Los Angeles Air Quality
Los Angeles County, California · 13.2M population
Moderate
2024 avg
98
Annual AQI
2024 average
11.5 μg/m³
PM2.5
2.3× WHO limit
142
Clean Days/yr
AQI ≤ 50
87
Unhealthy Days/yr
AQI > 100
Monthly AQI Pattern
Monthly average AQI — peaks July–September (ozone + wildfire season)
Why Is LA So Polluted?
The Basin Trap
The Los Angeles Basin is surrounded on three sides by mountains — the San Gabriels, Santa Monicas, and Santa Anas. This geography creates a natural pollution trap. Temperature inversions (warm air sitting above cool air) cap the basin at low altitude, preventing pollutants from dispersing vertically. The ocean to the west provides the only natural ventilation.
15 Million Vehicles
The LA metro has one of the world's highest vehicle-to-person ratios — roughly one car per adult. Freeways carrying 500,000+ vehicles per day emit NOx and VOCs that cook into ozone in summer sunlight. Despite California's strict emission standards (ZEV mandate), the sheer number of vehicles keeps ozone high.
Port Diesel
The Ports of Los Angeles and Long Beach together form the busiest container port complex in the Western Hemisphere. Ship engines, drayage trucks, and cargo equipment emit massive amounts of diesel PM2.5 and NOx. Port-adjacent communities (San Pedro, Wilmington, Long Beach) face some of the highest diesel exposure in California.
Wildfires: Escalating Threat
Climate change has dramatically increased wildfire frequency and intensity in Southern California. The January 2025 Palisades and Eaton Fires burned through Pacific Palisades and Altadena, pushing AQI above 200 across the LA Basin. Santa Ana wind events (October–December) create extreme fire conditions with single-day AQI spikes above 300.
LA Air Quality by Season
Winter (Dec–Feb)
Temperature inversions trap PM2.5 and local vehicle exhaust. Rain events dramatically improve air quality. Rare wildfire events possible in dry years.
Spring (Mar–May)
Improving from winter inversions. Coastal 'June Gloom' (overcast marine layer) actually traps pollutants near the surface some mornings. Ozone starting to build.
Summer (Jun–Aug)
Peak ozone season. Intense UV and heat convert vehicle emissions to ground-level ozone. Inland valleys (San Gabriel, San Fernando) hit AQI 150+ regularly. Limit outdoor exercise to morning hours.
Fall (Sep–Nov)
Wildfire smoke combines with late-season ozone. Santa Ana winds (Oct–Nov) create extreme fire weather. Individual wildfire events can push AQI above 200 for multiple days.
Frequently Asked Questions
What causes LA's air pollution?
LA's pollution comes from four main sources: (1) vehicle emissions from 15+ million registered vehicles in the metro area, (2) port diesel from the Ports of LA and Long Beach (world's busiest container complex), (3) refinery operations in the South Bay, and (4) wildfire smoke — increasingly the largest PM2.5 source. The mountain-enclosed basin traps all of these emissions.
When is LA air quality worst?
July and August are the worst months for ozone — intense heat and sunlight cook vehicle exhaust into ground-level ozone. September and October add wildfire smoke. Santa Ana wind events (October–December) can produce the single worst air quality days of the year by spreading wildfire smoke. December–February can bring winter inversions trapping PM2.5.
Is LA air quality getting better?
Yes — dramatically over decades, but the trend is now mixed. Ozone has decreased ~80% since the 1970s due to vehicle emission standards and fuel reformulation. PM2.5 has dropped significantly too. However, wildfire smoke has reversed some of this progress — the 2020 wildfire season and 2025 LA fires show how climate change threatens these gains.
Which LA neighborhoods have the worst air quality?
Communities near the Port of Los Angeles (San Pedro, Wilmington), the I-710 freeway corridor (Long Beach, Huntington Park), and the eastern San Fernando Valley (Arleta, Pacoima) face the worst chronic diesel and traffic pollution. The San Gabriel Valley (Pomona, Ontario) has the worst ozone due to being downwind and inland. Environmental justice is a major issue in LA air quality policy.
What is the AQI Action Day system in LA?
The South Coast Air Quality Management District (SCAQMD) issues Action Day advisories when AQI is forecast to exceed 100. On these days, residents are asked to avoid gas-powered lawn equipment, refueling cars, and using wood-burning fireplaces. Schools are notified to limit outdoor physical activity for students.
Health Advisory for LA Residents
On high AQI days (100+)
- Exercise outdoors in early morning (6–9am) before ozone builds
- Keep windows closed on hot, smoggy days
- Use air purifiers with HEPA+carbon filters indoors
- Wear N95/KN95 masks if AQI exceeds 150
- Sensitive groups should stay indoors above AQI 100
Wildfire smoke events (AQI 150+)
- Shelter indoors; close all windows and doors
- Run HVAC on recirculate — do not draw outside air
- N95 masks are effective against smoke particles outdoors
- Reduce indoor sources: no candles, no cooking smoke
- Check SCAQMD and AirNow for hourly updates