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Improving air quality has been a major public policy priority in the Bay Area for many decades. Even with significant population and economic growth, air pollution concentrations in the Bay Area have declined steadily over the past 50 years.
A 2011 report by the Air District summarized emissions levels and trends for the Bay Area between 1990 and 2011. It found that the Bay Area’s economy grew by 77 percent, the population increased by 23 percent, and the total number of miles driven increased by 30 percent. However, during that same period, emissions for the criteria pollutants measured by the Air District, including ozone and particulate matter, decreased by 60 percent. Find out more about the Bay Area’s air quality history and progress.
Though overall emissions have decreased, the number of Spare the Air Alerts have remained fairly steady, as air quality standards have become more stringent during the same period.
Among other actions taken to meet these strengthened standards and better protect public health, the Air District passed the Wood Burning Rule in 2008, making it illegal to burn wood, indoors or outdoors, on days when a Spare the Air Alert is in effect for fine particulate pollution. Wood smoke is the primary source of fine particulate matter, the leading wintertime air pollutant, and for many years Spare the Air wood burning bans were restricted to the months between November and February. In 2019, however, to address the hazards of wildfire smoke, the Air District amended the Wood Burning Rule to ban wood burning on any day throughout the year when fine particulate pollution reaches unhealthy levels.
Peak ozone concentrations in the Bay Area are well below those first recorded when air quality monitoring began a half-century ago. Today, average ozone levels are at least one-third lower than levels in the 1970s. Bay Area ozone levels are generally lower than most other major U.S. metropolitan areas.
These improvements are in part thanks to California’s strict motor vehicle emission standards and the development of cleaner cars and trucks that have helped lower emissions, despite the Bay Area’s growth in population, vehicles, and miles traveled.
Still, ozone concentrations vary from year to year depending on weather conditions. When weather is unfavorable, with higher temperatures and lower winds, concentration levels rise in the region’s most affected areas and can exceed the stringent federal standards for ozone.
Over the past 15 years, the Bay Area’s three-year annual average particulate matter concentrations have fallen from 12.3 micrograms per cubic meter to just 7.8 micrograms per cubic meter — remarkable improvement over a relatively short period of time. In comparison to other major U.S. metropolitan areas, the Bay Area falls in the middle of the pack for particulate matter concentrations.
Particulate matter poses a significant health threat and is a serious environmental issue for the Bay Area. The miniscule size of fine particles allows them to pass deep into the lungs where they can enter the bloodstream and trigger serious conditions like asthma, strokes, and heart attacks.
These fine particles are emitted from a wide range of activities and sources, including industrial activities, wildfires, and motor vehicles. However, residential wood burning remains by far the highest source of fine particulate matter emissions during winter.