What is the biggest source of indoor air pollution in a house?

In the homes we measure, combustion inside the envelope: a gas range, and more often than owners expect a gas water heater or furnace with a compromised exhaust path. It ranks first because it is both common and continuous, and because the fix is mechanical rather than a filter.

Does a gas stove raise CO2 in the house?

Yes. Burning methane produces carbon dioxide, so a lit burner is a direct source and the reading climbs while you cook. At household levels carbon dioxide is a signal rather than a hazard. During cooking it is tracking combustion rather than ventilation, and it is the easiest way to tell whether a range hood is actually capturing what the burner produces.

Does an air purifier fix indoor air pollution?

It handles the particle fraction in the room it is standing in, which covers part of cooking and part of outdoor infiltration. It does nothing about nitrogen dioxide from a burner, formaldehyde from cabinetry, or garage air being pulled through a shared wall.

Is indoor air really worse than outdoor air in Los Angeles?

Not automatically. EPA reference data shows that homes without combustion appliances run about half the outdoor nitrogen dioxide level, and homes without strong particle sources match or beat outdoor particle levels. Specific sources inside the house are what make the difference, which is why measuring beats assuming.

How do you tell which source is causing a reading?

By where and when. We read all six parameters room by room and watch how they move during the visit. A spike that tracks the air handler points at the ducts or the garage, one that tracks the kitchen points at the burners, and carbon dioxide that will not come down points at ventilation.

Sources of Indoor Air Pollution, Ranked

The main sources of indoor air pollution in a Los Angeles home are inside the house: combustion appliances, an attached garage, moisture, building materials and cooking. Outdoor air gets in too, and during fire season it dominates. In an ordinary week it is fourth or fifth in line, not first.

That ordering matters because it decides what you buy. A homeowner who believes the problem arrives from outside buys a filter. A homeowner who finds out the problem is a water heater pulling flue gas back into a hallway closet buys a different water heater. Both spend money. Only one of them changed the air.

Ranking the Sources of Indoor Air Pollution

The list below is ordered by how often each one turns out to be the dominant problem in the homes we measure, not by how dangerous it is in theory. The EPA's own reference data makes the same point from the other direction. In homes without combustion appliances, nitrogen dioxide runs about half the outdoor level. In homes without strong particle sources, particle levels are the same as or lower than outside. A house is not inherently dirtier than the street. Specific things in it are.

1. Combustion Inside the Envelope

Gas ranges, gas water heaters, gas furnaces, wood fireplaces. Anything burning fuel inside the pressure boundary of the house puts nitrogen dioxide, carbon monoxide and ultrafine particles into the room you are standing in, and the exhaust path is the only thing between that and your lungs.

A 2024 Stanford study in Science Advances measured this across American homes and found that typical gas and propane stove use raises a household's annual nitrogen dioxide exposure by about 4 parts per billion, which the authors put at roughly three quarters of the way to the level the World Health Organization treats as unsafe in outdoor air. Concentrations crossed health benchmarks in bedrooms within an hour of cooking and stayed elevated after the burners were off, including in bedrooms a long way from the kitchen.

The appliance that gets missed is the water heater. A gas water heater in a closet, with a bath fan and a clothes dryer running, can be pulled into backdraft, which sends flue gas into the house every time the house goes negative. The test for it is 30 minutes with a combustion analyzer and a manometer, run under worst case depressurization rather than on a calm day. Most homes in the Valley have never had it done.

Carbon dioxide belongs in this section rather than in the ventilation one. Burning methane produces carbon dioxide and water vapor, so a lit burner is a direct CO2 source in the room. It climbs reliably while a gas range is running, and that reading means something different from the same number at six in the morning in a closed bedroom. Overnight, CO2 is a ventilation marker: it tells you how fast a room exchanges air. During cooking it is a combustion tracer: it tells you the burner's exhaust is still in the kitchen instead of outside it.

The number itself is not the hazard. At the levels homes reach, carbon dioxide is a signal rather than a toxin, and we treat 800 parts per million as the point where ventilation needs attention. What makes it worth watching while you cook is that it moves with the things you actually care about, nitrogen dioxide and ultrafine particles, which are harder to measure and have no smell. A 2024 study in Atmospheric Environment measured CO2 emission rates from a gas range directly and found the oven emitted gaseous pollutants at 2.6 to 29 times the rate of the stovetop burners. That matches the field pattern: a two hour roast moves the numbers more than a quick saute.

It is also the cheapest way to audit a range hood. If the combustion markers climb the same way with the hood running as with it off, the hood is not capturing. In Singer's study of nine California homes the best performing hood cut concentrations by 80 to 95 percent, while four of the nine kitchens still exceeded the 100 parts per billion one hour standard for nitrogen dioxide. Hoods vary that much.

2. The Attached Garage

This is the source homeowners never nominate and it is consistently near the top. A car, a mower, paint, solvents and fuel cans sit on the other side of a shared wall and a door that is rarely gasketed. When the air handler runs, or the dryer runs, or a bath fan runs, the house goes slightly negative and pulls garage air through every gap in that wall.

Health Canada ran the intervention in 33 Ottawa homes. Installing an exhaust fan vented outdoors in the attached garage cut gasoline related VOCs (benzene, toluene, ethylbenzene and xylene) by 43 to 62 percent in the living room, by about 80 percent inside the garage, and cut carbon monoxide by 23 percent in the house. Nothing inside the living room was touched. The source was outside the living space the whole time.

Door gaskets, a bottom sweep, foam at the pipe and wire penetrations, and a check that no HVAC return is drawing from the garage or from a closet sharing a wall with it, get most of that result without a fan at all.

3. Moisture, and What Grows Because of It

Mold is a moisture problem wearing a biology costume. The EPA's guidance is to keep indoor relative humidity below 60 percent and ideally between 30 and 50 percent, because above 60 the water vapor in the air alone supplies enough moisture for growth. Los Angeles homes swing hard in both directions: dry during Santa Ana conditions and long cooling runs, damp in a bathroom whose fan moves no measurable air, or over a crawl space with a torn vapor barrier.

Soil gas belongs in this section. A crawl space or a slab is a pathway, and the stack effect pulls whatever sits under the house upward through every penetration in the floor. The EPA puts the average indoor radon level at 1.3 picocuries per liter against about 0.4 outdoors, which is the entire argument for testing instead of assuming. Radon testing is its own measurement, separate from an air quality assessment.

4. Building Materials and Furnishings

Formaldehyde from cabinetry and engineered flooring, VOCs from fresh paint and adhesives, off-gassing from new upholstery. This is the source that newer and recently renovated homes reliably have more of, which surprises owners who expect a new build to measure better than a 1965 ranch. Tighter construction and new materials arrive together.

The regulatory floor has moved, and it is worth knowing where it sits. Composite wood panels sold in the United States have had to be certified under TSCA Title VI since 2018 and 2019, and the EPA sets those limits at the same levels as California's CARB Phase 2 rule. So the formaldehyde problems we still find are concentrated in older cabinetry, in uncertified panels, and in the first months after an install, when emission rates are highest and the house tends to be closed up.

5. Cooking, Cleaning and the Episodic Spikes

Searing a steak produces fine particles on an induction range, not only on a gas one. Oven cleaner, aerosols, fragrance and nail products produce VOC spikes that read high and then decay. These are real and they are brief. They matter most in a house that has no way to clear them, which is why the answer here is ventilation rather than avoidance, and why the next section is about the fan you already own.

6. Outdoor Air Getting In

Traffic particles, pollen, and the one that is not ordinary: wildfire smoke. During a smoke event outdoor air is the dominant source and nothing else on this list is close. That is a filtration and envelope problem for a week or two at a time, worth being ready for before the sky turns. It is not the reason a Tarzana bedroom reads 1,500 parts per million of carbon dioxide at six in the morning in March.

The Ventilation You Already Own

The range hood is usually the highest flow fan in the house. Running it with a window open on the far side is the fastest way most homeowners can clear VOCs, carbon dioxide and nitrogen oxides out of a living space. Not only at the cooktop. As whole house exhaust. It is a makeshift ERV, and in this climate the energy penalty for most of the year is small enough to ignore.

Exhaust only ventilation is a legitimate strategy rather than a workaround. ASHRAE 62.2, the residential ventilation standard California builds to, lists exhaust, supply and balanced as the ways to meet whole house ventilation, and the hood is already exhaust, already ducted outdoors, already wired. What it is not is balanced, and that is the condition attached to it.

Pulling air out pulls air in somewhere. Depressurization is the mechanism behind two of the sources ranked above: a negative house draws garage air through the shared wall, and it is what reverses an atmospherically vented water heater or furnace. The model residential code sets the make-up air trigger at 400 cfm (IRC M1503.4), roughly the point where depressurization gets strong enough to backdraft a naturally drafted flue reliably. Plenty of kitchens here have hoods rated well above that.

So the rule we give homeowners is this. Run it, open a window or a door on the far side of the house while you do, and do not lean on it while a gas water heater or furnace is firing, unless that appliance is sealed combustion or the house has passed a worst case depressurization test. Used that way the hood is one of the most effective tools in the house. Used without a make-up air path, in a home with an atmospherically vented appliance, it can import exactly what you are trying to export. An ERV buys the same dilution without the pressure penalty, which is the honest reason to own one.

One check before you trust yours: rated airflow and delivered airflow are different numbers. A stuck damper, a crushed flex duct or a run that is too long for the fan means the figure on the box is fiction.

What This Changes About the Fix

Three tools cover six sources, and they overlap less than people assume.

Source control is the only thing that solves the first four. Seal the garage, correct the exhaust path, dry the crawl space, vent the range hood outdoors. Ventilation dilutes everything, which makes it the one intervention that helps with sources nobody has identified yet. Filtration handles particles, which is most of categories five and six and none of one through four except the particle fraction.

Which is why the order matters more than the list. Six parameters, PM2.5, VOCs, carbon dioxide, humidity, temperature and nitrogen oxides, read room by room, tell us which of the six sources is running the house in front of us. Ventilation is not metered directly. We read it off how carbon dioxide behaves across the visit, as long as nothing is burning while we read it.

None of this requires alarm. It requires knowing which of six things you are dealing with before you spend anything.

Sources of Indoor Air Pollution in LA Homes, Ranked