Improving Indoor Air Quality When LA Air Is Bad

The most effective response to a Los Angeles smoke or ozone event is layered: a MERV-13 or higher HVAC filter running in recirculation mode, a portable HEPA in the room you sleep or work in, a tightly sealed envelope, and awareness of when outdoor exposure is the real problem. Each layer catches what the previous one misses. Together they can keep indoor PM2.5 levels a fraction of what's outside — even during multi-day wildfire events.

Understanding what's actually in LA air

Southern California has three distinct indoor-air challenges:

1. Ozone — a summer smog issue driven by heat + traffic + industrial emissions. Indoor ozone infiltrates from outside; there's not much a homeowner can do to remove it, but envelope tightness helps.
2. PM2.5 particulates — fine particles from traffic exhaust, industrial sources, and wildfire smoke. Filterable with a high-enough-MERV filter.
3. Wildfire smoke events — episodic but severe. PM2.5 concentrations during major fires can hit "hazardous" ranges (AQI 300+) for days.

MERV ratings, plainly

MERV (Minimum Efficiency Reporting Value) rates a filter's ability to capture particles at various sizes. Higher = finer particles captured.

• MERV 6–8 — the cheap builder-grade filter. Catches lint and pet hair. Does nothing for smoke.
• MERV 11 — a common upgrade. Catches dust and some pollen. Marginal on smoke.
• MERV 13 — the smoke floor. Captures PM2.5 including combustion particles. Widely recommended by air-quality agencies during wildfire events.
• MERV 15–16 — hospital-grade. Excellent smoke filtration. Some residential blowers can't handle the static pressure — check with a technician before installing.

Note: MERV rating alone isn't enough. A high-MERV filter that's clogged or oversized for the blower reduces airflow to the point of hurting the system. Match the filter to your HVAC's static-pressure spec.

The smoke-event protocol

When AQI in your area hits 100+ (or you can smell smoke inside):

1. Close all windows and doors.
2. Turn off any fresh-air damper on the HVAC (some systems have one).
3. Set the HVAC to recirculation mode. Do NOT turn it off — running the filter continuously is the whole point.
4. Set the fan to "on" instead of "auto" so it filters continuously even when the thermostat is satisfied.
5. Run a portable HEPA purifier in the primary bedroom or living space.
6. Avoid activities that add particulates: no candles, no gas stove without the range hood, no vacuuming without a HEPA-filtered vacuum.
7. If AQI is above 150, minimize going outside. Wear an N95 if you must.

Why envelope tightness matters here

A leaky home pulls in smoke through every gap in the envelope. Every one of those infiltration paths is also an insulation loss and an energy-bill hit. Attic air sealing, weather-stripping, and duct sealing pay off in three ways:

1. Lower cooling and heating bills
2. Better filtration effectiveness (less infiltrated smoke for the filter to catch)
3. Faster smoke clearance after an event

See our under-insulated home article for the diagnostic checklist and the summer cooling article for the fix order.

Portable HEPA sizing

Portable HEPA purifiers are rated in CADR (Clean Air Delivery Rate) and covered room size. Rule of thumb: match the CADR (in CFM) to the room's square footage for at least 4 air changes per hour. A 200 sq ft bedroom wants 130+ CADR; a 400 sq ft open living/kitchen wants 260+. Bigger = quieter at any given air change rate.

What doesn't help

• Standing fans without filtration — they just move smoke around.
• Boiling water or bowls of vinegar — no evidence they reduce particulates or ozone.
• Cracking a window "for fresh air" — during a smoke event, that's the opposite of what you want.
• Cheap ozone-generator "purifiers" — they add ozone rather than remove it, which is the opposite of what you need in an already ozone-heavy air shed.

FAQ