What is Smog?

Smog is air pollution. Vehicle emissions react in sunlight to form "secondary pollutants" that hazy smog you see. Usually, smog forms on hot summer days.

Roll over the images to see the Faisal Mosque in Islamabad, Pakistan and the city of Beijing in China on a smoggy day:

What are Spare the Air Days?

They are days when the air quality is really poor. There are usually clear skies, hot temperatures, calm winds, and an atmospheric inversion (imagine a lid in the air). Pollutants are trapped at ground level, where we walk around and breath. These pollutants are primarily ground-level ozone and secondary aerosols. They impact human health (especially for people with respiratory problems) and impair visibility by creating haze and smog.

Why should I care? What should I do?

On Spare the Air Days you are asked to modify your behavior to help minimize pollution. Cut back on driving; instead use public transportation, bike, or walk. If you are sensitive to unhealthy air you should limit your time outdoors, especially in the afternoon hours when pollution is highest.

What is Spare the Air Winter?

Spare the Air Winter is a program initiated by the Bay Area Air Quality Management District (BAAQMD). During winter, a significant source of pollution is particulate matter, an air pollutant emitted from burning wood in fireplaces.

During still, cold winter nights with clear skies, the air closer to the ground can be cooler, thus heavier than the air above. The difference in temperature creates a physical boundary known as the stable boundary layer.

When a stable boundary layer exists, smoke from wood burning is not permitted to mix into the atmosphere and remains close to the ground. This results in increased concentrations of particulate matter.

How will I know it's a Spare the Air Day?

Visit www.sparetheair.org to sign up for an Air Alerts e-mail notification, which are issued the day before each Spare the Air Day. Many television and radio stations also carry the advisories, as well as several Bay Area newspapers. You can check the daily forecasts on the home page of the Spare the Air website, learn more about the or call the Air District's toll-free line at 1-800-HELP AIR. Also, you can learn about the Air Quality Index that Spare the Air uses by clicking here.

View the Five Day Forecast!

Video: Summer Spare the Air Days

Video: Winter Spare the Air Days

Additional Links

Spare the Air Program: http://sparetheair.org

Spare the Air Winter Website: http://sparetheair.org/Make-a-Difference/Spare-the-Air-Every-Day/Winter.aspx

Wood Burning Regulation: http://sparetheair.org/Stay-Informed/Particulate-Matter/Wood-Smoke/Regulation.aspx

Health Effects: http://sparetheair.org/Stay-Informed/Air-Quality-and-Your-Health/PM-Health-Effects.aspx