Infectious respiratory diseases such as the current COVID-19 have caused public health crises and interfered with social activity. Given the complexity of these novel infectious diseases, their dynamic nature, along with rapid changes in social and occupational environments, technology, and means of interpersonal interaction, respiratory protective devices (RPDs) play a crucial role in controlling infection, particularly for viruses like SARS-CoV-2 that have a high transmission rate, strong viability, multiple infection routes and mechanisms, and emerging new variants that could reduce the efficacy of existing vaccines. Evidence of asymptomatic and pre-symptomatic transmissions further highlights the importance of a universal adoption of RPDs. RPDs have substantially improved over the past 100 years due to advances in technology, materials, and medical knowledge. However, several issues still need to be addressed such as engineering performance, comfort, testing standards, compliance monitoring, and regulations, especially considering the recent emergence of pathogens with novel transmission characteristics. In this review, we summarize existing knowledge and understanding on respiratory infectious diseases and their protection, discuss the emerging issues that influence the resulting protective and comfort performance of the RPDs, and provide insights in the identified knowledge gaps and future directions with diverse perspectives.

Firefighters are exposed to complex combustion products and to contaminants carried on personal protective equipment (PPE). Occupational exposure as a firefighter is classified as carcinogenic. This review summarizes the current evidence on exposure environments, routes of uptake, contamination and secondary exposure from PPE, and the effectiveness and limits of decontamination approaches. Across incident types, smoke composition varies with the fuels and combustion conditions, but fine and ultrafine particles and semi-volatile organic chemicals are common. Biomonitoring confirms uptake after incidents. Self-contained breathing apparatus reduces inhalation exposure during active suppression, yet exposures persist through dermal absorption at ensemble interfaces and post-incident tasks. Protective ensembles can retain soot-bound polycyclic aromatic hydrocarbons, additive chemicals, and metals; volatiles and particles resuspension in vehicles and stations can extend exposure. Studies show that on-scene preliminary exposure reduction and laundering can lower contaminant burdens on PPE; however, removal remains incomplete and decreases when cleaning is delayed or when gear is aged. Emerging evidence raises additional concern for per- and polyfluoroalkyl substances from foams and coating materials, with limited data on exposure metrics and removability. The field lacks standardized, realistic contamination platforms and a dose-based definition of clean PPE. Integrated intervention studies linking exposure, secondary exposure pathways, biomarkers, and decontamination methods are needed to set performance-based targets and evaluate emerging hazards.

Fire smoke, rich in toxic ultrafine particles and polycyclic aromatic hydrocarbons (PAHs), poses significant health risks to first responders and vulnerable populations. In this study, a reproducible combustion–smoke simulation platform was developed to mechanistically quantify fire behavior, particle emissions, and PAH toxicity under controlled heat flux and oxygen conditions. Consistent combustion and smoke emissions were achieved by measuring heat release rate, particle mass, particle number concentration, and PAH concentration, with an overall average coefficient of variation below 15%. Systematic experiments with representative biomass (pine, oak) and plastics (PVC, polystyrene) demonstrate that fuel composition, heat flux, and oxygen availability jointly govern particle formation and PAH partitioning. Regardless of the combustion factors, ultrafine particles dominated the particle number concentration (55.5–86.2%). Plastic combustion generated 7 to 59 times particle mass, up to 260 times higher PAH emissions, and up to 58,500 times greater PAH toxic equivalent quotient (PAH-TEQ) than wood. Oxygen-deficient and smoldering regimes shifted emissions toward fine and ultrafine particles enriched in high-molecular-weight PAHs, revealing a coupled physical–chemical hazard not captured by bulk PM metrics alone. These results establish a quantitative framework linking combustion regime, particle size, and PAH toxicity, providing critical insight for exposure assessment, PPE design, and mitigation strategies in ventilation-limited and mixed-fuel fire scenarios.

Lithium-ion battery fires present complex aerosol hazards that vary substantially with oxygen availability; however, the effects of oxygen depletion on particulate emissions, soot transformation, and contamination pathways remain poorly characterized. Here, LiNi₀.₈Mn₀.₁Co₀.₁O₂ (NMC811) 18650 cells (2.5 Ah) were tested across a 12-condition state-of-charge–oxygen matrix (0–70% SOC; 20.95%, 7%, and 0% O₂) under 50 kW/m² radiant heating. Reducing O₂ decreased peak heat release rate by 47–71% but increased total particulate matter by 22–158% and particle number concentration by 84–134%. Ultrafine particles (≤100 nm) dominated particle emissions, accounting for 80–94% of total counts. Airborne metal emissions exhibited a non-monotonic oxygen response, reaching 11.90 ± 0.68 µg/m³ at 7% O₂ and 70% SOC, representing a 65% increase relative to normoxic (20.95% O₂) conditions. Multi-technique soot characterization (HRTEM, Raman spectroscopy, and XPS) revealed increased carbon ordering under oxygen depletion, evidenced by reduced D1/G ratios (0.386→0.261), decreased fringe tortuosity (1.37→1.13), and substantially lower oxygenated surface carbon. Soot–metal aggregate particle diameters increased from 47 ± 9 to 61 ± 11 nm. SEM–EDS identified Ni- and Co-rich particle deposition throughout firefighter PPE layers under reduced or absent flaming conditions. These results demonstrate that oxygen depletion shifts NMC battery-fire emissions toward enhanced ultrafine particle loading, persistent metal-containing aerosols, and more ordered soot structures, providing source-characterization data for battery-fire hazard assessment and for future validation of mitigation, exposure-control, and PPE-contamination protocols.

Wildland-urban interface (WUI) and lithium-ion battery-involved fires are emerging hazardous emission sources that generate chemically complex smoke containing carcinogenic and mutagenic polycyclic aromatic hydrocarbons (PAHs) together with toxic metal-rich aerosols. This combined organic–metal mixture creates an unresolved exposure-assessment challenge for firefighters and fire-affected communities because low-burden passive samplers have not been evaluated for simultaneous PAH and metal capture under fire-smoke conditions. Here, six chemically and structurally distinct candidate materials: silicone wristband (SWB), polyurethane foam (PUF), activated carbon felt (ACF), thermoplastic polyurethane nanofiber (TPU), cotton fabric (CF), and cotton ball (CB) were exposed to controlled smoke from pine, pine + polystyrene (PS), and pine + battery scenarios and benchmarked against active reference sampling. Active sampling confirmed source-specific hazards: total PAHs increased from 10.2 µg m⁻³ in pine smoke to 167.8 µg m⁻³ with PS, whereas the battery scenario produced a metal-dominated aerosol, especially nickel (~840 µg m⁻³). Passive capture was strongly material-dependent. PUF retained the highest total PAH loading (~1.50 µg cm⁻²), ACF selectively enriched volatile naphthalene, and SWB retained the highest battery-derived metal loading (~46 µg cm⁻²). SEM-EDS showed that PAH and metal retention followed distinct mechanisms: sorption and phase partitioning for PAHs versus deposition and surface retention of metal-bearing particles. Therefore, no single passive material represented the full combined hazard. These findings support a complementary, multi-material passive sampling strategy for source-resolved PAH-metal hazard assessment, protective-equipment evaluation, decontamination studies, and future exposure-reduction efforts in fire-affected settings.