The Clean Air Campaign issues Smog Alerts on days when the concentrations of either ozone or particle pollution -- or both -- are expected to reach unhealthy levels. Sign up to receive Smog Alerts in your inbox.

Every day, a team of forecasters reviews weather patterns and other factors to report the forecast of the next day's ozone and particle pollution levels. It is this forecast that determines if a Smog Alert will be issued.

The Air Quality Index (AQI) has five color-coded levels: the Green, or "good," category (AQI 0 to 50) is the best, followed by the Yellow, or "moderate" (AQI 51 to 100).

Smog Alerts start at Code Orange, or "unhealthy for sensitive groups" (AQI 101-150), and are also issued at Code Red, or "unhealthy" (AQI 151 to 200) and Code Purple, or "very unhealthy" (AQI 201 to 300). The hazards of poor air quality increase steadily as pollution levels increase. Each pollutant -- ozone or particle pollution -- carries its own associated health risks and effects.


Air Quality	Air Quality Index	Health Advisory
Good	0 - 50	Air quality is good. Enjoy activities.
Moderate	51 - 100	People who are unusually sensitive to air pollution should consider reducing prolonged or heavy exertion.
Unhealthy for Sensitive Groups	101 - 150	People with heart or lung disease (including asthma), older adults and children should cut back or reschedule strenuous activities.
Unhealthy	151 - 200	Everyone, especially people with heart or lung disease (including asthma), older adults and children should cut back or reschedule strenuous activities. Sensitive groups should avoid strenuous activities.
Very Unhealthy	201 - 300	Everyone, especially people with heart or lung disease (including asthma), older adults and children should significantly cut back on physical activities. Sensitive groups should avoid all physical activities.

To find out how many Smog Alerts have been issued this year, click here. To find out how many days the region has exceeded air quality standards this year or in past years, click here .