Natural gas leaks pose explosion risks, health concerns and contribute to climate change, said researchers who spent January and February 2013 driving all of the 1,500 miles of Washington, D.C., roads with an instrument that took methane readings close to the ground every 1.1 seconds.
Writing in the American Chemical Society journal Environmental Science & Technology, they reported finding nearly 6,000 leaks, with some locations showing concentrations of methane at about 45 times what would be expected with no leak.
Robert B. Jackson, an environment science professor at Duke University, was the study leader, with researchers from Boston University taking part.
At locations with high levels, probes put into manholes found concentrations 10 times the threshold at which explosions can occur at 12 sites, the researchers said.
"If you dropped a cigarette down a manhole ... it could have blown up," Jackson told USA today.
Despite reporting the leaks to the local gas utility, four months later nine of the sites were still emitting dangerous levels of methane, he said.
The researchers conducted a similar study in Boston in 2011 and found it had a similar number of leaks per mile as Washington but at lower concentrations.