Air quality index (AQI⁺) and PM2.5 air pollution in California •
When air quality in your area reaches unhealthy levels, we'll send you a quick alert, along with expert tips on how to reduce your exposure.
Real-time most polluted cities in California
Real-time cleanest cities in California
When outdoor air quality drops, the EPA recommends upgrading to a MERV 13 filter — or the highest-rated filter your HVAC system can handle — to help capture more of the fine particulate (PM2.5) that comes with poor air quality. MERV 13 is the highest rating most residential systems can run without restricting airflow.
AQI stands for Air Quality Index, the EPA's scale for reporting daily air quality. It runs from 0 to 500. The higher the number, the greater the level of air pollution. Values at or below 100 are generally considered satisfactory; values above 100 mean air quality is unhealthy, first for sensitive groups, then for everyone as numbers climb.
The AQI tracks five major pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. The reported AQI value reflects whichever pollutant is highest at that time — most often ozone or fine particles (PM2.5).
Monitoring stations measure pollutant concentrations, and the EPA converts each into a standardized 0–500 score using set breakpoints. Each pollutant gets its own sub-index, and the highest sub-index becomes the reported AQI. That's why a single number can represent different pollutants on different days.
An AQI of 0–50 is rated "Good" — air quality poses little or no risk. 51–100 is "Moderate," acceptable for most people though unusually sensitive individuals may want to limit prolonged outdoor exertion. Anything at or below 100 is generally considered satisfactory air quality.
100 is the dividing line: at or below it, air quality is satisfactory for the general public. From 101–150, air is "Unhealthy for Sensitive Groups" — people with respiratory conditions, older adults, and children. From 151–200, it's "Unhealthy" and everyone may begin to feel effects.
Short-term AQI spikes usually come from wildfire smoke, temperature inversions that trap pollutants near the ground, high-ozone weather (hot, sunny, stagnant days), or heavy traffic and industrial activity. Check a real-time source like the map on this page to see which pollutant is driving today's reading in your area.
Many runners follow the EPA category lines: below 50, no adjustment needed; 51–100, unusually sensitive people may shorten intense workouts; above 100, sensitive groups are advised to reduce prolonged or heavy outdoor exertion; above 150, that advice extends to everyone. Exercise increases how much air you breathe in, which is why thresholds matter more during workouts.
A common rule of thumb is to keep windows open when the AQI is under 100 and close them when it climbs above, especially during wildfire smoke events when fine particles are elevated. When windows are closed, running your HVAC system with a fresh filter helps capture particles circulating indoors.
Public health agencies generally suggest considering a well-fitted N95-style respirator outdoors when the AQI exceeds 150, particularly during smoke events. Cloth and surgical masks don't filter fine particles effectively. Follow guidance from your local health department, which may adjust recommendations during severe episodes.
Check real-time air quality across California in one place. Search your city, see today’s AQI number and color, and adjust plans fast.
In this blog, we’ll give tips on how to use the live map, what AQI levels mean for health, and the simple indoor and outdoor steps to take when air quality gets worse.
Open the live AQI map for your city and look again if the wind or weather shifts.
Shift outdoor effort when AQI rises, especially for children, older adults, and people with asthma or heart or lung disease.
Close the house, set HVAC to recirculate, use the right MERV filter, and run a HEPA purifier in one room.
Compare AQI along commutes and plan stops in cleaner locations.
AQI is a single number that sums up today’s outdoor air and what it means for health. It tracks pollutants like fine particles (PM2.5) and ozone, which can irritate eyes and throat and make breathing harder. Children, older adults, people with asthma, COPD, or heart disease, and people who are pregnant may feel effects sooner and should adjust plans when AQI rises.
A higher AQI means a higher chance of symptoms and a need to adjust plans. Use it to decide when to move workouts or practices, when to keep kids indoors, and when to close windows and set HVAC to recirculate. It also helps you choose the right filter at home and when to run a HEPA purifier in one room. Lower AQI means cleaner air and fewer limits on outdoor time.
Type your city or ZIP code to see current AQI and the main pollutant. Click nearby sites to compare conditions along your commute or school route. Recheck later if wind shifts, a marine layer burns off, or smoke appears on the horizon.
Coast vs inland: Morning marine layers can keep coastal AQI lower while inland basins warm and build ozone in the afternoon.
Basins and valleys: The South Coast Basin and Central Valley can trap pollution on calm days.
Wildfire season: Smoke can travel far from the fire and settle overnight.
Winter inversions: Cool, stagnant air can hold particles near the ground until mixing improves.
0–50: good for outdoor plans
51–100: most people are fine; sensitive groups should watch symptoms
101–150: sensitive groups should reduce hard outdoor activity
151+: everyone should scale back time outside
Morning: check the map before school runs, training, or job sites. Midday: ozone can rise inland; adjust workouts or yard work. Evening: smoke or dust can move in with wind changes; recheck conditions before outdoor events.
Close windows and exterior doors and set HVAC to recirculate. Use a snug, pleated filter that fits the return without gaps and change it on schedule. Keep one room cleaner with a True HEPA purifier and keep that door mostly closed. Reduce extra indoor sources by keeping frying brief and avoiding candles and incense. Run bath and kitchen exhaust fans only as needed so you do not pull outdoor air inside.
Check AQI along your route if you cross coastal and inland zones. In the car, keep windows up and set cabin air to recirculate. Choose fuel and rest stops where the map shows cleaner readings.
Post the day’s AQI before practices and field trips. Move vigorous activity indoors when levels pass 100 and give sensitive students options to sit out or relocate to cleaner spaces.
Choose a filter that fits tightly and suits your system.
MERV 8: everyday dust and lint with low resistance.
MERV 11: better capture for homes with pets or light allergy needs.
MERV 13: finer particles, including wildfire smoke, if your system maintains normal airflow and comfort.
Filterbuy makes pleated filters in MERV 8, 11, and 13 in standard and custom sizes, made in the USA with fast, free U.S. shipping. Auto Delivery keeps replacements arriving on the schedule you set.
One number that reflects current outdoor air quality and its likely health impact. Lower is better.
Look in the morning, then check again if wind shifts, haze appears, or temps jump.
Coastal marine layers, inland heat, valley inversions, and wildfire smoke can move or build within hours.
Often PM2.5 from smoke and dust, and ozone on hot, sunny afternoons.
Use extra care once AQI passes 100. Limit hard outdoor activity for kids, older adults, and people with asthma or heart/lung disease.
Close windows and doors. Set HVAC to recirculate. Use a snug pleated filter and keep one room on a HEPA purifier.
A well-fitting N95 helps with fine smoke particles. Cloth or loose surgical masks are not designed for that.
Check AQI along your route. Keep car windows up and set cabin air to recirculate.
MERV 8 for everyday dust, 11 for homes with pets or light allergies, 13 for smoke or high PM2.5 if your system maintains normal airflow.
Filterbuy offers pleated filters in MERV 8, 11, and 13 in standard and custom sizes with fast, free U.S. shipping and Auto Delivery to stay on schedule.
