Number of days per year with "Good" air quality on the EPA's Air Quality Index
Geographic Unit of Analysis:
Scientific studies consistently show an association between exposure to air pollution and significant human health problems. Most well known are the respiratory effects such as aggravated asthma, chronic bronchitis, and reduced lung function. Air pollution affects heart health and can trigger heart attacks and strokes that cause disability and death. Air pollutants may be a contributing factor to leading causes of death recorded for San Francisco’s population (ischemic heart disease; lung, bronchus, and tracheal cancers; cerebrovascular disease; chronic obstructive pulmonary disease; hypertensive heart disease and lower respiratory infection). Exposure to air pollutants that are carcinogens can also have significant human health consequences. For example, exposure to diesel exhaust is an established cause of lung cancer.
Between 1980 and 1998, the number of days with Good Air Quality increased, from 273 to 353, but has remained under 300 since 1999. Over the same period, the number of days where the maximum air quality index reading was deamed "unhealthy" or "unhealthy for sensative groups" has declined, the the lowest maximum readings in the 1990's. While the number of days with poor air quality has fallen since the beginning of the 2000's, the number of days with moderate air quality has in general increased. It is unclear at this point why the improvements in air quality that were experienced during the 1990's have since dissipated. One possibility is that the growth of the tech sector in the mid-90's brough additional jobs to San Francisco and with it, additional passenger cars.
In terms of sources of air pollution, San Francisco has increasingly fewer stationary sources of air pollution-- power plants in Hunters Point and Potrero Hill were closed in 2006 and 2010, respectively, and many industrial businesses have since left the city. However, air pollution from other stationary sources such as diesel generators, gas stations and dry cleaners continue to contribute to poor air quality in the city. Air pollution from cars, trucks, ships, emissions from construction equipment, and tire and brake wear on roadways contribute substantially to air pollution-related health outcomes. These mobile sources of air pollution are the biggest root cause of poor air quality in the city and addressing these should result in a new positive trend for air quality.
Data were downloaded from AirNow and graphed.
Air pollution as measured by the EPA's air quality index is sourced from a single monitor in the Potrero Hill neighbordhood. Thus, this measure does not capture geographic variations in air quality that exist within the City.
Air quality data from https://www.airnow.gov/