![]() ![]() 2 In November 2018, the Camp fire burned 153,336 acres of land and drove San Francisco’s AQI for the month up to 137, putting it into the “unhealthy for sensitive groups” bracket, which poses concern for young children, the elderly and those with a predisposition to respiratory ailments. In the last three years, five of the most destructive California wildfires were in relatively close proximity to San Francisco, greatly impacting the quality of air across the Bay Area. These inversions can therefore prolong and exacerbate existing air pollution and smog in San Francisco and the Bay Area during wintertime. This reversal, known as a thermal inversion, leads the warm layer of air to act like a “cap” or a lid, trapping the air beneath for long periods of time, usually until a weather change such as winds arrives to disperse it. During cold conditions, occasionally a layer of warmer air can become held above a cooler of ground-level air, when usually the opposite arrangement is true. Additionally, cold weather conditions can affect the behavior of air pollution particles. Without taking into account the effects of potential wildfires, winter months are commonly much more polluted than the summer season, often as a result of increased heating and wood burning taking place. 1 Wildfires, which are becoming increasingly common in the Bay Area, give rise to drastic spikes of air pollution, usually occurring during summer and fall. ![]() ![]() Air pollution in San Francisco comes primarily from transportation emissions, namely vehicles such as cars, motorbikes and trucks, as well as planes, and ships all contributing to the ambient levels of air pollution. The good air quality rating in San Francisco can be attributed to the city’s coastal location, as well as its natural topography and having a sparse level of factories and other industrial production plants around the city limits. For context, this compares to a similar rating in New York during 2019 (7 μg/m 3), and represents a healthier level of air quality than other cities such as Los Angeles (12.7 μg/m 3), London (11.4 μg/m 3) and Paris in France (14.7 μg/m 3). In 2019, San Francisco averaged an annual PM2.5 concentration of 7.1 μg/m 3, also achieving the World Health Organization’s (WHO) target for annual PM2.5, which is any annual reading below 10 μg/m 3. The air quality of San Francisco is rated on average as falling into the “good” bracket of the United States Air Quality Index (US AQI), which describes a level of fine particulate matter (PM2.5) between 0 to 12 μg/m 3. How good is the quality of air in San Francisco? ![]()
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