Philadelphia, PA – While legions of security personnel scan crowds, windows and rooftops for potential dangers during Pope Francis’ visit to Philadelphia, another group will be sniffing around for an invisible threat.

Philadelphia Gas Works, operators of one of the country’s oldest and leakiest natural gas distribution systems, plans to deploy teams of workers on foot and in specially equipped vehicles to sniff for gas leaks in areas where the pope plans to speak.

The sniffer patrols, first reported by the Allegheny Front, do not mean PGW is accelerating pipeline replacement because of the pope, the company said. But it will monitor for potentially explosive leaks at Independence Mall, the Ben Franklin Parkway and elsewhere the pontiff will appear.

“We are doing the same thing we do for any large event,” PGW spokeswoman Janis Shields told the Tribune-Review.

That includes sending out trucks with electronic “sniffers” that can detect a few molecules of highly explosive methane in the air and deploying workers with hand-held gas detectors to walk the streets in and around event spaces.

Explosions in the country’s natural gas distribution lines — the networks that bring gas from utilities to homes and businesses — killed more than 120 people over the past 10 years, the Tribune-Review found in its award-winning investigative series, The Invisible Threat.

Philadelphia has about 1,500 miles of gas pipes made of cast iron, the most corrosion- and leak-prone material still in use in the United States, the Trib found. Philadelphia’s inventory — which includes enough pipe to stretch from Pittsburgh to Albuquerque — accounts for about half of all cast iron pipe still in use in Pennsylvania.

A 19-year-old PGW gas worker died in an explosion while investigating a leak in 2011, just weeks before a gas-fueled blast leveled most of a block in Allentown, killing five.

In recent years, utilities around the state have accelerated their replacement of old cast iron pipeline, paying for the work with customer rate hikes allowed by the state Public Utility Commission. Because of the high cost of replacing pipe in Philadelphia’s dense, urban landscape, as well as the sheer volume of cast iron pipe it continues to use, PGW’s original replacement schedule would run through the end of the century.

That’s the longest timetable in the state and exceeds the average lifespan of people born today, the Trib found.

By comparison, Columbia Gas of Pennsylvania and Peoples Natural Gas Co., Western Pennsylvania’s largest gas utilities, plan to replace the last of their high-risk pipe in 2029 and 2031, respectively.

Under pressure from regulators, PGW submitted a plan to the PUC on Sept. 1 that would cut its replacement schedule to 48 years, which is 40 years shorter than the current plan. To pay for the accelerated schedule, the utility would need PUC approval to hike rates 2.5 percent. The commission has not voted on the plan.

The pope’s largest events will take place on Independence Mall and the Ben Franklin Parkway.

“We’ve already done most of our (gas line) replacement in those areas,” Shields said.