California Air Quality Guide

Overview: California's Unique Air Quality Challenges

California faces some of the most complex air quality challenges in the United States. From devastating wildfires to valley inversions, from coastal marine air to desert dust storms, the state's diverse geography creates dramatically different air quality conditions across regions.

Key factors affecting California air quality:

• Wildfire smoke: Increasingly severe and prolonged fire seasons
• Geography: Mountains and valleys trap pollution
• Vehicle emissions: High population density and car dependency
• Agriculture: Dust, pesticides, and ammonia from farming
• Ports and shipping: Diesel emissions in coastal areas
• Climate change: Exacerbating all of the above

Regional Air Quality Patterns

Southern California (Los Angeles, San Diego, Orange County)

Primary pollutant: Ozone (summer) and PM2.5 (year-round)

Characteristics:

• Historically known for smog; significantly improved since the 1970s but still struggles
• Ozone peaks during summer months (June-September)
• Traffic congestion creates persistent NO₂ and PM2.5
• Coastal areas generally have better air quality
• Inland areas (Riverside, San Bernardino) often have worst AQI

Worst months: June-September for ozone; year-round potential for poor AQI days

Best months: November-February typically cleaner (outside wildfire smoke events)

San Francisco Bay Area

Primary pollutant: PM2.5 (wildfire smoke) and PM2.5 (winter wood smoke)

Characteristics:

• Generally good air quality most of the year
• Marine air helps disperse pollution
• Wildfire smoke the biggest concern (August-October)
• Winter wood burning creates localized PM2.5 issues
• Traffic pollution concentrated near highways and urban cores

Worst months: August-October (wildfire season)

Best months: March-June typically excellent air quality

Central Valley (Fresno, Bakersfield, Modesto, Stockton)

Primary pollutant: PM2.5 and ozone

Characteristics:

• Consistently worst air quality in California
• Valley geography traps pollution from agriculture, traffic, and industry
• Winter inversions can last weeks, creating hazardous PM2.5 levels
• Summer ozone formation exacerbated by heat
• Agricultural dust and pesticide drift year-round
• Wildfire smoke frequently fills valley from surrounding fires

Worst months: November-January (inversions) and July-September (ozone + smoke)

Best months: March-May typically better (but not guaranteed good)

Sierra Nevada and Mountain Communities

Primary pollutant: PM2.5 (wildfire smoke and wood burning)

Characteristics:

• Generally excellent air quality most of the year
• Wildfire smoke biggest concern (directly affected by nearby fires)
• Winter wood smoke in mountain towns can create poor nighttime AQI
• Some communities see smoke "trapped" in mountain valleys

Northern California (Sacramento, Redding, Chico)

Primary pollutant: PM2.5 (wildfire smoke)

Characteristics:

• Moderate air quality most of year
• Severely impacted by wildfires (2018 Camp Fire created hazardous conditions for weeks)
• Sacramento experiences valley inversions but less severe than Central Valley
• Summer heat creates moderate ozone

Wildfire Season: California's Biggest Air Quality Challenge

Wildfires have transformed California air quality. What was once a 2-3 month concern is now a 5-6 month reality affecting the entire state.

Wildfire Season Timeline

• May-June: Fire season begins, particularly in Southern California
• July-August: Peak fire activity statewide
• September-October: Continues, often the worst smoke impacts
• November: Late-season fires still possible; improved conditions
• December-April: Typically fire-free (but exceptions occur)

How to Prepare for Wildfire Season

Before Fire Season (March-April):

• Purchase quality HEPA air purifiers for bedrooms and main living areas
• Stock up on N95 or KN95 masks (supply shortages common during fires)
• Replace HVAC filters with MERV 13+ rated filters
• Identify which rooms can be sealed as "clean air rooms"
• Create evacuation plan including go-bags with medications, documents
• Sign up for local air quality alerts

During Fire Season:

• Check AQI daily, multiple times per day during active fires
• Run air purifiers continuously when AQI > 100
• Keep windows closed during smoke events
• Limit outdoor activities when AQI > 150
• Wear N95 masks if you must be outside during poor AQI
• Consider temporary relocation if smoke is severe and prolonged

Best and Worst Cities for Air Quality

Best Air Quality (Usually)

• Coastal cities: San Francisco, Santa Cruz, Monterey, San Diego (coastal areas)
• North Coast: Eureka, Arcata (when not affected by distant wildfires)
• Generally benefit from marine air and better dispersion
• Still affected by wildfire smoke during major events

Worst Air Quality (Consistently)

• Central Valley cities: Bakersfield, Fresno, Visalia, Modesto
• Inland Southern California: Riverside, San Bernardino, parts of Los Angeles
• Geography and emissions combine for poor conditions
• Frequently exceed EPA standards

Month-by-Month Air Quality Calendar

January-February: Central Valley inversions; wood smoke; generally good elsewhere

March-April: Best months statewide; spring cleaning of air; minimal wildfires

May-June: Fire season begins; increasing ozone in inland areas; still relatively good

July-August: Peak wildfire and ozone season; expect multiple poor AQI days

September-October: Worst wildfire smoke historically; highest AQI readings

November-December: Fires decrease; winter inversions begin in valleys

Tips for California Residents

Everyone Should:

• Bookmark AirQuality.directory or AirNow.gov for quick AQI checks
• Set up air quality alerts for your area
• Invest in at least one quality HEPA air purifier
• Stock N95 masks and replace HVAC filters regularly
• Plan outdoor activities around AQI forecasts

If You Live in High-Risk Areas (Central Valley, Inland SoCal):

• Multiple air purifiers for bedrooms and living spaces
• Consider whole-house air filtration system
• Seal air leaks to prevent outdoor pollution from entering
• Have indoor exercise alternatives ready
• Work with healthcare providers if you have respiratory conditions

During Wildfire Season:

• Check AQI before every outdoor activity
• Keep "go-bags" packed in case of evacuation orders
• Create clean air room for extended smoke events
• Monitor both AirNow and PurpleAir for most accurate data
• Consider temporary relocation during severe, prolonged smoke

Resources for California Residents

Official Sources:

• California Air Resources Board (CARB): State air quality regulations and data
• Bay Area Air Quality Management District: Bay Area specific forecasts and alerts
• South Coast AQMD: Southern California air quality management
• San Joaquin Valley APCD: Central Valley air quality information

Real-Time Data:

• AirQuality.directory: Fast lookups for any California address
• AirNow.gov: Official EPA data and forecasts
• PurpleAir: Hyperlocal PM2.5 data from crowd-sourced sensors
• Fire.airnow.gov: Wildfire smoke map during fire season

The Future of California Air Quality

California faces ongoing challenges but also leads the nation in air quality improvements:

Reasons for concern:

• Climate change extending and intensifying wildfire seasons
• Drought conditions increasing dust and fire risk
• Population growth adding vehicle emissions

Reasons for hope:

• Aggressive clean vehicle standards and EV adoption
• Renewable energy expansion reducing power plant emissions
• Air quality has dramatically improved since 1970s despite population growth
• Ongoing research and policy innovation