Particulate Concentrations

How clean is the air we breathe?

Particulate Concentrations

Particulate matter concentrations refer to the amount of fine particulate matter (PM2.5) in the air we breathe. Fine particulate matter particles are very small, measuring less than 2.5 microns or 1/25 the width of a human hair – yet they pose a significant health risk. Air quality standards for particulate matter are designed to protect public health in two different ways: the annual average focuses on long-term (chronic) exposure, while the peak 24-hour average emphasizes short-term (acute) exposure.

Particulate matter poses a significant health and environmental threat to Bay Area residents. While these tiny particulates may seem innocuous, their minuscule size allows them to reach deep into the lungs and bloodstream, where they can trigger serious (or even fatal) conditions like asthma, strokes and heart attacks. Fine particulates are emitted by a wide range of activities and sources, including residential wood burning, industrial sources, wildfires and motor vehicles. Particulate matter levels are often elevated in close proximity to freeways and high-volume roadways.

Regional Performance
Fine particulate matter concentrations have declined significantly over the long term.

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.

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Historical Trend for Particulate Matter Concentrations - Bay Area

Local Focus
Over the long term, Point Reyes has had the lowest fine particulate matter concentrations in the Bay Area.

Local particulate matter concentrations are influenced by variations in emissions and local weather conditions. The monitoring station at Point Reyes in Marin County consistently has the lowest concentrations of fine particulate matter of any monitoring site in the region. This result at Point Reyes is a reflection of human activity having minimal influence on local air quality due to its location in the relatively pristine Golden Gate National Recreation Area – upwind of population centers and emission sources.

Although local concentrations may be higher in close proximity to major emissions sources, particulate levels are relatively similar at air quality monitoring stations throughout urbanized portions of the Bay Area. Outliers include West Oakland and San Jose, where heavy traffic volumes result in above-average particulate matter levels. Napa also experiences higher particulate matter levels, a result of its high levels of residential wood-burning activity. In contrast, Gilroy has consistently seen some of the lowest levels in the urbanized Bay Area.

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Particulate Matter Concentrations

Select sensors on the map to see detailed historical fine particulate matter concentrations.
National Context
Among major U.S. metro areas, Miami has the cleanest air in the country as a result of low levels of particulates.

Of the ten largest U.S. metro areas, Miami continues to have the lowest particulate levels. Miami’s strong performance in terms of regional air quality is a result of multiple favorable factors. Only lightly industrialized, Miami features a flat topography and favorable wind conditions, which mean that those particulates that are emitted tend to be blown away from populated areas. This stands in stark contrast to Los Angeles. There, unfavorable topographical and meteorological conditions result in the highest particulate matter concentrations among the nation’s largest metro areas. Meanwhile, the Bay Area falls in the middle of the pack of major metro areas in relation to particulate matter, though with significantly lower levels than Southern California.

The Bay Area is in the middle of the pack in terms of particulate matter concentrations. While the Bay Area has relatively low annual average levels of particulate matter, our region experiences higher peak levels than other major U.S. metro areas. These peak events, which can last for multiple days, are a result of winter conditions in which stagnant air close to the ground causes a spike in particulate matter concentrations.


Bay Area Air Quality Management District: Air Quality Sensor Data (1999-2015)

Environmental Protection Agency: Air Quality System (2013-2015)

Image: Flickr (Creative Commons license), Photographer: Kim Seng,

Methodology Notes: 

Data is collected at specific sensor sites operated by the Air District that capture the concentration of fine particulate matter (PM2.5) in the air; sensor locations may change over the years and thus minor relocations are shown as the same station in this analysis (e.g., sensor moved across the street from its prior location). Data either reflect the annual average or the peak 24-hour average (i.e., 98th percentile day; worst-case conditions); some data are reported as three-year rolling averages, which align with air quality standards and minimize the impact of short-term variations in weather conditions in any given year. The worst location site may shift from year to year depending on which sensor location had the highest levels of fine particulate matter over that particular time period. The regional trend reflects the average of eight sensors that were available throughout the analysis time period, using an iterated two-way analysis of variance (ANOVA) for interpolation in gap years. For current air quality data, refer to the Bay Area Air Quality Management District’s website: