Natural gas is escaping from more than 3,300 leaks in Boston’s underground pipelines, according to a new Boston University study that underscores the explosion risk and environmental damage from aging infrastructure under city sidewalks and streets.

The vast majority of the leaks are tiny, although six locations had gas levels higher than the threshold at which explosions could occur. Although there have been no reports of explosions in Boston from any of the leaks, the study comes three years after a Gloucester house exploded probably because of a cracked and corroded gas main dating to 1911.

The research, being published Tuesday in the journal Environmental Pollution, confirms what Bostonians sometimes smell on city streets: a telltale whiff of gas.


“It is something that is distributed across all neighborhoods in Boston,’’ said Nathan Phillips, associate professor in BU’s Department of Earth and Environment and a lead author of the study. Phillips and an assistant drove a black hatchback over every one of Boston’s 785 miles of roads to test methane, the primary ingredient in natural gas, in the air. “And we know once we go outside of Boston, Newton is just as leaky. . . . Any old, mature city is going to have this problem.’’

Gas companies and the state Department of Public Utilities say the risk of an explosion from the leaks is exceedingly small. Serious leaks are repaired right away — as were the six that Phillips’s research team discovered — and the remaining ones are not at levels high enough to cause an explosion.

“Do they pose a safety problem to the public? No, safety is our highest priority,’’ said Ann G. Berwick, chairwoman of the state Department of Public Utilities. She said the agency has a financial incentive program to encourage gas companies to replace aging mains.

Leaks can develop in corroded pipes but are most often caused by contractors or homeowners using excavation equipment, according to the public utilities department. During cold weather, frost can penetrate deep into the ground and shift the earth around mains to cause hairline fractures. Other cracks can form at joints where service lines to homes or businesses join mains.


A spokesman for National Grid, the predominant gas distributor in Boston, said it does not make sense to repair all the minor leaks if they are not causing a safety problem. Rather the company is working to replace aging pipes with more flexible plastic pipe as quickly as possible, at a rate of about 150 miles a year in Massachusetts. “It’s a better course of action,’’ said David Graves, the spokesman, adding that National Grid investigates every report of a gas odor — actually the rotten egg smell of mercaptan, an odor added to odorless gas.

“We always respond and always make the situation safe,’’ he said.

About 9 billion cubic feet of natural gas was unaccounted for in Massachusetts gas distribution system in 2010, which includes leaks, thefts, and purging of pipelines for maintenance, according to federal energy statistics. That represents about 5 percent of all greenhouse gas emissions each year in Massachusetts, Phillips said. Counting only leaks that utilities have documented, it would represent about $40 million worth of gas, according to an analysis by the Conservation Law Foundation, a Boston-based legal advocacy group. Many leaks are not known.

The problem is not one of safety alone. Leaking natural gas can damage vegetation and trees by displacing oxygen in the soil, scientists say. It is also a potent greenhouse gas that contributes to climate change: Methane is over 20 times more effective at trapping heat in the atmosphere than carbon dioxide, although it is not as long-lived.In fact, Phillips and co-author Robert B. Jackson of Duke University began their research in part with global warming in mind. Natural gas emits far less carbon dioxide than coal when it burns, and new reserves have driven down its price, greatly expanding its use in power plants, homes, and businesses. But Phillips and Jackson were curious whether leaks — and other emissions that occur during the extraction and transportation of natural gas — might mean the fuel is not as friendly to the climate.


“The question we are struggling with is how leaky is natural gas as a fuel source,’’ said Jackson.

To test the air, Phillips and an assistant drove about 15 to 20 miles an hour down one side of a street with a device in the back that measured methane in the air higher than 2 parts per million — the amount normally in the air. Sometimes when they got a high reading, they would get out of the car and test the air — in some cases they traced it to a specific manhole cover. They knew it was gas from pipelines, rather than landfills or other sources, because of its chemical signature, which showed it was an old fossil fuel rather than newer methane created from decaying garbage, for example.

Phillips, who credits gas companies for working hard to fix gas leaks, said his work captures only a hazy picture of leaking gas in Boston; he drove down only one side of streets, and gas readings depended heavily on wind and other weather. The amount of methane in the air also does not directly reflect what is happening underground. For example, a large amount of gas could be trapped underneath a street and only a small amount makes it to the surface.

“There are a lot of unknowns,’’ said Phillips. He and Jackson have put sensors on Boston buildings to understand the collective effect of gas leaks in the city — and how much escapes to the atmosphere.

Shanna Cleveland of the Conservation Law Foundation, which has a natural gas leak report coming out soon, said the state Department of Public Utilities could require timelines for gas companies to repair various grades of leaks and improve upon accelerated reimbursement rates for gas companies that replace old gas lines.

A bill sponsored by state Representative Lori Ehrlich of Marblehead that would require a timeline for the most serious leaks to be fixed and require utilities to notify police and fire of the locations of leaks unanimously passed the House in June and is before the Senate.

Ehrlich said better regulations are needed for gas pipelines. “If everyone could see methane, I doubt if there would be any leaks left,’’ Ehrlich said. “Can you imagine if it came out red?’’