The deadly explosion that ripped apart two buildings in New York's East Harlem on Wednesday morning brings new attention to a risk experts have been warning is especially great beneath the oldest U.S. cities: natural gas leaks.

Con Edison, the utility that services New York, said via Twitter that it was on the scene responding to a gas leak just before the explosion at about 9:40 a.m. The blast was so violent that it sent people "flying out the window," according to reporting by the New York Daily News.

At least two people were killed and 16 more were injured, some seriously, in the collapse of two buildings and fire, said the New York Fire Department. Authorities, still fighting the fire and searching for victims, had reached no immediate conclusion on the cause of the explosion, but Con Edison said it was working closely with the NYFD. (Related: Q&A: How Do Explosions Cause Building Collapses Like Harlem's?)

The explosion comes at a time when utility companies in many parts of the United States are grappling with a difficult problem: what to do about aging natural gas mains, many of them in difficult-to-access locations in older urban areas, which have deteriorated and are prone to leaking. The gas mains that are cause for most concern are made of cast iron, many of them dating back to before World War II. (See related quiz: What You Don't Know About Natural Gas.)

Although much work has been done to upgrade natural gas pipelines, U.S. government data shows that the cities that still have the highest percentage of cast iron mains, including New York, Boston, Philadelphia, and Chicago, have higher-than-average rates of "unaccounted for" gas. It is not clear whether that is gas lost due to leaks. (Some scientists who have been studying the issue say the "unaccounted for" numbers indicate cause for concern, although gas industry officials say no such conclusions should be drawn from the figures, which might merely indicate problems in measuring system inputs and outputs.) (See related graphic.)

Silent Leaks

The problem was highlighted by a study published in January by Duke University and Boston University researchers, who spent two months driving the streets of Washington, D.C., with mobile monitoring equipment. They found 5,893 leaks of methane, the main component of natural gas.

View Images More than 250 firefighters and dozens of pieces of equipment were called to the scene of the deadly blast Wednesday. PHOTOGRAPH BY ANDREW BURTON, GETTY

One dozen of those sites had underground methane concentrations as high as 500,000 parts per million, ten times the threshold at which an explosion can occur. "If there's a telecommunications manhole there, and you get a spark or a short, the air can ignite," Duke environmental sciences professor Robert Jackson, who led the study, explained in an interview after publishing the study. "If someone drops a cigarette butt down a manhole, it can do the same thing."

Such leakage can cause catastrophic explosions.

In 2011, for example, a natural gas explosion in a residential neighborhood in Allentown, Pennsylvania, killed five people. A state investigation discovered that the cause was a crack in a main that had been installed in 1928.

Still, natural gas mains can be quietly releasing a lot of methane without triggering explosions. In the Washington, D.C., survey, four individual street-level leaks measured by the team had emissions ranging from 9,200 to 38,200 liters of methane per day. That's comparable to the amount of natural gas used by two to seven typical homes, according to a Duke University press release.

"Our study highlights the systemic nature of these leaks," Jackson said. "People don't understand how pervasive they are."

Methane is an odorless gas, but natural gas is treated with a substance that produces a characteristic odor so people will be aware of a leak. Con Edison said it was responding to such a report at the time of the East Harlem explosion. (See related story: "Methane: Good Gas, Bad Gas.")

It's not known how many such leaks exist nationwide, since there hasn't been any systematic effort to search for them. But in a 2012 study of Boston, which like Washington has aging natural gas infrastructure, Jackson and his colleagues found 3,356 leaks under the city's streets.

The root of the problem is the cast-iron and wrought-iron pipe that gas companies used to build their distribution networks in many parts of the nation back in the early to mid-20th century. While strong, the iron pipes are vulnerable to corrosion, and their rigidity makes them susceptible to stresses, including pounding from construction and vehicle traffic above, according to David Graves, a spokesman for National Grid, a multinational utility company that provides natural gas in Massachusetts and elsewhere in the northeastern United States, interviewed before the East Harlem incident.

Utilities have replaced most of the old iron infrastructure over the past several decades. But according to the federal regulator, the U.S. Pipeline & Hazardous Materials Safety Administration, nearly 46,000 miles of iron mains and service lines to individual homes were still in place in 2012, the most recent year for which data is available. Even though only 2.6 percent of the nation's distribution lines are still made of cast iron, 11 percent of the incidents that caused accidents or fatalities have involved cast-iron pipes.

According to PHMSA data, the large city systems with the highest percentage of cast-iron mains were Philadelphia, with 50 percent; Washington, D.C., with 35 percent; and Boston, with 33 percent. Con Ed was also in the top ten, with 30 percent of its pipelines in New York City made of cast iron. About 43 percent of the natural gas lines in the Con Ed service area including Manhattan date to before 1940, the PHMSA data shows. (See related post about a new natural gas pipeline from New Jersey to New York: "Another Pipeline, Another Controversy.")

PHMSA lists 34 other states with obsolete iron lines as well.

The biggest problem outside the nation's capital seems to be New England, where Rhode Island (27 percent of mains), Connecticut (19 percent), and Massachusetts (18 percent) all have high concentrations of iron in the ground. There's an additional complication because some of the region's infrastructure was originally was designed to distribute coal gas, which has higher moisture content than natural gas. After utilities switched over to using the same pipes for natural gas in the 1950s, the lower-moisture fuel dried out the caulking that connects the joints of the pipe sections, making them more prone to failure, Graves said.

Utility companies have two options for dealing with obsolete iron pipes, according to Lori Traweek, a senior vice president and chief operating officer for the American Gas Association, a Washington, D.C. industry group representing local gas delivery and service companies. (She was interviewed before the Harlem accident occurred.) Pipes don't all deteriorate at the same rate, and "there is some cast iron pipe that looks as if it was put in yesterday," she noted in an interview before the New York incident. If a section is in relatively good shape, it may be possible to repair it by inserting a protective liner. If it's unsalvageable, the utility can replace it with modern polyethylene plastic pipe.

The Repair Challenge

But because many of the deteriorating legacy lines lie beneath streets in older, high-density urban areas, fixing or replacing them can be a slow, arduous process, according to National Grid spokesman Graves. "A lot of the work has to be done with hand equipment, since you're very close to sewer and water lines," he said. "You're not really sure what you're going to find when you go into the ground." Additionally, digging up the streets tends to disrupt local traffic, putting additional pressure on the utilities. "Just getting construction permits from the city can be a lengthy process," Graves said.

Because of that complexity, utilities often have a tough time estimating the cost of the repairs, which can make financing them a challenge, he added.

But companies might get some relief in that area from the federal government. U.S. Sen. Edward Markey, D-Massachusetts, has introduced legislation that would provide federal funding and allow price increases to speed the overhaul, in exchange for ending the practice of charging consumers for lost gas.

"We understand that for the utility companies, the problem is that they have to put up so much capital up front, and it takes them a long time to make back that investment," explained Markey spokesman Eben Burnham-Snyder in an interview last month. "We're saying that we'll help you get back this money quicker, but that you can't keep charging people for gas that they're not getting. In the end, reducing inefficiencies in the system will work out to be a net positive for consumers."

Leakage of natural gas is not only an explosion issue; it is a greenhouse gas issue. The latest science shows that methane is 34 times more potent a heat-trapping gas than carbon dioxide over a 100-year time frame. For that reason, as well as explosion risk, a number of natural gas utilities are working with Washington State University and the nonprofit Environmental Defense Fund on a nationwide field study to better understand emissions associated with the distribution of natural gas. (See interactive map: Four Ways to Look at Global Carbon Footprints.)

The American Gas Association, in statements prior to the East Harlem incident, said that natural gas utilities are installing modern plastic pipes at a rate of 30,000 miles per year, and cathodically protected steel mains used for high-pressure delivery at a rate of 1,500 miles per year, connecting new customers as well as upgrading existing pipeline infrastructure. In the past decade, natural gas utilities have added 300,000 miles of distribution mains to serve 17 million more customers.

"A concerted effort by natural gas utilities to upgrade and modernize our nation's pipeline network to enhance safety has also contributed significantly to a declining trend in emissions from the natural gas system," said the AGA in a recent statement about its efforts to address leakage. (See blog post, "Methane Emissions Far Worse Than U.S. Estimates, But Study Concludes Natural Gas Still Better Than Coal.")