Methane at 10 percent (100,000 ppm) gas concentration measured in a telecommunications manhole in Washington, D.C. Researchers found nearly 5,893 natural gas leaks from the city’s aging pipe system. (Robert B. Jackson)

Researchers who conducted a street-by-street survey of the District found nearly 6,000 natural gas leaks from the city’s aging pipe system, including 12 in manholes where methane had collected to potentially explosive levels, according to a study published Thursday.

The team, after warning Washington Gas about those 12 locations, retested them four months later and discovered eight places where the gas buildup was still at concentrations that could cause explosions.

Some of the thousands of gas leaks found throughout the city were at levels many times those normally found in the environment. But they posed no direct health or safety risk at those concentrations and the gas plumes dispersed into the air, according to team leaders Robert B. Jackson of Duke and Stanford universities, and Nathan G. Phillips, who teaches at Boston University.

Still, the gas emissions have other effects: They contribute significantly to climate change by trapping heat, they help form ground-level ozone when they react with sunlight and other gases, and they cost consumers money. In most cities, including Washington, consumers are charged for gas that is lost along transmission lines.

Methane is much more effective at trapping heat than carbon dioxide, the more common greenhouse gas that is the major culprit in global warming, according to the study.

The researchers expressed concern about the methane buildup in 12 of the 19 manholes that merited examination.

“It’s kind of like an earthquake,” Phillips said of the likelihood of an explosion. “You don’t stop living your life . . . but somewhere, this is likely to happen at some time. It’s a low-probability, high-impact kind of event.”

In 2000 and 2003, explosions that occasionally sent heavy manhole covers flying through the air rattled Georgetown and other parts of the city, causing damage but no injuries. Some of the blasts were attributed to ignition of naturally occurring gas and sewer gas by sources that included underground electric lines. The incidents led to a $30 million project, completed in 2005, to upgrade Georgetown’s underground power, gas, sewer, water and phone lines. Like many older cities, the District and its suburbs also are plagued by aging water mains that rupture and cause outages during colder months.

In Rio de Janeiro, manhole explosions became so frequent between 2010 and 2012 that a Facebook video game was created in which players try to dodge them.

The new study said that while pipeline safety has improved in recent years, incidents involving natural gas transmission systems in the United States cause an average of 17 fatalities and $133 million in damage every year.

The study was published in the journal Environmental Science & Technology. The authors presented some of their findings at a conference last March.

The researchers, who drove every street in Washington last January and February with a car-mounted device that detected 5,893 leaks, believe most were caused by breaks in the city’s aging cast-iron pipes, Jackson and Phillips said. They found that the District had the highest proportion of cast-iron gas mains, 35 percent, when they compared its 419 miles of those pipes with gas transmission systems in 16 states.

They discovered similar leaks in a street-by-street survey of Boston a year earlier and assume they would reach the same results in most older cities where underground cast-iron pipes were used beginning in the early part of the last century to carry natural gas, Jackson said.

“Aging infrastructure is more than roads and bridges,” Phillips said. “It’s the infrastructure that we can’t see.”

Washington Gas officials declined to answer questions about the study, but Eric Grant, the utility’s vice president of corporate relations, issued a statement saying that it maintains “rigorous inspection programs, operating procedures and record-keeping protocols. Washington Gas practices exceed the leak detection and repair procedures that are required by code, enforced by federal agencies and overseen by each state’s public service commission.” The statement said the company has 13,000 miles of distribution mains and more than 940,000 service lines in its system.

Betty Ann Kane, chairman of the District’s Public Service Commission, said there has never been an explosion of leaking natural gas in her eight years on the panel. Under orders from the commission, Washington Gas is aggressively replacing older cast-iron mains and a type of pipe coupling that has leaked elsewhere, she said.

Natural gas used in homes and businesses is largely composed of methane. The researchers analyzed the gas they collected to make sure it was coming from pipes and not natural sources.

They drove more than 1,500 miles of city streets and found 1,122 leaks greater than 5 parts per million of methane, or 2.5 times the background concentration. There were 334 leaks greater than 10 parts per million and 67 leaks of more than 25 parts per million, according to the study.

There was no correlation between leaks and geography, income or anything else except aging pipes, Jackson said. “It’s not a social justice issue,” he said.

In areas where high levels of methane were noted, the researchers put probes into 19 manholes and found 12 where methane had built up to more than 50,000 parts per million, or 5 percent gas, known in the industry as the “lower explosive limit” of methane in air, according to the study. One contained 50 percent gas.

When they went back in June, the methane level in eight was still above that threshold, one had declined to 40,000 parts per million — the lower limit for “an existing or probable hazard to persons or property,” according to industry standards — and two others were reduced substantially. The researchers could not get to one other manhole because of construction.