The CAAA directs EPA to "reduce overall risks to human health and the environment."13 EPA has attempted to characterize emission levels and exposure pathways in each use sector in order to minimize environmental impacts. Thus, EPA first looks for the outliers such as the perfluorinated carbons (PFCs), which have atmospheric lifetimes in excess of 3,000 years and are virtually indestructible.14 Yet, because PFCs have a favorable toxicity profile, EPA recognizes that they can play a role in fire protection applications where other agents are not suitable for either technical or safety reasons. Thus, EPA has listed PFCs as acceptable with certain contingent restrictions. Likewise, although HFC-23 (CF3H) has a 300-year lifetime,15 it is a by-product of the manufacture of HCFC-22 (CF2HCl), which will continue to be produced as an intermediate for the manufacture of polymers such as Teflon™, and EPA thus has placed no restriction on its use as a fire protection agent.
In response to concerns about environmental effects and efficacy, fire protection manufacturers are also developing several new alternative fire protection technologies, including inert gas systems, water mist systems, and powdered aerosol systems. These non-halocarbon alternative agents require a different means of determining risk during use. Some of the newer non-halocarbon alternative agents—the inert gas systems—limit but do not entirely remove the oxygen available to a fire. The most important condition for the safe use of such agents is the stipulation that the amount of oxygen remaining in the area of release is sufficient to maintain central nervous system function and that reduced oxygen does not impair escape from the area if people are exposed.
Powdered aerosol systems present still other risk assessment issues. The conditions determining the safe use of these agents must account for potential deposition in the respiratory tract of inhalable particles, ranging from very small particles that may be deposited in the alveoli to large particles capable of irritating the upper nasal passages. The size of such particles may be the most significant factor determining risk. Water mist systems using pure water pose little risk, although additives must be evaluated on a case-by-case basis to determine potential health hazards. A concern with both mist and powdered aerosol systems is the visual obscuration that occurs during discharge and that may potentially limit individuals' ability to leave the area.
Because the risk analyses for alternative fire protection technologies differ somewhat from standard EPA risk assessment procedures, EPA has encouraged the formation of ad hoc workshops and medical peer-review panels to characterize the risks presented by each new technology and to help delineate the appropriate exposure limits for different clinical groups. Conditions for the appropriate use of inert gases with limited oxygen have been evaluated by special medical panels, and EPA has also solicited guidance from OSHA on conditions of use, since OSHA will ultimately determine the proper use of all fire suppressant systems. Workshops and panels have been formed to analyze issues concerning powdered aerosols and water mists.
The EPA has been largely successful in identifying several agents and technologies that can be used in most total flooding and streaming fire protection applications. There are still some application areas that pose technical challenges, however, including aviation (both civil and military), military tanks, some military shipboard uses, and explosion inertion applications. The U.S. military has been a leader in research and development efforts, e.g., the selection of HFC-125 (CHF2CF3) for the design of fire protection systems on new military aircraft and the selection of HFC-227ea (CF3-CHFC-CF3; or FM-200™) for new shipboard machinery spaces. For commercial aircraft, the Federal Aviation Administration (FAA) is spearheading an industrywide R&D effort to identify effective substitutes for halon as a fire suppressant. Once an agent is identified for complex systems, much work still remains to design, manufacture, and certify the fire protection system (see Chapters 1 and 2 for details).