Modern firefighting has become more challenging and riskier than ever because of the changes in our living environment and thermal conditions, which cause firefighters to be exposed to a wide range of toxic substances. In June 2022, the International Agency for Research on Cancer (IARC) officially classified occupational exposure of a firefighter as carcinogenic to humans (Group 1) based on sufficient evidence for cancer in humans.1
Firefighters encounter toxic substances in the form of gas, liquid and smoke particulates. Smoke particulates can also carry gases and liquids to penetrate through a firefighter’s personal protective equipment (PPE). Typical firefighter turnout gear has a moisture barrier that may reduce smoke penetration; however, a conventional hood, made of two layers of loose knits, has little ability to stop toxic smoke from penetrating.
The dermal exposure data in a study suggests that the neck skin is the most exposed part of a firefighter’s body, absorbing combustion products.2 The penetration rate can even be several times greater when temperature and humidity are extremely high as experienced during firefighting. Other studies have shown that polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (VOCs) found in firefighter urine likely come from the neck being the primary site of exposure and absorption.
Comparing barriers used in hoods
Two types of barriers are generally known to be used to block smoke particulates from entering through a firefighter hood– polytetrafluoroethylene (PTFE) membrane or a nanofiber-based nonwoven. Both are flame resistant and are stable at high temperatures.
The PTFE membrane used in hoods is known as a microporous film that blocks particulate ingress at the surface of the membrane by making pores significantly smaller than the size of the particles.
In contrast, a nanofiber-based particulate barrier is a nonwoven consisting of a network of continuous nanofiber filaments that entrap smoke particles in its tortuous maze. The tortuous maze is designed so deep and long that the particles are not capable of escaping. Figure 1 shows the size of a nanofiber compared to a human hair (about 60X to 100X smaller in size). Figure 2 shows a typical nanofiber barrier hood with nanofiber nonwoven being sandwiched between two layers of knit fabrics.
Figure 3 clearly shows the superior particulate filtration efficiency of firefighter hoods made with a nanofiber particulate barrier using Nomex® Nano Flex. As shown, even after 250 washes, the hoods made using Nomex Nano Flex had higher than 99% particulate filtration efficiency, including at the seam.
These same hoods were then subjected to a smoke simulation test in a room filled with fluorescent aerosols. As shown in Figure 4, the hood made with Nomex Nano Flex shows no signs of submicron particle penetration, while the hood made without a nanofiber-based nonwoven shows significant penetration of submicron particles.
In addition to particle filtration efficiency, the other significant difference between a PTFE membrane and a nanofiber-based barrier is breathability. With a PTFE membrane, the pores are very small, limiting the ability of air and moisture vapor to exchange, which limits metabolic heat dissipation from a heat-stressed firefighter.
In contrast, a nanofiber-based barrier is remarkably breathable. In fact, that’s the reason why nanofiber was widely used in facemasks during the COVID-19 pandemic. What’s more, placing a nanofiber nonwoven between two layers of a knit firefighter hood may improve thermal insulation by 10% to 20%, only adding roughly 6% by weight.
In the fire service, comfort is typically measured by air permeability and resistance to evaporative heat transfer (RET). A material having higher air permeability and low total heat loss (THL) is indicative of comfort and breathability. Figure 5 shows the difference in comfort when comparing a nanofiber-based hood to PTFE membrane hoods, as measured by air permeability and RET.
NFPA standards
Since the inception of National Fire Protection Association (NFPA) 1971-2018 standard, which implemented the barrier hood as optional PPE, early adopters have had very good things to say about the benefits of wearing them.
The barrier hoods keep their upper jaw and neck clean, areas that have some of the highest absorption rates for harmful smoke particulates. And wearing barrier hoods does not cause physiological challenges. In their study of firefighting hoods, Kesler, et. al. concluded that soot contamination levels measured from neck skin were lower when firefighters were wearing laundered particulate-blocking hoods compared to when they were wearing new knit hoods. Physiological responses were similar.3
The positive experiences of firefighters wearing barrier hoods, combined with solid research findings, have compelled the NFPA 1970 technical committee to change particulate barrier hood from optional to mandatory for structural and proximity firefighting. As part of the consolidation plan, NFPA 1970 combines NFPA 1971, NFPA 1975, NFPA 1981 and NFPA 1982 into one standard on Protective Ensembles for Structural and Proximity Firefighting. In this new standard, a hood (seam included) shall meet not only the minimal 90% filtration efficiency but also THL ≥325 W/m2 and thermal protection performance (TPP) ≥20 cal/cm2. It is important to mention that the hood material is subject to meeting the requirement for restricted substance level, which is a newly added prerequisite to ensure new firefighter PPE does not contain toxic substances.
Conclusion
Firefighting will always be a dangerous profession; however, important advancements in PPE – such as nanofiber-based hoods – and updated standards are helping to address the long-term health risks associated with exposure to many toxic substances found in our environment. Nanofiber-based hoods have been proven in the lab and, more importantly, in real-world firefighting conditions. Compared to PTFE membrane hoods, they provide superior protection against particulate penetration and are much more comfortable to wear.
References
- International Agency for Research on Cancer, ‘IARC Monographs Evaluate the Carcinogenicity of Occupational Exposure as a Firefighter’, 2022, https://www.iarc.who.int/news-events/iarc-monographs-evaluate-the-carcinogenicity-of-occupational-exposure-as-a-firefighter/
- Fent, K. et al. ‘Systemic Exposure to PAHs and Benzene in Firefighter Suppression Controlled Structure Fires’, The Annals of Occupational Hygiene, Volume 58, Issue 7, August 2014.
Kesler, R. et al. ‘Effects of Firefighting Hood Design, Laundering and Doffing on Smoke Protection, Heat Stress, and Wearability’, Ergonomics, Jun 64(6), 2021.
About the Author
Jian Xiang
Jian Xiang is Global Technology Leader for Emergency Response, DuPont Personal Protection.
