Over the past 16 years, regional annual average particulate matter concentrations have fallen from 13.7 micrograms (μg) per cubic meter to just 8.3 micrograms per cubic meter – a remarkable improvement in a relatively short time. Peak particulate matter levels have also declined significantly over this time period, as indicated by the peak 24-hour average trend. Recently, however, particulate matter levels have ticked slightly upwards due to unfavorable weather conditions. Longer stagnant periods with light winds and no rain contributed to higher Bay Area particulate levels in 2015, as winter weather conditions allowed for particulate matter to accumulate in the air basin.
Thanks to progress in reducing particulate emissions, the Bay Area has been successful in meeting national air quality standards in recent years. However, there is no 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.