Wikipedia:Smog Watch

Smog Watch, also known as the Anti-Idling Sign, is a voluntary government protocol to prevent the Seattle, Washington, region from exceeding the federal health standards in relation to ozone. It aims at protection of public health during summers, where ozone levels are often found to be the highest. At certain times during the summer, agency meteorologists will issue a smog watch for 48 hours when there are elevated smog levels. During this 48-hour period, citizens of the Seattle district are encouraged to lower smog levels by taking steps to decrease pollution, including by: driving less, refueling when it is cool, refraining from using lawn mowers, partaking in pollution-free recreation, turning off car engines while idling, and volunteering for environmental sustainability programs.