In the early morning of October 9 2017, a series of fatal wildfires broke out across Northern California’s wine country. High winds and dry weather exacerbated the spread of the flames throughout the day, killing at least 17 people and evacuating more than 20,000.

Source: NASA Worldview

Wildfires have a devastating effect on the areas they strike, as homes are destroyed, citizens evacuated, and lives uprooted. In addition, smoke and particulate matter from the blaze can have a far-reaching impact on air quality.

When the fires began in the middle of the night, the prevailing wind was blowing towards the Bay Area. This meant that San Francisco saw a severe peak of PM2.5 particulate matter in the early hours of the fire.

This harmful airborne pollutant consists of tiny dust or soot particles, often small enough to penetrate the bloodstream through the lungs. As well as from wildfires, they come from road vehicles and combustion due to heating and other sources in urban areas.

However, as the wind direction changed in the afternoon, smoke from these fires was largely blown out to sea, rapidly reducing the intensity of the peak over the city. You can see the shift in wind direction on the graph below as the background color changes.

Source: Plume Labs

If you are in an area affected by wildfires, the American Red Cross advises keeping windows and doors closed to prevent smoke entering your home. Also, avoid adding to indoor air pollution by using candles, fireplaces, or gas stoves.

Those with specific respiratory conditions should take extra care and follow their healthcare provider’s advice.