Radon gas in homes may not be quite as dangerous as previously believed, but in high concentrations it can cause lung cancer, experts said Friday.

A study issued by the National Research Council said that earlier estimates of the risk of lung cancer in homes may have overstated the danger by 20 to 30 percent.Estimates by the National Council on Radiation Protection and Measurements and other agencies compared the danger of developing lung cancer from exposure to radon in homes with the risks of radon-caused lung cancer among mine workers.

Several factors overlooked in the previous studies, the NRC said, caused the final estimates to be too high.

Radon gas is a byproduct from the decay of uranium-238, which is present in most soils and rocks. The invisible substance often is strongly present in some mines and can collect in unventilated homes built on some types of soils.

Radon decays into a group of short-lived radioisotopes that can attach themselves to dust particles and then be inhaled.

High levels of lung cancer among some miners is attributed to their daily contact with radon, and some earlier studies found that the risk was as great or greater for residents in homes where radon was concentrated.

But the new NRC study said that when comparing how the radon particles would be inhaled and absorbed, the risk of cancer "tends to be lower for the home environment - by about 30 percent for adults of both sexes and by 20 percent or less for infants and children."