Air quality in San Diego

How is the air quality in San Diego?

San Diego air quality suffers from frequent and prolonged periods of unhealthy ozone pollution.

Ozone is a corrosive gas pollutant composed of three oxygen atoms. Due to ozone’s highly reactive nature, prolonged exposure can cause damage to the lung tissue, leading to the early aging of the lungs and the worsening of chronic lung disease.¹ Short term effects can trigger asthma attacks, coughing and difficulty breathing. These effects are more acute in sensitive groups including children, the elderly, and individuals with preexisting heart and lung conditions.

On average, San Diego experiences 43.3 days of unhealthy ozone annually, far exceeding the 3.2-day federal target.² Its severe nonattainment status has resulted in an “F” rating by the American Lung Association (ALA). There is a concerning trend of worsening ozone in San Diego. Since 2015, the number of unhealthy ozone days has risen by 42 percent.

Fine particulate matter, or PM2.5, is another key contributor to San Diego’s air pollution mix, adversely affecting resident health. PM2.5 is a term for a suspended mixture of tiny particles, including a wide range of chemical compositions and sources. Different materials include metals, soot, soil, dust, and salt. Breathing PM2.5 has been directly linked to adverse health effects, such as heart and lung diseases, cancer, and premature mortality.

The ALA has rated San Diego a “C” for 24-hr particle pollution. The barely passing grade is a result of the City’s average 2 annual unhealthy PM2.5 days. While the city does meet annual attainment levels for PM2.5 over the span of a year, it had failed to do so in the previous two years in 2017 and 2018. Since 2018, PM2.5 pollution levels in San Diego have improved by 32 percent. This improvement, however, may not be indicative of a year-over-year trend, but rather more ideal weather and environment conditions.

For the most up-to-date air quality data in San Diego, consult the information displayed at the top of this page. Using San Diego forecast air quality data is a good tool for planning ahead to reduce pollution exposure.

Why is air quality bad in San Diego today?

Despite relatively similar day-to-day emissions from vehicles, buildings, and industry across San Diego, pollution levels vary. The reason for bad air quality in a given area often has more to do with climate, geography, and weather patterns than local spikes in emissions.

San Diego is part of the San Diego Air Basin (SDAB), a subtropical climate zone characterized by hot, dry summers, and mild, wet winters.³ Light winds and stagnant air, combined with little precipitation and more than 170 days over 70 degrees, combine to create ideal conditions for the creation and accumulation of ozone pollution.

San Diego’s location on the coast gives way to pollution-accumulating weather events known as marine inversions. Marine inversions describe a reversal of normal temperature patterns in which cooler ground level air near the ocean becomes trapped by warmer, denser air above. The result is the trapping and accumulation of ground-level emissions until normal temperature conditions return. In San Diego, temperature inversions are a result of the city’s geography near the Pacific Ocean. The ocean cools the air in the lower atmosphere, which then gets trapped by warmer inland air. Light winds blowing in from the west can further aggravate this condition, by driving air pollutants and the cold surface level air inland, toward the foothills, furthering the effect of the marine inversion.

Winds flowing down from the South Coast air basin and Los Angeles can also play a role in significantly elevating San Diego air pollution levels. Los Angeles ozone is the worst in the nation. State studies identify transported air pollution as an additional challenge for San Diego in meeting federal ozone attainment levels.

Local variances in San Diego’s AQI may be attributable to a number of reasons. Use the San Diego air pollution map to understand where emissions are flowing in from and what weather events may be contributing or alleviating air pollution.

Is there smog in San Diego?

San Diego smog, or visible air pollution, is among the nation’s worst. Smog is composed of a combination of air pollutants, primarily ozone and particle pollution. For both pollutant measures, San Diego ranks poorly.

For the 5th year in a row, San Diego is rated 6th nationally for worst ozone pollution out of 229 included metropolitan areas. All five regional areas which rank higher than San Diego are located in California. Los Angeles, located 120 miles to the northwest, has held the number one position for worst ozone every year but one in 20 years. In 2019, San Diego air quality fared better than Visalia air quality, Bakersfield air quality, the Fresno-Madera-Hanford area air quality, Sacramento air quality, and Roseville air quality.

The majority (42 percent) of the pollutants that contribute to ozone formation in San Diego County come from tailpipe emissions from motor vehicles. The city’s population and emissions are primarily concentrated in the western part of the County, particularly in the inland areas near the Cuyamaca Mountains. According to a state study, Alpine, Borrego Springs, and Warner Springs have the highest levels of ozone pollution in San Diego County.⁴

Particle pollutants, such as PM2.5, have been described as ‘visibility reducing particles’ for their contribution to regional smog. In 2019, San Diego PM2.5 varied from concentrations from a low of 6 μg/m³, “good,” in March, to a high of 15.9 μg/m³, “moderate,” in November.

The months of August through January have traditionally been the most polluted in San Diego for PM2.5 pollution. Whilst the summer months of March through September are the most polluted for ozone. When these peaks overlap, heavy smog is common.

According to the American Lung Association, San Diego’s smoggy days are up 42 percent over the last five years.⁵ Warming temperatures as a result of climate change are thought to be one of the drivers for this increase, and presents a challenge for tackling ozone in the future.

How polluted is San Diego?

In 2019, San Diego averaged an air quality index of 39 (“good”). This was a slight improvement in overall air quality compared to years prior. Both 2018 and 2019 experienced an average AQI of 52, “moderate,” just exceeding federal guidelines.

The AQI, or Air Quality Index, translates sometimes unintuitive pollutant concentrations into one easy-to-understand scale in order to convey associated health risk of an environment with multiple airborne threats. The index accounts for six criteria pollutants measured by the Environmental Protection Agency (EPA), including PM2.5, PM10, carbon monoxide (CO), sulfur dioxide (SO₂), nitrogen dioxide (NO₂), and ground level ozone (O₃).

San Diego annual AQI indicates that, on average, pollutants are within healthy ranges. Sweeping annual averages, however, tend to smooth over periods of unhealthy pollution events, which are considerably common in San Diego.

On average, San Diego experiences 43.3 days of unhealthy ozone and 2 days of unhealthy PM2.5 annually. The city’s ozone levels represent numbers more than 13 times the national target, advising no more than 3.2 unhealthy days. By this examination, San Diego still has a very long way to reduce air pollution and its threat on residents.

Currently, persistent high ozone levels in the summer may be contributing to the County’s exceedingly high asthma rates. Asthma rates in Barrio Logan, just south of the downtown area, are among the highest in California.⁶ A 2014 report conducted by the California’s Office of Environmental Health Hazard Assessment (OEHHA) found that asthma-related hospital visits in this area are in the highest 10th percentile in the state.

Tackling ozone levels by transitioning more residents to fuel-efficient, lower-emission vehicles as well as increasing the availability and attractiveness of public transportation present opportunities for managing the largest contributing source to San Diego ozone, and improving the health of San Diegans.

+ Article Resources

[1] San Diego County Air Pollution Control District. (2016). 2008 eight-hour ozone attainment plan for San Diego County.
[2] American Lung Association. (2020). State of the air – 2020.
[3] HELIX Environmental Planning. (2016). San Ysidro community plan update and San Ysidro historic village specific plan.
[4] Schroeder J. (2018, January 30). Interactive map shows which San Diego areas have the worst air quality.
[5] Anderson E. (2019, April 24). San Diego smog among the nation’s worst.
[6] Bailey T. (2015, May 27). Air pollutants contribute to Barrio Logan asthma rates.