The Winter Spare the Air Program began in 1991. The table below lists the number of days on which Winter Spare the Air Alerts were issued for the Bay Area each year since then, as well as the number of days on which particulate matter (PM) concentrations exceeded the state and federal health-based air quality standards.
PM 10 refers to particulate matter that is 10 microns in size or smaller (about 1/7 the size of a human hair), and PM 2.5 refers to particulates that are 2.5 microns or smaller (about 1/28 the size of a human hair). The PM 2.5 standard was introduced by the federal government in 2000, and then strenghtened in 2006, as research demonstrated the significant public health impact of these "fine particles." Particles between 10 microns and 2.5 microns in size are sometimes referred to as "coarse particles."
Winter Spare the Air Alerts are issued when violations of national health standards are predicted by the Air District during the Winter Spare the Air season, which generally extends from November through February.
Note: The chart below has been updated as of January 11, 2010 to reflect the Winter Spare the Air Alert for January 10, 2010.
By Season
| Nov.-Feb. 2009-2010 | 7 | 0 | 0 | 6 |
| Nov. - Feb. 2008 - 2009 | 11 | 0 | 0 | 13 |
| Nov. - Feb. 2007 - 2008 | 6 | 0 | 3 | 7 |
| Nov. - Feb. 2006 - 2007 | 30 | 0 | 6 | 20 |
By Year
| 2010 | 4 | 0 | 0 | 4 |
| 2009 | 9 | 0 | 0 | 8 |
| 2008 | 6 | 0 | 5 | 12 |
| 2007 | 23 | 0 | 4 | 14 |
| 2006 | 12 | 0 | 15 | 10 |
| 2005 | 0 | 0 | 6 | 0 |
| 2004 | 0 | 0 | 7 | 1 |
| 2003 | 0 | 0 | 6 | 0 |
| 2002 | 1 | 0 | 9 | 7 |
| 2001 | 0 | 0 | 12 | 5 |
| 2000 | 0 | 0 | 7 | 1 |
| 1999 | 0 | 0 | 12 | |
| 1998 | 0 | 0 | 5 | |
| 1997 | 0 | 0 | 4 | |
| 1996 | 0 | 0 | 3 | |
| 1995 | 0 | 0 | 7 | |
| 1994 | 3 | 0 | 9 | |
| 1993 | 0 | 0 | 10 | |
| 1992 | 1 | 0 | 18 | |
| 1991 | 11 | 2 | 18 | |
24-Hour PM Standards
The national 24-hour air quality standard for PM 10 was set at 150 micrograms per cubic meter of PM 10 in the ambient air, averaged over 24 hours.
The California 24-hour standard for PM 10 is set at 50 micrograms per cubic meter in the ambient air, averaged over 24 hours.
The national 24-hour standard for PM 2.5 is set at 35 micrograms per cubic meter of PM 2.5 in the ambient air, averaged over 24 hours. (Prior to 2006, the standard was set at 65 micrograms per cubic meter.)
The California 24-hour standard for PM 2.5 is currently in the process of being determined.
Annual Average PM Standards
Although they don't appear in the table above, there are also annual average standards for PM, listed below.
The national annual air quality standard for PM 10 was set at 50 micrograms per cubic meter of PM 10 in the ambient air, averaged over the entire calendar year.
The California annual air quality standard for PM 10 is set at 20 micrograms per cubic meter in the ambient air, averaged over the year.
The national annual air quality standard for PM 2.5 is set at 15 micrograms per cubic meter of PM 2.5 in the air, averaged over the entire calendar year.
The California annual air quality standard for PM 2.5 is set at 12 micrograms per cubic meter in the ambient air, averaged over the year.
Visit the Air District's web site for more information about the state and federal ambient air quality standards and the Bay Area's attainment status, and to see ten years of annual air quality summary information about Bay Area air quality.