In mid-August of 2020, California cities dominated the entire top 10 most polluted cities in the United States and, in some cases, the entire world due to the outbreak of several major wildfires inNorthern California.

Pictured: World AQI Rankings with real-time most polluted city rankings in California. Air quality ratings n Winters and Nevada show as "Hazardous", while Castro Valley, Fairview, Monte Sereno, Hayward, Los Alto Hills, Ashland, Portola Valley, and Paso Robles are "Very Unhealthy".

Cities in Northern Californialike Castro Valley and Hayward faced dangerous levels of air quality, breaking into some of the highest ratings for dangerous air pollution on the Air Quality Index (AQI):

• 200+: considered “unhealthy” and linked to health risks for anyone exposed to this level of air pollution
• 300+: considered “hazardous”, with air pollution levels reaching emergency conditions that can cause long-term health effects even after short periods of exposure

Pictured: the IQAir AirVisual air quality index (AQI), based on US AQI standards per the Environmental Protection Agency (EPA)

Large swathes of the southern Bay Area, home to famous tech hubs like San Jose, Cupertino, and Mountain View, experienced air quality well into the 200s and 300s, prompting local officials to set up clean air and cooling centers and declare emergency alerts as well as evacuation orders.^1,2

Pictured: Air quality in the southern Bay Area reached the upper limits of the air quality index, considered unhealthy for all populations. Source: IQAir AirVisual

Cities as far away as Truckee near Lake Tahoe, CA also experienced extreme air pollution from PM2.5 (particulate matter with a diameter size of 2.5 microns or smaller) as a result of smoke drifting from Northern California through California and Nevada.

Cities far south of the Bay Area also faced elevated air pollution levels as wildfire smoke traveled hundreds of miles from Northern California fires down the California Coast – typically pristine wine country resort towns like Paso Robles and wealthy beachside destinations like San Luis Obispo experienced dangerous PM2.5 pollution from the Bay Area fires.

These Northern California fires were the first major blazes of the 2020 wildfire season in mid-August for the region, with several major fires breaking out during the week of August 16-22, 2020 with containment slow to progress as blazes began burning out of control within a matter of days.

Follow real-time air quality data updates in Northern California fires using the IQAir AirVisual Map to see the latest local air quality.

Click the links below to see real-time air quality for other Northern Californiacities impacted by these fires:

• Oakland
• San Jose
• Cupertino
• Mountain View
• Palo Alto
• Los Gatos
• Pleasanton
• Dublin
• Livermore
• Atherton
• Fremont
• Stanford
• Milpitas

How did the Bay Area fires start?

Several major conglomeration of wildfires were referred to by names given in reference to the units of Cal Fire, the California state firefighting organization, that battled the blazes:³

• CZU Lightning Complex fires near Santa Cruz and San Mateo Counties
• SCU Lightning Complex fires near Santa Clara, Contra Costa, and Alameda Counties
• LNU Lightning Complex fires near Napa, Sonoma, and Solano Counties

Shortly after the fire broke out, the LNU Lightning Complex spread into nearby Yolo and Solano counties, home to the suburban areas of Vacaville just northeast of the Bay Area past other major suburban cities such as Vallejo and Fairfield, the latter near Travis Air Force Base.

Here are some of the fires that broke out in Napa and Sonoma Counties – click the links below to see live air quality for cities previouslyaffected by the Bay Area fires:

• Hennessey Fire near St. Helena, CA on Hennessey Ridge Road and Chiles Pope Valley Road, east of St. Helena
• Gamble Fire near Brooks, CA on Berryessa Knoxville Road
• Green (15-10) Fire near Putah Creek Bridge and Berryessa Knoxville Road
• Markley Fire close to the Monticello Dam
• Spanish Fire close to Spanish Flat
• Wallbridge Fire near Healdsburg, CA
• Myers Fire near Jenner, CA

Here are some of the fires that were burning in Santa Cruz and San Mateo Counties:

• 5-15 Fire close to Dearborn Park Road and Butano Truck Trail near Portola Valley
• 5-18 Fire close to China Grade Road and Butano Truck Trail
• 5-14 Fire close to Butano Fire Road Olmo Truck Trail and Butano Fire Road
• Waddell Fire close to Highway 1 and Old Coast Roa

Here are some of the fires that were burning near Santa Clara, Alameda, Contra Costa, San Joaquin, and Stanislaus Counties:

• Deer Zone fires, including the Briones Fire, Marsh Fire, Palm Fire, and Round Fire
• Calaveras Zone fires, including the Ohlone Fire, Arroyo Fire, Kilkare Fire, Reservoir Fires, Welch Fire, and Mill Creek Fire
• Canyon Zone fires, including Del Puerto Fire, Peg Leg Fire, and Terraville Fire

How do the Northern California fires impactair quality?

The Northern California fires in August of 2020 impacted the air quality of nearly 7.8 million residents in the San Francisco Bay Area alone, home to billion-dollar and trillion-dollar tech industry titans like Apple, Google, Tesla, Salesforce, and hundreds of others.

Millions more outside the Bay Area in Northern California and beyond, especially in big suburban enclaves like Pleasanton, Dublin, Livermore, and Reno were also affected by the smoke.

These fires came amidst a confluence of planned outages meant to help the region confront a series of record heat waves during a particularly hot week in August 2020. Temperatures well into the 90s and 100s along with extremely low levels of humidity created conditions ripe for the outbreak of wildfires across the Bay Area region.

These immense blazes resulted in some of the worst air quality in the world, illustrating just how quickly fires can break out during California’s infamously dangerous wildfire season. They also overshadowed the 2019 Kincade fire, previously the record holder for the biggest Bay Area fire to date.⁴

Wildfire smoke pollution is especially dangerous because it contains not only particulate pollutants like PM2.5 and ultrafine particles (UFPs) from combustion but also chemicals from the burning of buildings that release compounds from toxic building materials, including formaldehyde, that can be especially dangerous to human health.

Wildfire smoke can also travel for hundreds of miles beyond the source of the blaze. The Northern California fires burning during August of 2020 provide a stark illustration of this – all across California, wildfires have impacted air quality in all of the state’s major urban regions, including Los Angeles, San Francisco, and San Diego, but also drifted as far away as Las Vegas, Nevada and Phoenix, Arizona during the peak of fires, such as the Apple Fire in Cherry Valley, CA.

Pictured: Air quality measurements throughout the state of California, showing the impact of Bay Area fires on the entire state. Source: IQAir AirVisual

What are the health impacts of wildfire smoke?

The Center for Disease Control and Prevention says that wildfire smoke can cause illness in older adults, children, pregnant women, and individuals with pre-existing respiratory and heart conditions.⁵

Additionally, all people impacted by poor air quality from wildfires may experience immediate health issues, including:

• coughing
• breathing difficulties, including wheezing and shortness of breath
• dry, stinging eyes
• runny nose
• throat scratchiness
• headaches
• sinus irritation
• chest pain
• rapid heartbeat
• asthma attacks
• tiredness

Under these conditions, it's recommended to limit your exposure to the smoke by staying indoors or leaving the area if necessary. Monitoring your air quality, using air purification devices, and using properly worn N95 masks (respirators that meet U.S. quality standards and filter at least 95% of 0.3 microgram-sized particles)can all help safeguard your health.

Wildfires are an important part of California's ecology. Stay up to date on how Northern Californiafires and California wildfires in generalimpact your air quality with. Keepinformed about how your air quality is affected by wildfire smoke by visiting AirVisual Map for the most accurate real-time air quality data, powered by IQAir AirVisual.