Environmental group mapping gas leaks across Mass.

Thousands of natural gas pipeline leaks have been identified in Boston and across Massachusetts that not only cost consumers millions of dollars in lost fuel, but also can pose a threat to public safety and the environment.

Now, using public records, a local environmental advocacy group has begun mapping such leaks and publishing the results online at www.clf.org/map.

Working with Boston University, the Conservation Law Foundation has located more than 4,000 leaks reported over several years by National Grid and a smaller utility that serves Fall River, North Attleboro, Plainville, Somerset, Swansea, and Westport. The leaks — some of which may have been fixed — are graded by severity.

More attention has been paid to leaky gas pipelines in recent years as blasts have claimed lives. Just last week, an explosion caused by a gas leak hit buildings in New York’s East Harlem area, killing several people. A pipeline explosion caused a deadly fire in San Bruno, Calif., in 2010 and another leak rocked a Gloucester home in 2009.

The Conservation Law Foundation said it is trying to obtain leak data from the other utilities, including Northeast Utilities and Unitil.