A few years ago, researchers at UCLA set out to answer a specific question: when a household switches from gas appliances to electric, what actually changes in the air inside?
To find out, they measured it. Participating households hosted three monitors each, two indoors and one outdoors, for two-week periods in both winter and summer. The result was one of the more detailed real-home data sets on residential combustion air quality produced in California.
The project runs through UCLA's Institute of the Environment and Sustainability. The formal title references disadvantaged communities because that is where the policy question lives. But the chemistry it documents is the same in any home with a gas appliance, including the well-maintained ones.
What Combustion Produces Inside a Home
Natural gas burns. When it burns inside a conditioned space, it produces nitrogen dioxide (NO₂), carbon monoxide (CO), formaldehyde, and fine particulate matter (PM2.5). These are not trace residues from equipment failure. They are the byproducts of normal, intended operation.
The findings from the UCLA research and related California public health work are direct. After cooking for one hour with a gas stove and oven, peak NO₂ levels inside the kitchen exceeded California and federal outdoor air quality standards in more than 90 percent of the homes modeled. Homes with gas stoves had nitrogen dioxide concentrations between 50 and 400 percent higher than comparable homes with electric stoves.
That is indoor pollution generated during a normal dinner. Not a malfunction. Not a leak. A Tuesday.
The Appliances That Get Less Attention
The range gets most of the attention in this conversation, but the UCLA research frames the question more broadly: gas furnaces, water heaters, and clothes dryers all contribute to a home's total combustion load.
Gas furnaces are particularly relevant because they operate directly inside the conditioned envelope. A properly installed furnace with an intact heat exchanger vents combustion gases outside. An aging unit with a cracked exchanger or compromised flue introduces those gases into the air stream the home's HVAC system then distributes throughout every room.
Most homeowners do not know the condition of their heat exchanger. It is not something visible during a filter change. It requires either a direct inspection or a CO sensor near the air handler to flag when something is wrong. It is exactly the kind of thing a Baseline assessment surfaces, and exactly the kind of thing that goes undetected in homes that have never been measured.
Ventilation Is the Main Variable
The UCLA research identifies ventilation as the primary determinant of how much combustion pollution accumulates. Homes with better mechanical ventilation, more air changes per hour, and appropriate filtration show meaningfully lower buildup from the same appliances.
In the Los Angeles context, this is not a simple equation. Opening a window or running an exhaust fan introduces outdoor air, but LA's ambient air quality is its own variable. On days when outdoor PM2.5 is elevated, pulling in outside air to dilute indoor combustion products is a tradeoff, not a clear win.
This is why point-in-time readings tell only part of the story. The UCLA study's strength was its continuous two-week measurement window. That duration captures peak exposure periods: a Sunday morning with bacon and toast, an evening with four burners running, a cold week when the furnace cycles every 20 minutes. Single snapshots do not show that. Continuous sensors do.
What This Means for Your Home
If your home has gas appliances and the air inside has never been measured, you have no exposure baseline to work from. You do not know whether your kitchen is ventilating adequately after cooking, whether your furnace is combusting cleanly, or whether your filtration system is handling what the appliances produce.
The pollutants the UCLA research documents are the same ones that appear in a Baseline assessment: NO₂, CO, PM2.5, CO₂, and ventilation rate. When any of these are elevated, the assessment report identifies the likely source and ranks it by priority. A cracked heat exchanger shows up differently in the data than poor range ventilation. The pattern matters.
For homes with ongoing gas appliance use, a monitoring plan provides the continuous visibility that a single assessment cannot. A sensor positioned near the kitchen and mechanical room gives a running picture of what combustion events actually produce in that specific home, under real conditions, across seasons.
Measurement comes before optimization. That is true whether you are thinking about electrification, improving your ventilation, or simply trying to understand what is happening in a home you have lived in for years without ever looking.
Source: The Healthy Home Study, UCLA Institute of the Environment and Sustainability