GAS STOVES — EVERYDAY CARBON MONOXIDE EXPOSURE?

Gas Cooking Releases Combustion Products Directly Into the Home

Natural gas and propane are burned directly inside the kitchen. This combustion produces several air pollutants, including nitrogen dioxide (NO₂), carbon monoxide (CO), particulate matter, formaldehyde and, as newer research has shown, benzene.

Much of the recent discussion about gas stoves has focused on NO₂ and benzene. Carbon monoxide has received less attention, even though it is a well-established product of gas combustion and is routinely measured in homes using gas appliances.

Unlike an occasional environmental exposure, cooking can occur every day. A person who cooks breakfast, lunch and dinner with gas may therefore experience repeated combustion-related exposure inside the home throughout the week.

How Much Carbon Monoxide Can a Gas Stove Produce?

Carbon monoxide levels from gas appliances vary enormously depending on the burner, adjustment of the flame, ventilation, room size, duration of use and condition of the appliance.

The U.S. Environmental Protection Agency reports:

• Homes without gas stoves: approximately 0.5–5 ppm CO
• Near properly adjusted gas stoves: often 5–15 ppm CO
• Near poorly adjusted gas stoves: 30 ppm or higher

These values are particularly important because they show that measurable carbon monoxide is not limited to a broken appliance or an obvious poisoning event. Even a properly operating gas stove can create concentrations in the several-ppm range near the source.

Real-world measurements support this. A recent residential kitchen study found that gas-stove use produced the largest deterioration in measured kitchen air quality among the activities studied, including an average 24.4% increase in CO during gas-stove use.

A study of hundreds of Canadian homes similarly found that having a gas stove was associated with a 43% increase in peak hourly CO concentrations during winter.

The WHO 24-Hour Guideline Is About 3.5 ppm

The World Health Organization recommends a 24-hour carbon monoxide guideline of 4 mg/m³, which is approximately 3.5 ppm under normal indoor conditions.

This number must be interpreted correctly. A measurement of 10 ppm beside a stove for a short period is not the same as a 24-hour average of 10 ppm. Short cooking peaks and daily averages are different measurements.

But repeated peaks matter because every period of elevated CO contributes to the total daily exposure. If cooking occurs repeatedly, ventilation is poor, or CO from one cooking period has not fully cleared before the next exposure begins, the average exposure over the day rises.

WHO 24-hour guideline: approximately 3.5 ppm CO average

The important question is therefore not only “How high does CO rise while the stove is burning?” but also “How often does this happen, how long does the CO remain in the home, and what is the total exposure over the entire day?”

Carbon Monoxide Has Been Known About for Decades — But Mostly Through Acute Poisoning

Carbon monoxide from gas appliances is not a newly discovered problem. What has changed much less is the way its health significance is usually assessed.

Conventional carbon monoxide toxicology focuses heavily on carbon monoxide binding to hemoglobin, forming carboxyhemoglobin (COHb), reducing the oxygen-carrying capacity of blood and eventually causing tissue hypoxia. This explains the major features of acute carbon monoxide poisoning.

However, carbon monoxide does not interact only with hemoglobin. It is distributed throughout the body and can also bind to other heme-containing proteins, including cytochrome c oxidase in mitochondria. Official toxicological guidance recognizes that non-hypoxic mechanisms can also contribute to carbon monoxide toxicity.

This distinction matters when considering repeated low-level exposure. A concentration may be far below the level expected to cause dramatic acute CO poisoning while carbon monoxide molecules are still entering the bloodstream and tissues.

Biological Effects Have Been Observed Below Traditional Acute Poisoning Levels

Experimental research shows that biological effects of carbon monoxide occur well below the concentrations normally associated with collapse, unconsciousness or fatal poisoning.

In one chronic low-level animal study, exposure to 25 ppm CO increased several markers of oxidative stress. Nitrotyrosine increased significantly, including within blood vessels, indicating that biological changes were occurring during prolonged exposure at this concentration.

Separate experiments using isolated human muscle mitochondria demonstrated direct inhibition of mitochondrial cytochrome c oxidase. After five minutes of experimental CO exposure, cytochrome c oxidase activity decreased by approximately:

• 20% at 50 ppm
• 42% at 100 ppm
• 55% at 500 ppm

These mitochondrial experiments were performed in vitro, so 50 ppm in the experiment cannot be interpreted as equivalent to breathing 50 ppm room air. Likewise, the 25 ppm animal study cannot establish a human household safety threshold.

What these studies demonstrate is something more fundamental: carbon monoxide has direct biological effects beyond simply producing very high COHb during acute poisoning.

Gas-Stove Exposure and Experimental Effect Levels Are Not Completely Separate Worlds

EPA reports 5–15 ppm near properly adjusted gas stoves and 30 ppm or more near poorly adjusted appliances. Experimental biological effects have been demonstrated in the tens-of-ppm range, although under very different experimental conditions.

This does not prove that normal gas cooking at 5 ppm causes the same effects observed experimentally at 25 or 50 ppm. It does show why repeated household exposure deserves attention rather than being dismissed simply because concentrations are far below those associated with immediate carbon monoxide poisoning.

The exposure pattern is also fundamentally different. A poisoning accident may involve one large exposure. A gas stove can create small, repeated exposures every day for years.

Gas Stoves Are Already Linked to Health Problems

Gas cooking has repeatedly been associated with respiratory effects, particularly in children. A large meta-analysis found that children exposed to gas cooking had higher odds of asthma, although the studies could not determine which individual combustion pollutant was responsible.

A later U.S. analysis estimated that 12.7% of current childhood asthma could be statistically attributable to gas-stove use based on gas-stove prevalence and previously reported asthma associations. That estimate has been debated because a population-attributable fraction depends on assumptions about causality, and later reviews have warned against interpreting it as proof that gas stoves directly cause exactly 12.7% of childhood asthma.

The broader finding nevertheless remains important: gas cooking has repeatedly been associated with childhood respiratory disease.

Most discussions of this relationship focus particularly on NO₂. That is reasonable because gas stoves can produce substantial NO₂ exposure and NO₂ has established respiratory effects. But children using the same kitchen are not exposed to NO₂ alone. Gas combustion produces a mixture that can also contain carbon monoxide, benzene, particles and other combustion products.

A gas stove does not expose a person to one pollutant. It creates a repeated mixture of combustion pollutants inside the living environment.

New Research Has Increased Concern About Other Gas-Stove Pollutants

Recent research has shown that gas and propane stoves can generate benzene at concentrations capable of spreading beyond the kitchen and remaining elevated elsewhere in the home for hours.

Other studies have found that typical gas cooking can substantially increase indoor NO₂ exposure. These findings have driven much of the recent concern about gas stoves.

Carbon monoxide has not received the same renewed attention because its presence was already known and its risk continues to be discussed largely through the traditional framework of acute CO poisoning.

But the fact that CO is an old, familiar pollutant does not mean every biological consequence of repeated low-level exposure has already been established.

Ventilation Changes the Exposure

A range hood vented outdoors can remove part of the pollution generated during cooking. Opening windows and increasing air exchange can also reduce indoor concentrations.

However, ventilation does not change what the flame produces. It changes how quickly those combustion products are removed from the room.

The effectiveness of range hoods also varies considerably according to airflow, installation, burner position and whether the hood actually exhausts outdoors. Gas combustion still occurs inside the home, and whatever is not captured by ventilation remains available to spread through the indoor environment.

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