Projo 7 to 7 News Blog

When methane mixes with air in a confined space at concentrations of under 5 percent or over 15 percent, no explosion can take place, Dillon said.

It's when the concentration lurks in the 5-to-15 percent range that bad things can happen. The same is true with gasoline. A mixture of gasoline and air too rich or too lean will burn but not explode.

Dillon said firefighters rarely encounter truly dangerous gas leaks, because the stink of mercaptan usually alerts residents, who then call the Fire Department or gas company before the mix can reach the dangerous range.

The first thing firefighters do, he said, is "remove the life hazard."

That means getting people out of the building.

Then firefighters check the gas level with their portable meters.

"We get a 2-percent level, we know we got a problem," he said. "The first thing we do after that is shut off the gas at the meter inside the house."

So long as the gas concentration has not reached the dangerous level, or has exceeded it, it's safe for firefighters to do that, he said.

"If it's at the explosive level, we get out and contact the gas company to shut it off through a valve in the street," he said. "Most of the time when we get very low readings, it is usually due to a faulty gas appliance. We shut off the gas and have them contact the gas company."

Leaks outdoors can be a very different breed of cat, however.

"A very hazardous situation is a problem out in the street where there's a cracked pipe," he said. "The gas can travel along the sewer service and get into your house and build up. The problem is, as the gas goes through the sewer the mercaptan is leached out, and there is no odor, while the gas is building up all the time."

That was the likely scenario in 1998, when a house in Attleboro blew up and killed two city workers.

A crew of seven public works employees had been at work repairing a drain in the street in front of the house. The area they excavated had a "no gas" sign, an indication that it was safe to dig there. But the sign was wrong. They unknowingly fractured a gas line.

The workers took a break, which allowed gas to escape for 90 minutes. It got into the house. No one reported any smell of gas. The furnace in the basement kicked on, creating a spark.

Five other workers and two people in the house were injured in the ensuing blast.

Dillon said an outdoor gas leak is hard to pin down, because the atmosphere dilutes the gas to the point where it is undetectable with the equipment carried by firefighters.

"If the gas is leaking outdoors, it's 99.999 percent air, and we're not picking that up," he said.

Gas company personnel, he said, are equipped with devices far more sensitive.

"The first thing we do when we think we have an outdoor leak is check all the buildings in the area," Dillon said. "When the gas company comes, they dig up the ground and repair the leak."

But if the concentration of gas builds in a basement or other confined space it won't explode, even if at the right mix, unless there is a source of ignition. Unfortunately, Dillon said, such sources are common.

"It could be your boiler kicking on, or static electricity, or flicking on a light switch _ anything will ignite it. And when it's in a confined space it ignites with explosive force."

If the gas is not confined, he said, it will burn but not explode. That's why you can heat a kettle of water without a boom.

Strange as it may seem firefighters sometimes have no choice but to create an explosive mix while trying to avoid a disaster.

Dillon said that if firefighters find a house where the gas-air mix is too rich, they will open the doors and windows to ventilate the house. As the mix drops, it passes down through the 5-to-15-percent range where it is capable of exploding. But if there is no ignition source, the gas-to-air ratio will continue to drop until there is too little gas to cause a problem.

Firefighters carry radios that are designed to give off no sparks. Unfortunately, Dillon said, many household appliances -- electronics particularly -- cause a spark when turned on. That can ignite gas at the right mix.

He said if firefighters enter a house and find a gas pipe burning in the cellar, they will let it burn until the gas supply can be shut off.

"Often it's better than to extinguish the fire and have an uncontrolled gas filling up the house and creating an explosive hazard," he said. "If the fire is impinging on other appliances, we can protect them with a hose stream and keep them from burning until we get control of the gas.

"The gas company is very, very good to work with, but if they're tied up at other emergencies, for the most part we will let it burn until we can stop the flow of gas."