All it takes is gas and a spark.

Fire safety experts say the aftermath of an explosion in Harford County shows the magnitude of what gas leaks can do: A Bel Air residence was leveled, killing both the homeowner and a Baltimore Gas and Electric Co. contractor. The blast also spread debris throughout Arthurs Woods Drive and rendered 16 homes uninhabitable.

Crews started clearing debris Monday as investigators from the Maryland state fire marshal’s office, as well as the federal National Transportation Safety Board and Bureau of Alcohol, Tobacco, Firearms and Explosives, worked to figure out what exactly led to the Sunday morning blast.

Meanwhile, officials who have handled previous explosions said a resident who smells something off in their home should try to find the source immediately.

And if the smell is rotten eggs or sulfur — it’s likely mercaptans, the pungent compounds added to fuel gases to make leaks noticeable  — residents should get out and call both 911 and their utility company.

All gas explosions contain the same two elements — a release of gas and an ignition source. What matters the most to investigators, after determining that fuel gas caused the explosion, is how the gas got out.

“As long as the gas stays where it belongs, these things — natural gas, propane — are safe,” said Richard Summerfield, a retired ATF special agent who now works as a consultant for a private fire and explosion investigation firm.

It could be as simple as a stove’s gas valves being left open, building pressure inside over time before a spark or flame sets it off. It could be intentional, as investigators believe was the case when they found an unscrewed cap on a gas line at the scene of an April explosion in Essex.

But gas explosions frequently involve some kind of flaw with a gas delivery system, whether it be damage to a pipe or other malfunctioning equipment, like a pressure regulator.

“The origin gives you a lot of clues,” said Brian Glasser, a Washington attorney who led the legal team representing survivors of a 2016 gas explosion that partially destroyed a Silver Spring apartment complex, killing seven people and injuring more than 60.

After investigating that case for nearly three years, the NTSB concluded that the explosion at the Flower Branch Apartments was most likely caused by a faulty Washington Gas regulator that was left unconnected to a vent pipe in a basement meter room. They also noted lapses in communication between the complex’s owner, Kay Management, the fire department and the utility, which wasn’t notified of residents who had reported smelling gas.

The landlord and utility eventually settled with more than 150 plaintiffs affected by the explosion under confidential terms. Glasser, who also represented people affected by the 2018 gas explosions in Massachusetts, declined to comment specifically on the Flower Branch case, citing confidentiality restrictions.

How gas gets loose

The federal board could not determine what ultimately ignited the gas in the Silver Spring case. Most of the time, the ignition source doesn’t matter much — the minimum amount of energy needed to set off natural gas is “so low that it’s often a waste of time to look for it,” Summerfield said.

The problem at hand is that gas was able to get out of its natural environment.

Leaks can happen due to pipes being damaged by natural causes, ongoing utility work or, sometimes, gas infrastructure simply getting old. Gas lines corrode over time, and many were installed decades ago — older cities like Baltimore have some cast iron pipes dating back to the 19th century. Once a line leaks, gas can migrate “considerable distances” into homes and buildings through the ground or even through sewer lines, Summerfield said.

Investigators determined that a Columbia office building had filled with natural gas after a faulty electrical service line beneath the parking lot damaged a gas line leading to the building in 2019. Firefighters noted the pavement had swelled up, cracked and smelled like gas as the Lakeside Office Park building filled and ultimately exploded, injuring no one but destroying the building and damaging several storefronts.

That explosion led to BGE being fined more than $437,000 by the Maryland Public Service Commission, whose investigators found the utility had violated safety standards and noted that fire officials estimated damages at roughly $25 million. The utility also was required to conduct safety upgrades on about 1,000 similar sites. A lawsuit against BGE was ultimately dropped after the utility went into private mediation with the building owners’ insurance company, according to court records.

While investigating Sunday’s explosion in Bel Air, investigators will likely be interested in finding out what the BGE contractors on scene were working on, Glasser and Summerfield noted. The utility said Sunday that contractors were responding to an electrical issue. One of them, identified as 35-year-old Jose Rodriguez-Alvarado, died in the blast.

“As a general rule, you always want to inquire as to the pre-event,” said Summerfield, who noted he had no knowledge of the Harford explosion. The work could be a coincidence or could have contributed to the ignition, but it could also be related to the leak, especially if it was close to a gas conduit.

After a 2020 natural gas explosion in Northwest Baltimore killed two people, investigators pinned the cause of the blast on HVAC work that had been done the day before. They said gas that built up overnight was ignited by a stove the next morning, leveling three two-story rowhomes. BGE had said soon after the August explosion that the utility’s equipment was working properly and that customer-owned equipment was to blame.

Lessons learned

Some work is ongoing to prevent more leaks. Maryland legislators have worked to encourage utilities to repair aging gas pipelines. But costs associated with that work have caused rate increases, as regulators and utilities debate the future of natural gas.

After the NTSB released its final report on the Silver Spring explosion, Maryland legislators passed the Flower Branch Act in 2021, requiring the utility to initiate plans to relocate indoor gas regulators to the outside of multifamily homes. Moving the regulators outdoors can ensure gas doesn’t build up inside a home if the equipment fails.

Some work related to that plan prompted an outcry last summer when the utility placed external regulators outside some historic, single-family homes in Baltimore.

BGE advises those who smell gas to get at least 100 feet away from the area and call 911 as well as the utility provider at 877.778.7798.