She said Seattle has 10% more air pollution than most American cities of its size because of industry sources and the transportation corridor. Hammes also said fuel-powered vehicles, industry landscaping equipment and big fireworks shows are all large polluters.

A rating anywhere between 301 and 500 is considered “hazardous,” and air quality at that level will trigger health warnings. At that level and beyond, everyone should stay indoors and reduce activity levels. “Use the same information that is for the 'hazardous' category,” the E.P.A. advises.

For each pollutant an AQI value of 100 generally corresponds to an ambient air concentration that equals the level of the short-term national ambient air quality standard for protection of public health. AQI values at or below 100 are generally thought of as satisfactory.