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The stunning impact of COVID-19 social distancing on air pollution

How efforts to flatten the curve of COVID-19 have reflected air quality in the San Francisco Bay area.

Comparing current Bay Area air pollutant levels to years past

Weekday air pollutant levels drop following Bay Area COVID-19 response.


We focused on weekdays to highlight changes during the workweek when commuter and other commercial traffic would be most affected. Looking at year-over-year differences helps to control for the seasonality associated with air pollution levels. There was a notable decrease in the levels of these air pollutants measured during 2020 to those from 2017 through 2019, with the exception of O3. In fact, NO2, PM2.5, CO and BC levels were from 16 percent to 29 percent lower during the first two weeks of the regional COVID-19 response.

The 'Weekend Effect': As NOx goes down, ozone temporarily goes up

NO2 levels measured by Aclima

Aclima measures 10% drop in carbon dioxide across the Bay Area

CO2 levels across Bay Area
  • Ozone (O3) is formed when sunlight reacts with vehicle pollution and is a key component of smog as well as a greenhouse gas that contributes to climate change. Breathing ozone irritates your respiratory system and can reduce lung function, aggravate asthma and cause permanent lung damage over time.
  • Carbon monoxide (CO) is a colorless, odorless gas released when fuels are burned. Most CO in urban areas comes from vehicles. When inhaled, carbon monoxide displaces oxygen in the blood and deprives the heart, brain and other vital organs of oxygen. High levels of CO typically are not found outdoors. It also reacts with other molecules in the atmosphere to create carbon dioxide, a greenhouse gas.
  • Carbon dioxide (CO2) is the primary greenhouse gas resulting from fossil fuel combustion. CO2 doesn’t directly cause human harm when encountered outdoors. The primary health concern with CO2 is its role in climate change. The main sources of CO2 include transportation, electricity production and industry.
  • Fine particulate matter (PM2.5) is emitted directly from a combustion source or formed in the atmosphere from complex chemical reactions. Fine particulate matter (PM2.5) is one of the primary pollutants public health officials care about because it is linked to asthma, lung cancer and deaths from cardiopulmonary diseases.
  • Nitrogen dioxide (NO2) is a gaseous pollutant that is formed from fossil fuel combustion and is a key component of smog. Indoors, NO2 can be emitted from unvented gas appliances. NO2 irritates the respiratory system and can lead to coughing or wheezing, increased asthma attacks and reduced lung function.
  • Black carbon (BC) refers to sooty black particles emitted from diesel engines, heavy oil or coal-fired power plants, biomass burning and other sources that burn fossil fuel. Black carbon is associated with health problems including respiratory and cardiovascular disease, cancer and even birth defects.

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