The Philadelphia Health Department measured an “elevated” level of deadly hydrogen fluoride gas outside the South Philadelphia refinery during a fiery accident last month, but the reading was dismissed as a “false positive” and no actions were taken to protect residents, according to a Drexel University environmental engineer.

A city inspector, using a handheld monitoring device, measured the elevated gas reading at a location near the Philadelphia Energy Solutions (PES) refinery complex, according to Peter DeCarlo, a Drexel environmental engineering professor, who submitted testimony to a state legislative committee that held a hearing Wednesday on the June 21 refinery explosion.

“The positive measurement should have been cause for proactive measures to protect residents," DeCarlo said in his written testimony to the Senate Environmental Resources and Energy Committee, which conducted the hearing at the University of Pennsylvania’s Law School.

The incident was cited by several environmental and health advocates on Wednesday who suggested that regulators need to correct weaknesses in air-monitoring and response plans for industrial sites such as PES, which is surrounded by densely populated residential areas. They said the city’s current network of fixed air-monitoring equipment was not well-positioned to detect the impact of the huge smoke cloud that drifted eastward during the refinery fire.

The city’s health department downplayed the incident Wednesday, saying its Air Management Services (AMS) inspectors suspected the gas meter was not properly calibrated, and requested that the refinery and the U.S. Environmental Protection Agency double-check the tests.

“Both confirmed that there was no HF present in the air," James Garrow, the health department spokesperson, said in an email. "The AMS inspectors took the improperly calibrated meter out of service.”

At the time of the fire, a shelter-in-place order was put in place for residents near the refinery, but no evacuation ordered.

PES, which announced five days after the fire that it would shut down and put itself up for sale, filed for bankruptcy protection this week while it awaits resolution of $1.25 billion in insurance claims for damage and lost business. Most of its workforce of 1,100 are set to get laid off on Aug. 25.

The current air-monitoring system provided the public with a “false reassurance" based on inadequate monitoring, testified Dr. Marilyn V. Howarth, director of community engagement at the University of Pennsylvania’s Center of Excellence in Environmental Toxicology.

Charles N. Haas, the head of Drexel’s Civil, Architectural, and Environmental Engineering Department, suggested that a more modern air-monitoring system could be installed at the refinery’s fence line.

“The development of sensors is progressing very rapidly to deploy, where you might consider a network of dozens or hundreds of sensors in the local community to detect particular materials, much the way we use smoke detectors to protect a house," he said.

» READ MORE: How Philly dodged a catastrophe with the refinery explosion

The explosion, which emitted a fireball so large that it was visible to weather satellites, involved a processing unit that uses deadly hydrofluoric acid, or HF, as a catalyst (in its gaseous state, HF is called hydrogen fluoride). Under worst-case scenario, an HF gas cloud could travel seven miles in 10 minutes, potentially exposing more than a million residents. The toxin causes skin and respiratory irritation at low exposures, and is fatal in large doses.

Five workers suffered slight injuries from the incident and were treated on the scene, but there was no suggestion that HF caused any injuries at the refinery or in the public.

“No one, whether it be PES or any of the agencies investigating the fire, has detected HF with a functioning meter, at any point,” said Garrow, the health department spokesman.

During the emergency, refinery operators triggered a safety mechanism that quickly emptied the alkylation unit of HF acid to reduce the chance of a release. The “rapid deinventory” equipment was installed in 2006 by a previous owner, Sunoco Inc.

PES is currently developing plans to neutralize the 30,000 gallons of hydrofluoric acid that remain safely stored in the tank, said Joseph Adams, the deputy secretary for field operations for the Pennsylvania Department of Environmental Protection.

State Sen. Anthony Williams, whose district includes the refinery, said he organized the hearing as a means to focus on the environmental consequences of the accident — whether additional regulatory measures need to be taken, or special care directed at refinery employees or emergency responders who might have been exposed.

Some speakers took the opportunity to air long-standing grievances about the refinery, its emissions and it strained relations with surrounding communities, despite the committee’s attempt to keep the hearing tightly focused on the accident. Williams chided PES for failing to send a representative to the hearing.

The city’s focus now is primarily on what happens to the refinery complex going forward, though the company’s bankruptcy and announcement that it wants to sell the site makes the path forward unclear, Brian Abernathy, the city’s managing director, said in written testimony.

The refinery could be sold and put back into service, or reused for a different purpose. “Or it could be orphaned with no responsible party operating it,” said Abernathy.

“The city has limitations over what type of control it can exert over a privately owned and controlled site," he said. "We are firmly committed to ensuring that the site remains an important part of Philadelphia, and that its future is one that is clean and safer than its past.”