A study published by the Johns Hopkins Bloomberg School of Public Health has revealed that rising levels of radon in Pennsylvania homes is linked with the rise in fracking operations in those areas.

Radon is an odorless radioactive gas that is thought to be the second leading cause of lung cancer in the world after smoking.

The study has reported that 42% of household radon readings in Pennsylvania surpass what the US government considers safe.

Since 2004, the readings have been increasing sharply and around the same time the fracking industry aggressively began to drill natural gas wells in the state.

Over the past decade, buildings that are located in counties where fracking operations are more active have seen significantly higher readings of radon compared with buildings in low-activity areas. The differences in county readings prior to 2004 did not exist.

Study leader Brian S. Schwartz, MD, a professor in the Department of Environmental Health Sciences at the Bloomberg School said, "One plausible explanation for elevated radon levels in people's homes is the development of thousands of unconventional natural gas wells in Pennsylvania over the past 10 years. These findings worry us".

For the study, the researchers analyzed more than 860,000 indoor radon measurements included in a Pennsylvania Department of Environment Protection database from 1989. They evaluated associations of radon concentrations with geology, water source, season, weather, community type and other factors.

The findings revealed that on average houses and other buildings in the study using well water had a 21% higher concentration of radon than those using municipal water.

Houses and buildings located in rural and suburban townships were found to have a 39% higher concentration of radon than those in urban areas.

The study was conducted with Pennsylvania's Geisinger Health System and published online on April 9 in Environmental Health Perspectives.