Air pollution isn't just an outdoor problem. Unhealthy fumes may be emitted inside your own home if you're cooking over an unvented gas stove.

Almost two-thirds of California households using gas burners in the winter without venting range hoods are exposed to gases that can cause breathing problems, according to a new study by a team at Lawrence Berkeley Laboratory.

"Homes are exceeding air quality standards, exposing people to toxins who shouldn't be," said scientist Brett Singer, who contributed to the study.

The Bay Area has strict rules about outdoor air pollution, with 18 "Spare the Air" alerts issued so far this season to ban the burning of wood or manufactured fire logs. There are also state ventilation rules for indoor furnaces and hot water heaters.

Stovetop’s gas flame. (Bay Area News Group)
Stovetop's gas flame. (Bay Area News Group)

But our stoves are largely overlooked.

And while the majority of California homes have a vented range hood installed, research suggests that a minority of households use them during all cooking, according to the team.

The Berkeley researchers concluded that 62 percent of households using gas burners without venting in the winter are routinely exposed to excessive levels of nitrogen dioxide, 9 percent to carbon monoxide and 53 percent to formaldehyde, gases that can trigger respiratory problems and aggravate asthma and cardiovascular disease, especially in young children and older adults.

"If these pollution levels were outdoors, the state would be required by law to submit a plan for how to clean up the air," Singer said. "But they are inside a home."

Indoor pollution levels drop by half in the summer, when doors and windows are more likely to be open. The study, led by research scientist Jennifer Logue, is published in the journal Environmental Health Perspectives.

The findings underscore the importance of using a range hood while cooking, they note. Although the recommended size of a hood is influenced by range size, heat output and the size of the kitchen, the best hoods move at least 300 cubic feet of air per minute and are verified by the Home Ventilating Institute.

If hoods are quieter, they are more likely to be used, the researchers note. They also urged improving hoods' ability to capture gases.

The team is developing a simple standardized test to measure hood effectiveness to help consumers.

The scientists measured the pollutants emitted while cooking in a test kitchen set up in the small tightly sealed Building 51F in the Lawrence Berkeley Lab's parking lot. As they cooked hamburgers and stir-fried green beans with a variety of range hoods, operating at different settings, a computer measured the components of the smoky emissions.

They also collected data about cooking patterns in more than 6,000 Southern California households.

Combining those two sources of information, they created a mathematical model to determine exposure to air pollution. Then they compared these indoor levels to outdoors air quality standards.

"If it is unsafe to breathe outside, it is probably not a good idea to breathe it at home," said Singer.

Some of the fumes are emitted simply by burning natural gas, especially when it hits a cooktop pot or the metal inside an oven. Additional toxins are emitted by the cooking of food, whether fried, grilled or toasted.

About half of homes in California and 34 percent of homes nationally have natural gas cooking burners. It is unknown what fraction of these are vented -- as opposed to recirculating. Previous research found that a minority of households routinely use range hoods during all cooking.

In all, the scientists estimate that as many as 12 million Californians are exposed to levels of nitrogen dioxide above health standards as a result of cooking with gas burners. Nationally, there could be tens of millions more.

"If every one of those homes were to use a range hood that exhausts to the outside and is even moderately effective, the number of homes exceeding the standards would drop," Singer said. Contact Lisa M. Krieger at 650-492-4098.

CUTTING Kitchen pollution

n Always use your range hood. Ideally, it will cover the entire cooktop
n Turn hood fan to highest setting
n Cook on back burner to reduce inhalation
n Clean grease traps periodically
n If you don't have a hood, put an exhaust fan in the ceiling of the kitchen or open the windows