Indicator En.4.a Air quality

Descriptive Title: Proportion of population living in an area with 10 ug/m3 or higher PM 2.5 concentration and proportion of households living in an area with a total cancer risk greater than or equal to 100 cases in 1 million

Geographic Unit of Analysis: 20 meter receptor grid

Download PDF map.

Download PDF map.

Why Is This An Indicator Of Health and Sustainability?

Motor vehicle emissions, power plants, and refineries are the predominant sources of fine particulate air pollution (PM2.5). Several large-scale studies demonstrate that increased exposure to PM2.5 is associated with detrimental cardiovascular outcomes, including increased risk of death from ischemic heart disease, higher blood pressure, and coronary artery calcification.a

Motor vehicles and other forms of fossil fuel combustion emit several toxic air contaminants that are either known or probable human carcinogens, including benzene, formaldehyde, acetaldehyde, and 1,3-butadiene. The U.S. Environmental Protection Agency (EPA) estimates that “mobile sources of air toxics account for as much as half of all cancers attributed to outdoor sources of air toxics.”b

Additionally, in June 2012, the International Agency for Research on Cancer, part of the World Health Organization, updated their classification of diesel engine exhaust from a probable human carcinogen to carcinogenic to humans. The update was based on sufficient evidence that exposure to diesel exhaust can increase the risk of developing lung cancer and is positively associated with an increased risk of bladder cancer.c

Interpretation and Geographic Equity Analysis

The above maps illustrate average PM 2.5 concentrations in San Francisco and cancer risks from total organic gases (TOG) and diesel particulate matter (DPM) exposure, with the areas in red being of the highest public health concern. Interstate 80 and 280 are the most notable for particulate matter concentrations at or greater than 10 ug/m3 and the potentially exposed areas surrounding them. There are several neighborhoods in the city that are impacted by high particular matter concentrations. The Financial District, Civic Center/Downtown, South of Market, Bayview, Bernal Heights, Mission, Mission Bay, Excelsior, Potrero Hill, Visitacion Valley and Western Addition neighborhoods have populations that are potentially exposed and affected by high particulate matter concentrations (≥ 10 ug/m3). Those same neighborhoods, in addition the Haight Ashbury and Marina neighborhoods, have populations that may have a cancer risk at or greater than 100 cases per one million as a result of exposure to TOG and DPM pollution.

In June 2008, San Francisco Supervisor Tom Ammiano proposed an air quality ordinance requiring public agencies take regulatory action to prevent future air quality health impacts from new sensitive uses proposed near busy roadways. The regulation requires that developers screen sensitive use projects for proximity to traffic and calculate the concentration of PM 2.5 from traffic sources where traffic volumes suggest a potential hazard. If modeled levels of traffic-attributable PM 2.5 at a project site exceed the current action level, developers would be required to incorporate ventilation systems to remove 80% of PM2.5 from outdoor air. The regulation does not place any requirements on proposed sensitive uses if modeled air pollutant levels fall below the action threshold. This ordinance is currently under revision to include cumulative impacts, which include point sources of PM2.5      and cancer risk. These indicators reflect anticipated threshold revisions in the amended air quality ordinance.


Models were created by SFDPH in conjunction with the Bay Area Air Quality Management District to quantify total PM 2.5, TOG and DPM concentrations from all sources in the city.   Cancer risk was calculated based on the cumulative TOG and DPM exposures. Estimated PM 2.5 concentration and cancer risk were assigned to points on a 20 meter grid. PM 2.5 values were interpolated using spline interpolation. Population exposure estimates to PM 2.5 were created using the ArcGIS zonal statistics spatial analysis tool using residential lots as the “zones.” Lots with a maximum PM 2.5 concentration value of 10 ug/m3  or more were selected as falling within an air quality hazard zone. The population that was estimated to have a cancer risk of 100 or more in 1 million was determined by performing a “Select by Location” query on city residential lots that were within 20 meters of a location that was modeled to have a cancer risk of 100 or more per 1 million. The population that was estimated to be living in those lots was summed by neighborhood and divided by the total population in the neighborhood to calculate the percent of the population exposed.


Weather, wind, geography, and buildings all affect how emissions are dispersed and their resulting concentrations in the air. While computer modeling can help estimate airborne concentrations of pollutants from different pollution sources, it may not completely reflect pollution concentrations that people are exposed to on a daily basis.

The importance of the exposure to health risk also depends on the combination of multiple air pollutants, the relative toxicity of the pollutants, and many other factors. For more information, visit: Exposure also varies depending on the activities of the exposed individual.

Data Source

PM 2.5 and cancer risk data provided by Bay Area Air Quality Management District and the San Francisco Department of Public Health, 2010

Population data, 2010 Census

Map prepared by City and County of San Francisco, Department of Public Health, Environmental Health Section using ArcGIS software.

Table data is presented by planning neighborhood. While planning neighborhoods are larger geographic areas than census tracts, census tracts do not always lie completely within a planning neighborhood. SFDPH used ArcGIS software and a dasymetric mapping technique to attribute Census block group data to residential lots. We then assigned residential lots to planning neighborhoods to calculate Census population totals within the neighborhoods.

Detailed information regarding dasymetric mapping, census data, geographic units of analysis, their definitions, and their boundaries can be found at the following links:

  1. Simkhovich BZ, Kleinman MT, Kloner RA. Air Pollution and Cardiovascular Injury: Epidemiology, Toxicology, and Mechanisms. J Am Coll Cardiol. 2008;52(9):719-26.

  2. USEPA Environmental Fact Sheet. (August 1994). Air Toxics from Motor Vehicles. Available at:

  3. International Agency for Research on Cancer. Diesel Engine Exhaust Carcinogenic. 12 June 2012. World Health Organization Press Release No. 213. Available at: