Air Pollution In San Diego: How Bad Is It?

San Diego's air quality is generally considered acceptable, but certain groups may experience symptoms due to the city's air pollution. The air quality is largely dependent on climate, geography, and weather patterns. The city's subtropical climate, characterized by hot, dry summers and mild, wet winters, creates ideal conditions for the accumulation of ozone pollution. Fine particulate matter, or PM2.5, is another key contributor to the city's air pollution, adversely affecting the health of residents.

Ozone pollution

San Diego is one of the most ozone-polluted cities in the United States. In 2023, it ranked eighth in the country for ozone pollution. The city's air quality suffers from frequent and prolonged periods of unhealthy ozone pollution, with an average of 43.3 days of unhealthy ozone annually, far exceeding the 3.2-day federal target. This has resulted in an "F" rating by the American Lung Association (ALA).

Ozone is a corrosive gas pollutant composed of three oxygen atoms. Due to its highly reactive nature, prolonged exposure can cause damage to lung tissue, leading to the early ageing of the lungs and the worsening of chronic lung disease. Short-term effects can trigger asthma attacks, coughing, and difficulty breathing, with more acute impacts on sensitive groups, including children, the elderly, and individuals with pre-existing heart and lung conditions.

The high levels of ozone pollution in San Diego are influenced by a combination of geographical, climatic, and weather-related factors. The city's location within the San Diego Air Basin (SDAB), a subtropical climate zone with hot, dry summers and mild, wet winters, provides ideal conditions for the creation and accumulation of ozone pollution. Light winds, stagnant air, and minimal precipitation further contribute to the build-up of ozone. Additionally, San Diego's proximity to the coast gives rise to pollution-accumulating weather events known as marine inversions, where cooler ground-level air near the ocean becomes trapped by warmer, denser air above, trapping ground-level emissions.

Transportation, particularly heavy truck and port activities, play a significant role in ozone pollution in San Diego. Tailpipe emissions from motor vehicles contribute about 42% of the pollutants that form ozone in the region. The city's population and emissions are concentrated in the western part of the county, especially near the Cuyamaca Mountains. Specific areas with the highest levels of ozone pollution in San Diego County include Alpine, Borrego Springs, and Warner Springs.

Particle pollution

San Diego's air quality is considered to be poor, with the American Lung Association (ALA) listing it as one of the most polluted regions in the country. The city's air quality issues are largely attributed to ozone and particle pollution, which are caused by a combination of emissions from vehicles, buildings, and industry, as well as geographical and weather factors.

In 2019, San Diego's PM2.5 levels ranged from a low of 6 μg/m3 ("good") in March to a high of 15.9 μg/m3 ("moderate") in November. While the city has met annual attainment levels for PM2.5 in recent years, it failed to do so in 2017 and 2018. Since then, PM2.5 pollution levels have shown improvement, with a 32% reduction since 2018. However, this improvement may be due to favourable weather and environmental conditions rather than a consistent downward trend.

The ALA has given San Diego a "C" rating for 24-hour particle pollution, reflecting the city's average of two annual unhealthy PM2.5 days. This rating is only just within the acceptable range, indicating that particle pollution remains a significant issue in San Diego.

The city's geography and climate also play a crucial role in the accumulation of particle pollution. San Diego's location on the coast and its subtropical climate, characterised by hot, dry summers and mild, wet winters, create the ideal conditions for the formation and accumulation of pollution. Light winds and stagnant air, combined with low precipitation, further contribute to the build-up of particle pollution in the region.

Climate, geography, and weather

San Diego is part of the San Diego Air Basin (SDAB), a subtropical climate zone characterised by hot, dry summers and mild, wet winters. The city experiences more than 170 days a year above 70°F, with light winds and stagnant air. These conditions, combined with a lack of precipitation, create an ideal environment for the formation and accumulation of ozone pollution.

Ozone is a corrosive gas composed of three oxygen atoms. It is highly reactive and can cause damage to lung tissue, leading to premature ageing of the lungs and exacerbating chronic lung disease. Inhaling ozone can trigger asthma attacks, coughing, and difficulty breathing. These effects are more pronounced in vulnerable populations, including children, the elderly, and those with pre-existing heart and lung conditions.

The geography of San Diego, situated near the Pacific Ocean, also contributes to its air pollution issues. Marine inversions, a reversal of typical temperature patterns, occur when cooler ground-level air near the ocean becomes trapped by warmer, denser air above. This phenomenon traps ground-level emissions, including pollutants, until normal temperature conditions resume. The ocean cools the air in the lower atmosphere, which is then trapped by warmer inland air.

Additionally, winds blowing in from the west can drive air pollutants and cold surface-level air inland, exacerbating the marine inversion effect. Winds from the South Coast Air Basin and Los Angeles, a city with notoriously poor air quality, can significantly impact San Diego's air pollution levels. The combination of geographical location, climate, and weather patterns in San Diego creates a unique set of challenges for managing air quality and protecting the health of its residents.

Marine inversions

San Diego's location on the coast, with its subtropical climate, gives rise to a weather phenomenon known as marine inversions. This occurs when the normal temperature gradient is reversed, with cooler ground-level air near the ocean becoming trapped by a layer of warmer, denser air above. The ocean cools the air in the lower atmosphere, which is then held in place by the warmer inland air. This temperature inversion acts like a lid, trapping and accumulating ground-level emissions until normal temperature conditions resume.

The geography of San Diego plays a crucial role in the formation of marine inversions. The city's proximity to the Pacific Ocean results in the cooling of the lower atmosphere, setting the stage for temperature inversions. Additionally, light winds blowing in from the west can drive air pollutants and cold surface-level air inland towards the foothills, further exacerbating the inversion's impact.

Winds from the South Coast Air Basin, including Los Angeles, can also significantly influence San Diego's air quality. The inflow of air from regions with high levels of ozone pollution, such as Los Angeles, contributes to the overall air pollution levels in San Diego. These external factors, combined with local emissions, create a complex challenge for maintaining healthy air quality in the region.

While marine inversions can have negative consequences for air quality, they also play a role in moderating temperatures and influencing the region's climate. The trapped cool air can limit the sun's heating of the ground, maintaining cooler surface temperatures. This process can perpetuate the marine inversion, leading to periods of extended cool and hazy weather along the coastal areas.

Health effects

San Diego's air quality is generally acceptable for most individuals. However, the city does experience frequent and prolonged periods of unhealthy ozone pollution, which can have adverse health effects on everyone, especially sensitive groups.

Ozone is a corrosive gas pollutant composed of three oxygen atoms. Due to its highly reactive nature, prolonged exposure can cause lung tissue damage, leading to early lung ageing and the worsening of chronic lung disease. Short-term effects of ozone exposure include triggering asthma attacks, coughing, and difficulty breathing. These effects are more acute in children, the elderly, and individuals with pre-existing heart and lung conditions. On average, San Diego experiences 43.3 days of unhealthy ozone levels annually, far exceeding the 3.2-day federal target. This has resulted in an "F" rating by the American Lung Association (ALA).

Another key contributor to San Diego's air pollution is fine particulate matter, or PM2.5, which includes metals, soot, soil, dust, and salt. Breathing PM2.5 has been linked to serious health issues, including heart and lung diseases, cancer, and premature mortality. While San Diego meets annual attainment levels for PM2.5, it failed to do so in 2017 and 2018. Since then, PM2.5 pollution levels have improved, although this may be due to temporary favourable weather and environmental conditions.

The city's geography near the Pacific Ocean also contributes to air pollution through temperature inversions or "marine inversions." This phenomenon traps cooler ground-level air near the ocean with warmer, denser air above, leading to the accumulation of ground-level emissions. Light winds can further drive air pollutants inland, exacerbating the issue. Additionally, winds from the South Coast Air Basin and Los Angeles, which has the worst ozone levels in the nation, can significantly elevate San Diego's air pollution.

The health effects of San Diego's air pollution are evident in the region's high asthma rates. Barrio Logan, just south of downtown San Diego, has asthma-related hospital visit rates in the highest 10th percentile in California, according to a 2014 report by the California Office of Environmental Health Hazard Assessment (OEHHA). Transitioning to fuel-efficient, lower-emission vehicles and improving public transportation can help tackle the city's ozone levels and positively impact residents' health.

Frequently asked questions