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Phosphorus tribromide (PBr3) has effective fire fighting properties which make it a possible replacement tor Halon 1301. Results of genotoxicity testing using bacterial Salmonella strains indicted PBr3 is not a mutagen for either frame shift or base-pair substitution tester strains in both buffered and unbuffered solutions. Acute aquatic toxicity testing determined a 96-hour LC50 value for fathead minnows at 71 mg/L (25 mg/L NOEC) and a 48-hour EC50 value for Daphnia magna at 22.6 mg/L (6.25 mg/L NOEC). Application of 10 ul neat PBr3 to intact skin of an anesthetized NZW rabbit caused edema and necrosis of the treated skin within 10 minutes of dosing. Toxicity was not observed in rats exposed for 4 hr to 0.4 mg/L in an inhalation exposure. Male rats (5/group) were exposed to PBr3 vapor, 4 hr/d for 5d, at 0, 0.06, 0.16 and 0.51 mg/L PBr3; 0.06 mg/L was the NOAEL. Rats (10/sex/group) were exposed to PBr3 vapor, 4 hr/d, 5 d/wk, for 4 wk at 0, 0.03, 0.01 and 0.3 mg/L; the NOAEL was 0.1 mg/L. Phosphorus tribromide reacts with moisture in the air to produce phosphonic acid and hydrogen bromide gas (HBr). A 10 cc cartridge of PBr3, tested as fire suppressant, would react to form an estimate 25 g of HBr. If uniformly distributed in a room 10x10x5 m in size, the maximum HBr concentration would be 0.05 mg/L (50 mg/in3); if in a 61.5x46.2x14.8 m hanger, it would be 5.9x10-4 mg/L (0.59 mg/ni3). Since the exposure limit for HBr is 10 mg/m3, the maximal hanger concentration would be below the action level, indicating that an acceptable exposure level during usage on aircraft is possible based on the physical characteristics of PBr3 and the small amount necessary for fire suppression. Dermal and eye contact with PBr3 should be avoided due to its corrosive behavior.

The objective of this program was to demonstrate the effectiveness and to determine the toxicity and associated risks of labile bromine compounds as halon replacements for suppression of engine nacelle and dry-bay fires in military aircraft. Tests performed during this program have shown that labile bromine compounds generally and phosphorous tribromide (PBr3) in particular, are more effective than Halon 1301 for both engine nacelle and dry-bay fire extinguishment and have neither ozone depletion nor global warming potentials. Theoretical electronic structure calculations and experimental chemical kinetics investigations were performed to enhance the understanding of the chemical action of PBr3 fire suppression. Independent toxicity studies supervised by the tri-services toxicology laboratory (AL/OET) at Wright Patterson Air Force Base, determined that PBr3 was neither mutagenic nor toxic to aquatic organisms. Additional data on the toxicological properties of PBr3 were acquired by AL/OET including dermal irritation and acute and subchronic inhalation toxicity. These results formed the basis of their independent risk assessment, which concludes that use of PBr3 as a halon replacement is safe in normally unoccupied spaces such as engine nacelles or dry-bays.

Phosphorus tribromide (PBr3) is being considered by the DOD as a possible replacement for Halon 1301. This study was designed to determine the effects following single, high-concentration exposures which could occur in accidents, as well as repeated, low-concentration exposures which could occur on flight lines or where maintenance commonly occurs. Application of 10 or 50 microliters neat PBr3 to intact skin of an anesthetized NZW rabbit caused edema and necrosis of the treated skin within 10 minutes of dosing. Microscopic examination confirmed necrosis of the skin and underlying areas, including the skeletal muscle of the subcutis.