The blue flame on your kitchen stove looks harmless enough.
After all, it's supposed to be there.
But natural gas seeping out from a manhole at a busy Manhattan intersection? That's not supposed to be there.
"It's not at the explosive limit," said Alan Boyd of Con Edison, as NBCNewYork tagged along with the utility's Gas Leak Survey team. The crew had stopped on 9th Avenue at 24th Street one recent morning, after the high-tech sensors detected gas leaking at street level.
It turned out that leak was just a little bit of natural gas which had seeped into a water main control box. In fact, most of the nearly 2500 leaks found in the five boroughs and Westchester last year were in that category. Relatively minor, considered "Grade 3" by Con Edison.
Such leaks mean it's a low-level amount that's not hazardous to the public. But 17 percent of the leaks found last year were either "Grade 2"-- a moderate leak that could become dangerous in the future-- or, "Grade 1"-- which is an immediate risk, and potentially explosive.
Kevin Fagan, at Con Edison's Gas Operations Center in the Bronx, said, to paraphrase an NYPD slogan: If people smell something, they should say something. "People should never assume something isn't a leak," said Fagan. "If they have a suspicion by all means call."
What causes all the leaks? A combination of the sheer volume of the system, a brutal winter, and the nature of natural gas itself. To supply this valuable but flammable resource to the metropolitan area, 7300 miles of mains and service pipes transport one billion cubic feet of natural gas every day. The gas comes in from the Gulf of Mexico and from Canada. And the pressure is constantly monitored.
Survey teams are out almost daily from January to September to make sure that gas, as much as possible, is staying in the pipes and not leaking out into neighborhoods.
"Not knowing where this was coming from," said Boyd, referring to the leak found on 9th Avenue, "we had to make sure the area was safe."