Sealing up houses to improve energy efficiency also traps more radon inside and may lead to a higher risk of lung cancer, according to a new study based on modeling.
Guidelines suggest people install ventilation systems when they try to reduce heat loss from their homes.
Many energy efficiency measures, like putting draft strips along doorframes, reduce air exchange, study author Paul Wilkinson said.
"Moreover, even where trickle vents (small vents in windows or bricks) are fitted, a proportion will not be used or will be left closed," said Wilkinson, from the London School of Hygiene and Tropical Medicine.
Radon, a gas produced from naturally occurring uranium in soil and water, is known to increase the risk of lung cancer. It is present in many homes in varying amounts.
The U.S. Environmental Protection Agency (EPA) estimates radon contributes to about 21,000 lung cancer deaths each year, mostly among people who smoke.
Using physics models, Wilkinson and his coauthors calculated what would happen if homes in England used available methods to become more airtight and reduced air leakiness by about one half.
Increasing air tightness without taking any other measures would increase indoor radon levels by an average of more than 50 percent, according to their model, described in the British medical journal BMJ. That would mean the number of homes above the radon action level would increase from 0.6 percent to 2 percent - corresponding to an extra 750,000 people in those homes.
If the homes also installed purposeful ventilation methods that worked most of the time, radon levels would still increase, but to a lesser extent, the researchers found.
Mechanical ventilation and heat recovery systems would also do a great deal to mitigate radon risks, unless the systems failed.
The authors estimated current levels of radon account for about 1,000 deaths per year in England. In their worst-case scenario - more air tightness without extra ventilation measures - they predicted an additional 278 radon-related deaths per year.
"Regardless of type of home construction or perceived air tightness of a home, the first step is to test the home, or other structure, for radon," R. William Field said. He studies the health effects of radon at the University of Iowa College of Public Health in Iowa City but wasn't involved in the new study.
"Do it yourself test kits are inexpensive. If a home tests high for radon, increasing ventilation rates and/or installing a radon mitigation system will lower radon concentrations substantially," Field told Reuters Health in an email.
In the U.S. there is little awareness of the dangers of radon, he said, and only about 30 percent of homes have been tested.
According to an EPA map, states in the Midwest and New England tend to have the highest radon levels.
For most people, the increased risk of radon with more air tightness would likely be small, Wilkinson said.
"These small risks add up to an appreciable burden at (the) population level, however, if a high proportion of the housing stock is retrofitted," he said.
Installing better insulation fabric in walls can increase efficiency without necessarily increasing radon levels, he noted.
Homeowners should check radon levels before and after installing energy-efficiency retrofits, according to Wilkinson.
He suggested remediation measures like fitting extraction pumps under the floor for homes with high radon levels. When building a home, there are special membranes impermeable to radon that can be built into floors, he added.