Metro

In a catastrophe this big, why were there so few injuries and deaths?

LAWRENCE — It could have been the time of day, when many residents were still commuting home. It could have been the overwhelming smell of rotten eggs, a telltale sign of a natural gas leak, or the difficulty of achieving the precise conditions for methane to become combustible.

For Paul Brennan, Lawrence General Hospital’s head of emergency management, the best explanation for why the calamity in the Merrimack Valley claimed only one life and resulted in few serious injuries was simply this: “We were very lucky.”

“I could speculate,” said Brennan, who marveled that so few people were hurt as a result of some 70 fires and explosions that rocked Lawrence, Andover, and North Andover on Thursday. “But a lot of it was just pure luck.”

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Lawrence General reported a short list of casualties, given the size of the catastrophe that unfolded across three communities. The hospital treated 13 victims with injuries ranging from smoke inhalation to blast trauma. On Friday, one person remained hospitalized and was undergoing surgery. Other hospitals also treated some patients, but none with injuries considered life-threatening.

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The man who died, Leonel Rondon, 18, of Lawrence, was sitting in a car that was hit by a falling chimney after a nearby house exploded.

A day after the explosions, others pointed to the time of day as the best explanation for the limited number of casualties. The quick succession of fires began around 4:30 p.m.

“Many people hadn’t returned home from work yet,” said Guy Colonna, senior director of technical services for the National Fire Protection Association.

The balmy weather, he suggested, could have also been a factor.

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“For many people who had returned earlier, the weather was decent enough that they found themselves outside,” Colonna said.

And the quick response of police, firefighters, and others — warning residents to leave their homes immediately — also probably helped save lives.

“People took it seriously, listened, and stayed away and began letting the response entities do their thing,” Colonna said.

Bob Ackley, president of Gas Safety USA, a leak-monitoring company in Southborough, said he thought many residents may have fled to safety after recognizing the odor of natural gas. Gas companies are required by law to add a chemical that makes the gas detectable by smell.

Although it remains unclear what caused the explosions, Ackley and others surmised that it was the result of too much pressure in the region’s gas mains. Natural gas often comes to communities via high-pressure pipes, often at around 60 pounds per square inch. From there, it’s routed through a substation that reduces the pressure to about a quarter of a pound per square inch.

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If the gas wasn’t reduced in pressure before entering the affected homes, it may have been easier to detect.

“Those who survived or didn’t have a fire got very lucky that there wasn’t a spark,” Ackley said.

At the same time, it’s also possible that some homes were filled with too much gas to ignite. Methane, the principal component of natural gas, will only burn with a specific amount in the air. It also requires some kind of spark, which could be from a light switch, dryer, or even a ringing cellphone.

Another factor may have reduced the casualties: Many of the fires began in basements, where gas often enters a home from the utility’s network of pipes.

“People don’t tend to be hanging out” there, said Audrey Schulman, president of the Home Energy Efficiency Team, a Cambridge nonprofit that has mapped tens of thousands of gas leaks around the state.

She also believes the time of day helped. “Thank God it didn’t happen at night when people were sleeping,” she said.

At a press conference on Friday at Lawrence General, Dr. Earl Gonzales, chief of trauma surgery, said he was also surprised at the relatively small number of serious injuries.

“We were expecting a lot more,” he said.

Lawrence’s fire chief, Brian Moriarty, said he was too busy dealing with the aftermath of the disaster to comprehend any silver lining.

“We were lucky,” he said.

David Abel can be reached at dabel@globe.com. Andy Rosen can be reached at andrew.rosen@globe.com.