Safety Week: Study Sets Sights on SCBA Lens
Perhaps to no surprise, a study has found that the weakest link in a firefighter’s personal protective equipment ensemble is the facepiece lens on the breathing apparatus, and a federal agency is developing recommendations to improve firefighter safety.
Motivated by firefighter fatalities in which it appeared the lenses of self-contained breathing apparatus (SCBA) facepieces thermally degraded while they were still flowing air, the National Institute of Standards and Technology (NIST) found that the polycarbonate used in virtually all facepieces can't take as much heat as the rest of the ensemble.
High heat flux environments, frequently encountered by interior firefighters, can damage lenses to the point of failure, said Nelson Bryner, deputy chief and a chemical engineer in the Fire Research Division (FRD) of the Engineering Laboratory (EL) at the National Institute of Standards and Technology (NIST).
"If a glove fails, you burn your hand," Bryner said. "If a facepiece fails, we're talking about our respiratory systems. That's a whole different story."
The federal National Institute for Occupational Safety and Health (NIOSH), Fire Fighter Fatality Investigation and Prevention Program, asked NIST to look at the lenses after it had investigated seven recent fatalities where it appeared the firefighters’ lenses had experienced bubbling and crazing, and in some cases, holes.
Bryner was asked to speak at the National Fire Protection Association's conference and expo on June 14 in Boston. During that presentation, he explained his finding in a presentation called, "Characterizing the High Temperature Performance of SCBA Facepieces."
He explained that virtually all facepiece lenses are made of polycarbonate, which affords firefighters high optical clarity, has good impact resistance and affords thermal resistance.
However, Bryner doesn't think it’s enough, and is proposing that SCBA facepiece lenses be subjected to more rigorous testing, and perhaps made of different materials that are even more heat resistant.
Bryner explained that current NFPA testing requires lenses to be exposed to simulated heat and flame that equals exposure to 200 degrees for 15 minutes.
"Firefighters routinely see temperatures far greater than boiling water," he said, noting that facepieces can begin to soften at 248 degrees.
The rest of a firefighter's ensemble is subjected to temperatures of 500 degrees for five minutes, and turnout gear is tested at much higher heat flux temperatures up to 10 times the rest of the ensemble to help firefighters survive flashovers.
"Facepieces’ lenses ought to be at least as good as the rest of the firefighters' ensembles," Bryner said.
Bryner brought his point home with time-lapsed videos of the testing done on facepieces. As temperatures increased for longer times, the lenses would start to soften and actually pulse with the breaths of the high-tech mannequin. Soon after, the video showed crazing - a cracking noticeable in the surface - followed by small bubbles, then big bubbles and eventually holes as the material thinned.
Additionally, some of that deterioration was experienced at temperatures lower than what are classified as Class III fire temperatures -- 160 degrees to 260 degrees -- categorized as ordinary/hazardous conditions. That thermal class can be encountered at totally involved fire experienced outside a burning room or small building.
Bryner said the purpose of the research was multi-fold and included the need to improve SCBA facepiece design, and to develop more representative and realistic testing that mimics real-life fire conditions. It was also designed to help develop new designs and use of materials and coatings. It was focused exclusively on the lenses of SBAC and not any of its ancillary components like the silicon molded face seals. The findings of the study will help the NFPA 1981 committee develop new standards for SCBA facepieces.
Part of the recommendations will be for new testing that includes radiant heat panels that will more realistically mimic heat flux and temperatures experienced in fires.
Bryner is also proposing that additional testing be created to simulate firefighters wiping the facepiece with a gloved hand. As the polycarbonate material degrades, it becomes milky in color and firefighters may try to wipe lenses, which could cause substantial damage when the material is softened.
Interestingly, the testing showed that a facepiece could develop a hole of up to a half-inch in size and still maintain life, provided there was a positive flow of air in the mask. In fact, the flow of air cools the facepiece lens and could make it last longer, Bryner said.
There are other kinds of materials that could work for lenses, he said. Polyethersulfone has a deflection temperature of 401 degrees compared to 284 degrees for polycarbonate, and has a melting temperature of between 644 degrees and 734 degrees, compared to 437 degrees and 599 degrees for polycarbonate.
"Polyethersulfone has significantly better performance, but it does have a little bit of color to it," Bryner said. "There is a trade off that the committee will have to review."
There are some other products that might work as well, but they have to be flat to work at their peak and that would require significant redesign of the facepiece, and may prove unacceptable to firefighters who have become accustomed to wide views and peripheral vision.
He said material that resembles bulletproof glass may also provide a solution to more heat-resistant lenses, but the price may be prohibitive. Initial inquiries suggest the price would be $1,400 for an SBCA facepiece lens.
It is not, however, the mission of NIST to consider price when doing research, he said, noting that the organization is devoted to the science and testing of materials. The political aspects and tradeoffs come in when the rules and standards are made.
Other options include metal coatings of the lens, Bryner said, noting that firefighters in Europe and South America are used to gold-coated lenses. That however, comes at an optical price with a 50 percent reduction in transparency.
"We think there are some good tradeoffs to be made to keep firefighters safe," he said.