For years, the federal government has been advising people to test their homes for cancer-causing radon gas in winter.
That's because it was thought that tightly sealed doors and windows were likely to trap the radioactive gas and produce the highest readings on test equipment.
Radon experts now say there are so many factors that can contribute to high indoor radon readings -- including rainfall, soil type, vegetation, climate and construction quality -- that homeowners should invest in multiple tests.
"What you really want to do is test your house two times a year, or quarterly," said Daniel J. Greeman, a Penn State University geologist who spoke at the Omni Hotel in Baltimore last week at the regional meeting of the Geological Society of America.
But the most important thing scientists have learned recently about radon is its unpredictability.
"Everyone should test for radon because you never know who is going to have a high radon sample," Greeman said.
Radon is a colorless, odorless radioactive gas produced naturally by the decay of radium in the soil. The radium, in turn, is produced by the decay of uranium.
Radon is a potent cause of lung cancer that kills an estimated 20,000 people annually in the United States.
Many state and local radon-hazard maps in the United States show the highest radon risks in regions with higher concentrations of uranium-bearing rock. But the maps may be misleading.
Scientists at the conference stressed that indoor radon readings can reach hazardous levels almost anywhere if other conditions conspire to concentrate the gas and move it into dwellings.
"We're at the stage of this science when we're just starting to figure these things out," said Alec E. Gates, of the New York Geological Survey.
In a study of several thousand homes in Maryland and Virginia, a team of geologists from George Mason University in Fairfax, Va., found that rain and snow were potent factors in increasing indoor radon concentrations.
Douglas Mose, who led the study, said rain and snow tend to "cap" the soil, preventing its escape upward into the atmosphere and forcing it to move sideways into basement walls and floors.
Greeman said radioactive elements also tend to stick to the surfaces of damp, highly weathered soils and clays, and become more concentrated in those soils as other soil components are washed away.
"On top of that, plants think radium is calcium, and they love it," he said. So vegetation also tends to concentrate radium, and therefore radon as well, in upper-level soils, even when it is relatively rare in the underlying bedrock.
Douglass E. Owen, of the USGS, said wetlands, especially peat bogs, are particularly good at concentrating uranium and radium, raising serious questions about the environmental consequences of draining bogs rich in uranium, radium and radon for development, or of mining and marketing their peat.
Other studies have revealed that small-scale geologic features, such as faults that might provide conduits for the movement of radon, or phosphate-rich sediments that are also rich in uranium, may result in high radon readings in one home, while neighbors enjoy relatively low readings.
Once radon is concentrated in the soil, it can be drawn by a variety of mechanisms through cracked or porous foundations into the home.
Homeowners can purchase inexpensive radon test kits at many supermarkets and hardware stores.
When indoor concentrations reach levels above 4 picocuries per liter, the U.S. Environmental Protection Agency recommends that homeowners take steps to seal the foundation against radon intrusion. Low-cost methods for doing so are described in pamphlets available by mail from the EPA.
Geologists at the conference urged that all homeowners test their homes for radon. If you've already tested your home, they said, do it again.
"Don't make any decisions based on one test, whether the readings are high or low," said L.T. Gregg, director of geologic services for Atlanta Testing and Engineering, a Georgia consulting firm.