The finding is potentially important to many American homeowners because it concerns alpha radiation, which is emitted by radon, a radioactive gas released into the air from soil. Residents of a house exposed to radon have an increased risk of lung cancer.
All radiation risks commonly are assessed by a simple, logical method called linear extrapolation. It starts with the known damage done by high levels of radiation and assumes that the damage is reduced as the dose is reduced: one-tenth the dose causes one-tenth the damage.But now a team at the Columbia University Center for Radiation Research reports that low levels of alpha radiation don't follow that rule. When a single alpha particle hits a living cell, it causes damaging mutations not only in that cell but also in a large number of neighboring cells, says a report in the Dec. 4 issue of the Proceedings of the National Academy of Sciences.
"The implication is that down at a low dosage, where only a small fraction of cells in a population get traversed by an alpha particle, the damage is more than predicted by linear extrapolation," says Dr. Eric Hall, director of the Center for Radiation Research.
"If you hit only 10 percent of the cells, your damage is comparable to what would happen if every cell in the population is hit," says study leader Dr. Tom K. Hei, professor of radiation oncology and public health at Columbia. "There's a huge bystander response."
Some as yet unidentified cell-to-cell signaling mechanism causes the bystander response, he says.
The finding adds a new element to an ongoing debate about the validity of the linear extrapolation method. Some scientists have argued that the risk of low doses of radiation is smaller than the method would predict.
But the Columbia finding "looks like there might be a steeper risk slope for very low doses," says Dr. Howard L. Liber, associate professor of radiobiology at Harvard Medical School and a member of the National Council on Radiation Protection and Measurements, a nonprofit group that advises the government on radiation safety. "You can't necessarily extrapolate from higher doses linearly. At very low doses, the risk may be higher than we thought."
It's not known whether the finding applies to other forms of radiation, such as X-rays, Hall says. The alpha particle experiment was done with a highly specialized device called a single particle microbeam, which can direct a single radioactive particle at an individual cell. So far, the device cannot be used with other forms of radiation, although "we're trying to do it with electrons, particles that are set in motion by X-rays," Hall says.
Liber says more research is essential because "the bystander effect is one of the more exciting things to come out in this field for a long time. It is a critical area for investigation."
Homeowners concerned about radon can have their residences checked, Hall says. A number of states, including New York, require a radon test when a home is sold and protective devices can be installed. "In a new home, it is cheap. In an existing home it can be very expensive," Hall says.
"There is a big argument about how important a problem radon is," he says. "Any potential lung cancer risk from radon is swamped by the risk of smoking. About 10 percent of lung cancer cases are attributed to radon, and most of those occur in smokers. There is a synergistic effect."
What To Do
The first, basic step to prevent lung cancer is to avoid smoking. Checking a house for radon and installing a protective device if levels are high also can help.