The Bay Area Indoor Air Quality Guide
Smoke, damp, pollen and older homes — what actually works
Bay Area indoor air has its own particular set of problems: smoke seasons that can last weeks, coastal damp that feeds mould, long pollen seasons, and a lot of housing stock built before anyone designed for filtration. This guide covers what actually moves the needle, in the order worth doing it.
- Work in order: control the source, then filter, then ventilate, then manage humidity. Buying equipment first is how people spend money without feeling a difference.
- MERV 13 is the practical target for wildfire smoke, but only if your system can move air through it — have static pressure checked first.
- Keep indoor relative humidity roughly between 30% and 50%; sustained higher levels invite mould.
- On smoke days, recirculate and filter rather than ventilate. Opening windows brings the smoke indoors.
- Leaking ducts pull air from attics and crawlspaces into the air you breathe, which is an air quality problem as much as an efficiency one.
Why this is a local problem
The EPA has long noted that indoor air can be several times more polluted than outdoor air, and the Bay Area adds its own contributors:
- Wildfire smoke. Fine particulate travels a long way and gets indoors through every gap in the envelope. See how wildfire smoke affects HVAC systems and our smoke season guide.
- Coastal moisture. Mild damp air is comfortable and also ideal for mould in bathrooms, closets and crawlspaces.
- Long pollen season. Our mild climate stretches the growing season, and the allergy load with it.
- Older housing. Many homes were built for a climate that rarely demanded cooling, which means limited filtration, undersized returns and, often, no mechanical ventilation.
- Microclimates. A fog-belt house and an inland valley house three miles apart do not have the same problem to solve.
The four levers, in order
1. Source control
Cheapest and most effective, and consistently skipped. Vent cooking and bathroom moisture to the outside rather than recirculating it. Fix the leak feeding the damp patch. Store solvents and paint outside the living space. Manage pet dander at the source. No filter compensates for a source you have left running.
2. Filtration
MERV rates how much a filter captures: higher numbers catch smaller particles. MERV 8 handles dust and lint; MERV 13 captures a meaningful share of the fine particulate that makes up wildfire smoke. That makes MERV 13 the sensible target here — with one important caveat below.
Filters only work when air passes through them, so a system that runs in short bursts filters less. Continuous low-speed fan operation on a variable-speed system filters far more air per day than an on-off system does.
3. Ventilation
Filtration removes particulate; ventilation dilutes what filtration does not catch — carbon dioxide, cooking odours, volatile compounds off new furnishings. On a normal day, outdoor air is the easy answer. On a smoke day it is exactly wrong, because you are importing the problem. That conflict is why balanced mechanical ventilation with good filtration is increasingly specified in California: it lets you ventilate without opening a window.
4. Humidity
Target roughly 30–50% relative humidity. Too dry is uncomfortable; sustained too damp grows mould and dust mites. Humidity also changes how temperature feels, which is why an oversized system that cools fast but runs briefly can leave a house cold and clammy. See how humidity affects HVAC performance.
Equipment that helps, and what it is for
| Option | Good for | Worth knowing |
|---|---|---|
| Better central filter (MERV 11–13) | Whole-home particulate, smoke | Only if static pressure allows; check first |
| Portable HEPA purifier | One room, quickly | Size to the actual room; most are undersized for the space they are put in |
| Whole-home media cabinet | High filtration with less airflow penalty | Needs space at the air handler and correct return sizing |
| Balanced mechanical ventilation | Fresh air without opening windows | Filter the incoming air, or it defeats the purpose on smoke days |
| Dehumidification | Persistent damp, mould risk | Address the moisture source first |
| Duct sealing | Stops attic and crawlspace air entering the airstream | Usually a bigger win than duct cleaning |
For product-level comparisons see best air purifiers for Bay Area homes and purifiers to pair with your HVAC system.
Your ducts are an air quality component
People think of ducts as an efficiency issue. They are also an air quality issue. A duct system under negative pressure pulls air from wherever it leaks — attic, crawlspace, wall cavity, garage — and delivers it to your registers along with whatever is in it: dust, insulation fibres, vermin residue, garage fumes. Sealing that is one of the highest-value air quality interventions in an older home. See why duct sealing matters, and if the ducts are past saving, our ductwork replacement cost guide. Because attic ducts sit in the hottest, dustiest part of the house, attic condition is part of the same conversation.
A smoke-season routine that works
- Fit the best filter your system supports before the season, not during it.
- When the AQI climbs, close up, set the system to recirculate, and run the fan continuously.
- Add a correctly sized portable HEPA unit in the bedroom — the room you spend eight hours in matters most.
- Skip anything that adds indoor particulate: candles, wood fires, frying.
- After the smoke clears, change the filter. It has been working hard.
- Have the system checked if airflow felt weak or the unit struggled — see our maintenance services.
When to bring in a technician
Call if airflow is noticeably weak at the registers, if there is a persistent musty smell when the system runs, if some rooms are always worse than others, if you can see dust discharging from registers, or if you want to know what filtration your system can actually carry before you buy anything. Those are all measurable rather than matters of opinion.
Related reading: signs poor air quality is affecting your home, ten ways to improve indoor air quality, and whether HVAC systems can improve air quality.
Get a free quote from a licensed Bay Area technician →
Frequently asked questions
What MERV filter should I use in the Bay Area?
MERV 13 captures a meaningful share of fine smoke particulate and is the usual recommendation for wildfire season — but only if your system can move air through it. Older equipment and undersized returns can struggle, which reduces airflow and can cause the coil to freeze. Have the static pressure checked before committing to a higher-MERV filter.
Does running the fan help on smoky days?
Yes, provided you have a good filter installed and the system is set to recirculate rather than draw in outside air. Circulating indoor air through a MERV 13 filter reduces indoor particulate. Running the fan with a cheap filter accomplishes very little.
Do air purifiers actually work?
A properly sized HEPA unit measurably reduces particulate in the room it is in. The common mistakes are buying a unit rated for a far smaller room than the one it is placed in, and expecting it to address gases or odours it was never designed for.
What indoor humidity should I aim for?
Roughly 30–50%. Below that, you get dry air and irritated sinuses; sustained above it, you risk mould and dust mites. Coastal Bay Area homes tend to fight the high end, inland homes the low end during heat waves.
Will duct cleaning fix my air quality?
Sometimes, but it is oversold. Duct cleaning helps when there is visible debris, evidence of vermin, or contamination after construction. It does not fix a leaking duct system, and sealing leaks usually does more for both air quality and comfort.
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