
Slow moving high pressure
Temperatures in the 80s to low 90s

Code Green
Good

Windy conditions
Partly sunny to cloudy skies or rain
Passing cold front
Temperatures in the upper 70s to mid 80s

Figure 8
EPA ground-level ozone forecast and action guide