The National Weather Service issued an air quality alert Wednesday for the Chicago area and northwest Indiana.

The Illinois Environmental Protection Agency declared an air pollution action day for the greater Chicago metro area, the weather service said. An action day is declared when ozone and/or particulate levels are at, or above, levels that are unhealthy for sensitive groups, such as children and adults with asthma.

Active children and adults with asthma or other respiratory diseases should limit prolonged outdoor activity, the weather service advised.

Illinois counties covered by the alert include Cook, DuPage, Kane, Kendall, Grundy, Will, McHenry and Lake, the weather service said. Lake and Porter counties in Indiana are also under the alert.