Checking air quality in Los Angeles takes about a minute once you know where to look: the government AQI map for your address, a neighborhood sensor for your street, and, if you want to know what you are actually breathing, a small monitor inside the house. The number to learn is the AQI. The rest of this post is where to get it, what it means for Pasadena, CA and the valleys, and what your heating and cooling system can do about it.
What the AQI number means
The Air Quality Index is the EPA's scale that turns pollutant readings into one number and a color, in six categories. From AirNow.gov:
- 0 to 50, green, Good. Little or no risk.
- 51 to 100, yellow, Moderate. Acceptable, but people unusually sensitive to air pollution may notice.
- 101 to 150, orange, Unhealthy for Sensitive Groups. Children, older adults, and people with heart or lung conditions may be affected.
- 151 to 200, red, Unhealthy. Some of the general public may be affected; sensitive groups more so.
- 201 to 300, purple, Very Unhealthy. Health alert for everyone.
- 301 and up, maroon, Hazardous. Emergency conditions.
The AQI covers five pollutants: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. For day-to-day decisions in LA, two of them matter most. Ozone builds on hot, still summer afternoons and is worst in the inland valleys, where the sea breeze pushes it against the San Gabriel Mountains. PM2.5, the fine particles, is the wildfire-smoke pollutant, and it is what turns the sky brown in fall when the Santa Ana winds are blowing and something is burning.
Where to check outdoor air quality
- AirNow.gov. The EPA site. Enter a ZIP code for the current AQI and a forecast. Its app does the same.
- South Coast AQMD. The local air district. Its current AQI map at aqmd.gov/aqimap takes a street address, and its iOS and Android app can be set to notify you when the air where you are reaches a level you pick. AQMD also emails air quality advisories during wildfires, dust storms and no-burn days; you sign up from the Air Alerts page on aqmd.gov.
- AirNow's Fire and Smoke Map. During wildfire season this is the one to watch, since it layers smoke plumes over the monitors.
- Neighborhood sensors. Low-cost community sensors such as PurpleAir units are dense across LA and show street-level PM2.5 that the official monitors, which are miles apart, can miss. They read high in humid air and are not calibrated like the official network, so use them for trends and compare to the AQMD reading before you make a decision on them.
- Local news and weather apps. Fine for a headline number, usually pulled from one of the sources above.
On a smoke day, check in the morning and again mid-afternoon. Wind shifts move a plume across the basin in hours.
Why LA air behaves the way it does
The basin is a bowl. Mountains on three sides trap what the freeways, the ports, and industry put out, and a temperature inversion, warm air sitting on cooler air, holds it near the ground. Summer sun cooks vehicle emissions into ozone, and the sea breeze carries it inland, which is why Pasadena and the San Fernando Valley often read worse than the coast on a hot afternoon. In fall the pattern flips: Santa Ana winds clear the smog and bring in smoke.
Monitoring the air inside your house
Outdoor readings tell you what is trying to get in. An indoor monitor tells you whether it did. A basic consumer monitor reads PM2.5, and many add carbon dioxide, humidity, and temperature. Place it at breathing height in a room you use, away from the stove, a window, a supply register, or a humidifier, all of which give false spikes.
What to watch:
- Indoor PM2.5 that climbs with the outdoor reading, windows closed, means the house is leaky or the filtration is not keeping up.
- PM2.5 spikes at dinner time are the stove. Run the range hood.
- CO2 that rises all evening with people home means the house is tight and could use fresh air on a clean day.
People with asthma or allergies often notice the difference between a PM2.5 reading in the teens and one in the 50s before the monitor does.
What your HVAC system can do on a bad-air day
The EPA's guide to air cleaners in the home says most furnaces and HVAC systems can take a MERV 13 filter without equipment problems, and to use the highest-rated filter the fan and filter slot will accommodate. A MERV 13 pleated filter captures a meaningful share of smoke particles that a cheap fiberglass filter passes straight through. On smoke days:
- Put a MERV 13 filter in, if the slot takes one, and check it often. Smoke loads a filter fast.
- Run the fan on "on" for a few hours so the air keeps passing through the filter even when the system is not cooling.
- Keep windows and doors closed and skip the range hood's outdoor-air intake if it has one.
- Run a portable HEPA cleaner in the room you spend the most time in. The EPA guide sizes them by clean air delivery rate (CADR): pick one rated for your room size, and go higher for tall ceilings.
- If the readings inside stay high anyway, the air is coming in somewhere other than the filter. In older Pasadena, Altadena, and San Fernando Valley homes that is usually attic ductwork with open joints pulling 130°F attic air, and dust, into the return side. Sealing or replacing leaking ducts fixes a problem no filter can.
A whole-home air purifier mounted at the air handler filters every cubic foot the system moves, which matters more here than in places without a smoke season.
What you can check yourself
- Look up today's AQI for your ZIP code, and set the notification level in the South Coast AQMD app.
- Find the filter slot and read the size printed on the frame. Check whether a 1-inch or a thicker media filter fits.
- Replace the filter if you cannot see light through it.
- Set the fan to "on" during smoke and back to "auto" after.
- Put an indoor PM2.5 monitor in the main living area and compare it to the outdoor reading with the windows shut.
- Walk the attic hatch: if you can see a supply or return duct hanging loose or crushed, note it for the tech.
When to call a licensed tech
Call when the filter slot only takes a thin filter and you want a deeper media cabinet, when a MERV 13 filter drops your airflow noticeably (the blower may need adjustment or the return may be undersized), when indoor readings stay high with windows closed and a fresh filter, or when you want a whole-home purifier sized to the air handler. A maintenance visit also cleans the coil and blower, which are where years of unfiltered dust end up.
Common questions
What AQI is too high to run outside?
By the AirNow categories, sensitive groups should cut back at orange (101 to 150), and everyone should at red (151 and up). Move a workout indoors or to the morning, when ozone is lowest.
Are the neighborhood sensor readings accurate?
They are good for trends and for spotting a plume on your street before the official monitor sees it. They are not calibrated the way the AQMD network is and read high in humid air. Compare them to the official map before acting on a single reading.
Does a MERV 13 filter help with smoke?
Yes, for the particle side of smoke, and the EPA guide recommends MERV 13 or higher where the system allows it. It does not remove the gases in smoke; for those, closed windows and time do more than any filter.
If you want the filtration side handled, book a $69 diagnostic. You are told the price before we schedule, and it is credited toward the work if you go ahead. Call Boost Services Heating and Cooling at 818-277-5056 or send us a message and we will usually be out within a few hours.
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.