FORT WORTH, Texas — Common household dust has long been known to carry pesticides, allergens and other irritants.

But the dust that coats your television sets may finally answer why virtually every American tested has traces of a chemical flame retardant that may be harmful.

The flame retardants have been used for decades in television sets, computer wire insulation, mattress stuffing, carpet padding and many other common household products. They have been found in household dust, but no one has been able to say how they got there and from what specific products.

Now, a study by researchers at Boston University's School of Public Health appears to have pinpointed the largest source of chemical flame retardants as the dust on television sets.

Using a portable X-ray device, researchers sampled 19 Boston-area homes and found large volumes of the flame retardants in television dust. One theory is that when the television heats up, the flame retardants in the TV vaporize into a gas, eventually settling into the dust in the air and on bookshelves, floors and appliances.

"I think this link between the flame retardants and dust in TVs is a really big deal," said Tom Webster, a Boston University epidemiologist who led the study.

That's true, in part, because millions of television sets are expected to be discarded in February when television broadcasts switch to digital signals, resulting in a much greater volume of flame retardants contaminating the environment. The flame retardants take decades to break down, and they have been shown to travel great distances in the air and water.

Here is a question-and-answer look at the problem:

Q. What are chemical flame retardants?

A. Synthetic chemicals called polybrominated diphenyl ethers, or PBDEs, that help prevent the spread of fire by impeding the chemical reaction that causes it. They are commonly found in polyurethane foam products, such as the padding in furniture, as well as in textiles, televisions and computers.

Q. What's the concern?

A. The main concern is that PBDEs build up in the body over time. Data on how they affect humans are slowly emerging. Animal studies have shown that PBDEs harm the nervous system and alter hormonal functions and the development of reproductive organs. The most common flame retardant in use today has been found to cause cancer in laboratory rats. Industry officials, however, have argued that the levels in people and the environment are low and that this commonly used flame retardant is safe.

Q. What's being done?

A. The average American household has more than two television sets. Many companies, including Sony Corp., are phasing out the use of PBDEs in televisions.

Q. What should you do?

A. The most important things are to wash your hands often and dust your house. A study last year by researchers at Boston University's School of Public Health linked the presence of the flame retardants found in people to exposure to common dust, which can be inhaled or ingested.