Over the past 15 years, regional 3-year annual average particulate matter concentrations have fallen from 12.3 micrograms (μg) per cubic meter to just 7.8 micrograms per cubic meter – a remarkable improvement in a relatively short time. Peak particulate matter levels also declined significantly over this time period, as indicated by the peak 24-hour average trend. The most recent downward tick in particulate matter levels has been attributable to emission reduction programs as well as favorable weather conditions. An extended period of light winds and no rain contributed to higher levels of particulate matter accumulating in the Bay Area air basin in 2015; the return of wet and windy winter weather conditions in 2016 provided a natural scrubbing that “washed” particulate matter from the air.
The Bay Area has made progress in reducing particulate emissions and has reported concentrations below national air quality standards in recent years. However, there is no known safe threshold for exposure to particulate matter. On days when fine particulate matter concentrations spike due to worst-case conditions, Bay Area residents face significant health risks.