Chapter 6: SCREENING TESTS FOR

CANDIDATE FIRE SUPPRESSANTS

Richard G. Gann, Ph.D. 

National Institute of Standards and 

Technology

J. Douglas Mather, Ph.D. 

Chemical Development Studies, Inc.

TABLE OF CONTENTS

6.1 The Need for Screening Tests..........................................................................................518 

6.2 General Approach ............................................................................................................519 

6.3 Agent Properties to be Screened......................................................................................519 

6.4 Fire Suppression Efficiency Screening............................................................................521 

6.4.1 Estimation of Values.............................................................................................521 

6.4.2 Cup Burner ............................................................................................................522 

6.4.3 Dispersed Liquid Agent Fire Suppression Screen (DLAFSS)..............................525 

6.4.4 Transient Application, Recirculating Pool Fire Apparatus (TARPF)...................535 

6.4.5 Summation.............................................................................................................557 

6.5 Atmospheric Lifetime Screen ..........................................................................................557 

6.5.1 Factors Affecting Atmospheric Harm...................................................................557 

6.5.2 Prior Estimation Methods......................................................................................558 

6.5.3 The Screening Protocol .........................................................................................561 

6.5.4 Removal by Hydroxyl Radical Reaction...............................................................561 

6.5.5 Measurement of Photolysis Rates .........................................................................565 

6.5.6 Estimation of Reactivity........................................................................................568 

6.5.7 Solution of Fluoroalkylamines..............................................................................571 

6.5.8 Reactivity of Alkyl Phosphates towards Radicals in Water..................................571 

6.6 Volatility Screen ..............................................................................................................572 

6.6.1 Extinguishant Volatility ........................................................................................572 

6.6.2 Extinguishant Evaporation Equilibrium................................................................572 

6.6.3 Extinguishant Evaporation Rate............................................................................574 

6.6.4 Boiling Temperature..............................................................................................576 

6.7 Toxicity Screen ................................................................................................................577 

6.7.1 Hazards and Regulation.........................................................................................577 

6.7.2 Screening Process..................................................................................................578 

6.7.3 Literature Values...................................................................................................579 

6.7.4 Quantitative Structure Activity Relationships.......................................................581 

6.7.5 Non-animal Laboratory Testing ............................................................................584 

6.7.6 In Vitro Tests.........................................................................................................589 

6.7.7 PBPK Modeling of Cardiotoxic Effects................................................................590 

6.7.8 Limit Tests.............................................................................................................596 

6.8 Application of Screen Tests.............................................................................................597 
518 Screening Tests for Candidate Fire Suppressants 

6.9 References........................................................................................................................598 

6.1 THE NEED FOR SCREENING TESTS 

A primary component of the Next Generation Fire Suppression Technology Program (NGP) was the 

survey of the "world" of chemistry, seeking to identify chemicals that merited examination under 

practical fire suppression conditions or to enable a finding that no previously unidentified such chemicals 

existed. This entailed consideration of thousands of compounds, a task detailed in the next chapter of this 

book. While full-scale testing of chemicals is eventually necessary to demonstrate acceptability, it is

impractical to use such tests to narrow the large list of candidates. Full-scale tests are strongly influenced 

by the initial and boundary conditions surrounding the fire and suppression event. Complex, non-linear

relationships among the agent, the flow field, and the fire cannot be unraveled from these full-scale 

suppression tests because critical parameters cannot be controlled tightly enough, and the number of tests 

is constrained by cost, time, safety and environmental considerations. Rather, performing this survey

required a protocol for rapid and inexpensive screening of this large number of chemicals to identify those 

relatively few agents worthy of further examination as alternative fire suppressants. 

There are multiple, critical chemical properties whose beneficial values promoted the widespread use of 

halon 1301 (CF3Br) and which would contribute to the consideration of alternative fire suppressants.

These properties were listed in Chapter 1 and will be discussed further below. A testing hierarchy was 

needed to ensure that the most important properties were considered first. Estimation of values of these 

properties, for comparison with appropriately selected target values, needed to be obtained using minimal 

mass of the chemicals since: 

• The purchase of testing quantities of this many chemicals would be prohibitively expensive, 

• Many compounds would need to be custom synthesized, at significant cost, and 

• Some compounds would be of unknown toxicity. 

Prior to the NGP, various methods had been used to screen chemicals. Some of these were explicit, 

others were informal or ill-defined. Shortly before the start of the NGP, a pilot effort had arrived at

preliminary screening procedures and criteria for alternatives to both halon 1301 and halon 1211 

(CF2ClBr).1

 These screening procedures were able to distinguish between gases and liquids of varying 

degrees of performance. The authors noted the importance of the purity of the chemical being examined. 

They also developed classes of performance for new chemicals, related to the values for halons 1211 and

1301. 

The NGP development of a screening protocol built on this and other formal and informal procedures. 

The first item of business was to review the existing protocols. In this pursuit, the NGP conducted a 

Workshop on Agent Compatibility with People, Materials and the Environment on November 14 and 15, 

1997.

2

 Approximately 40 experts from government, academia, and industry participated in the evaluation 

and revision of current test methods for application to screening candidates for the next generation of fire 

suppressants. A distinction was made between screening and evaluation/analysis tests. Screening tests are 

meant to provide guidance in the identification of suitable compounds (and the elimination of unsuitable 
Agent Properties to be Screened 519

compounds) from a long list of candidates, whereas evaluation/analysis tests are performed as part of the 

risk assessment process that is required for qualifying all halon replacement chemicals. This chapter 

describes the set of screening tests used by the NGP research team. 

6.2 GENERAL APPROACH 

For a single candidate suppressant, performing a full battery of screening tests for the 12 properties listed

in Chapter 1 could require about 4 moles of chemical, about 8 days, and $15,000 (in 1990 dollars).1

 This

was beyond the resources of the NGP. Thus, the first order of business was to identify which of the 

12 properties should be the subjects for initial screening. This decision was based on the properties 

deemed most important for a successful chemical, most likely to disqualify a chemical, and easiest to 

determine. A second task was to identify or develop efficient and reasonably accurate screening tests,

recognizing that these would not necessarily be the tests used in the final acceptance of an agent. The 

selection was based on ease of operation for these types of chemicals, the extent of prior data available,

the amount of chemical required, and the relevance of the figure(s) of merit to the NGP applications. 

Existing methods were adapted or new methods were developed to meet anticipated needs. The third task 

was to set an order of screening tests to perform. As will be seen in Section 6.9, not all of the screening 

tests were routinely used. 

For each property to be screened, a three-step process was followed:

1. Determine whether there was a value in the published literature. 

2. Estimate the value of the property from the values for analogous compounds. If available, 

make use of quantitative structure-activity relationships (QSARs). 

3. Perform the selected screening test. 

The expanse of organic and inorganic chemistry was sorted into chemical families. Each family was 

characterized by one or more chemical features, e.g., the presence of a carbon-carbon double bond or a 

bromine atom. Since a family might include hundreds of individual chemicals, a small number of most 

likely prospects were identified that had desirable values of the screening properties, estimated using the 

first two of the steps above. For instance, chemicals with low boiling points were preferred, since they

would be most likely to disperse rapidly (see below). If commercially available, small quantities of these

chemicals were purchased. Otherwise, source(s) for custom synthesis were identified. (This is detailed in

Chapter 7.) The laboratory tests were then performed as described in the remainder of this chapter. 

6.3 AGENT PROPERTIES TO BE SCREENED 

In reviewing the 12 properties identified as affecting the desirability of a chemical compound as a 

halon 1301 alternative, the following evaluation emerged: 

• Fire suppression efficiency. This was a property that needed to be screened. Halon 1301 

quenches most flames at a concentration of about 3 volume % in air. HCF-125 (C2HF5), 

noted in Chapter 1 as the best commercially available alternative under the DoD Technology

Development program, has acceptable values of other key chemical properties, but typically

quenches flames at about 8 volume % in air, with some modest variation depending on the 

fuel.3

 It was dissatisfaction with the additional required mass and storage volume that led to 

the formulation of the NGP. The target for the screen results of candidate chemicals was a
520 Screening Tests for Candidate Fire Suppressants 

fire suppression efficiency better than HFC-125 and preferably comparable to that of 

halon 1301. 

• Effective quenching of flame re-ignition. It was presumed that a chemically active 

suppressant (see Chapter 3) would require little agent mass beyond that needed for open 

flame quenching. A physical flame suppressant would need an increase similar to that for 

HFC-125. Thus, no screen testing was deemed necessary.

• Atmospheric lifetime. Environmental effects were the problem that provoked the search for

halon alternatives. Since the focus of the NGP was on identifying volatile candidate fire 

suppressants that would disperse well, compounds were screened for atmospheric effects 

only. Atmospheric lifetime became the key environmental figure of merit, since chemicals

with short residence times in the troposphere would make minimal contributions to ozone 

depletion, global warming, and other, as yet unknown, types of environmental pollution. The 

Environmental Protection Agency (EPA) required a low value for approval under its 

Significant New Alternatives Policy (SNAP) Program.

4,i

 Manufacturers could expect a

continuing market in a chemical with a low value (assuming desirable values of the other 

chemical properties). Similarly, fire suppression systems owners did not want to be required 

to replace their systems yet another time. For screening purposes, the NGP aimed for lifetime

estimates of the order of one month. Halon 1301 has an estimated atmospheric lifetime of 

65 years.5

• Effectiveness of low-temperature dispersion. Because in-flight fires often occur at 

operational altitudes where temperatures are low, a successful agent must have high 

volatility.

ii NGP experiments established that excellent dispersion was achieved for 

chemicals whose boiling points were below the ambient temperature in the engine nacelle or 

dry bay (Chapter 8). The military specification for this minimum operating temperature is 

-40 °C, although operating temperatures in actual halon 1301 releases have generally been

significantly higher (Chapter 2). Halon 1301 and HFC-125 have boiling points below -50 °C, 

and flashing does occur. Thermodynamic calculations showed that a typical suppressant

chemical will have an equilibrium concentration of at least 5 volume % at -40 °C if its boiling 

point is below 20 °C. However, at these temperatures, the evaporation rate may be too slow

for effective dispersion to all parts of the engine nacelle or dry bay before the agent is flushed 

from the space. Nonetheless, the NGP used a maximum boiling point criterion of 20 °C for 

screening purposes. As a peripheral benefit, a low boiling point generally means that an 

agent would eventually evaporate with little or no residue. Thus, no additional screen for 

agent residue was used. 

 

i

 The EPA is responsible for assuring that no halon replacement presents an unacceptable risk to human health or the 

environment. For toxicity, the primary focus has been to assure the potential for timely egress from the immediate environment 

generated by the discharge of a fire suppressant. Although the EPA has no requirements on screening candidates, the EPA 

does characterize the risks associated with halon replacement agents. As part of this process, information on the toxic 

properties of chemical agents is compiled and combined with the exposure assessment results to estimate risk. The EPA 

requests toxicity data on a case-by-case basis for the risk characterization. Because many of the current halon replacement 

agents have been halogenated hydrocarbons, information related to acute toxicity, cardiac sensitization, subchronic toxicity, 

developmental toxicity and genetic toxicity have been required. Inert gas agents have had a different set of requirements, such

as the concentration that produces hypoxia. The EPA, however, does not state specific methods to use for these tests. Ideally, 

the methods used to screen chemicals advanced as fire suppressants will address the endpoints which the EPA will use to 

regulate them. ii Halon 1301 is referred to as a total flooding agent. When discharged, it rapidly fills the space it is protecting, becoming able to

quench flames in virtually any location. This is contrasted with streaming agents, which are (as the name implies) squirted at a 

fire. For these fluids, low volatility generally enhances performance by allowing a greater focus of the stream without loss by 

evaporation. 
Fire Suppression Efficiency Screening 521

• Low toxicity of the chemical and its combustion and decomposition products. Most engine 

nacelle and dry bay fires occur during flight and no people are exposed to the released agent 

or its by-products. The same is true for in-flight accidental discharges (of which there are

very few) in unoccupied spaces. However, there is the potential for a fire or an accidental

discharge when the aircraft is on the ground, e.g., during maintenance operations or fire 

suppression system re-charging. Then, there is the possibility of exposure of people. For

halogenated compounds, the primary risk from exposure to low concentrations is cardiac 

irregularity. The NGP screening criterion was the suppressing concentration should be lower 

than the lowest observed acute exposure level (LOAEL) for inducing cardiac arrhythmia. 

These properties are not entirely independent. For example, the molecules in fluids with low boiling 

points are generally small, i.e., composed of relatively few atoms. Some of the molecular features that 

lead to short atmospheric lifetime, such as a carbon-carbon double bond, increase the number of atoms 

over the simplest single-carbon compounds, which generally increases the boiling point. The presence of

atoms that provide efficient flame extinguishment, such as a bromine atom, may increase toxicity. 

The following properties were not included in the initial screening. However, preferred measurement

methods were identified.2

• Low electrical conductivity

• Low corrosivity to metals 

• High compatibility with polymeric materials 

• Stability under long-term storage 

6.4 FIRE SUPPRESSION EFFICIENCY SCREENING 

6.4.1 Estimation of Values 

The first step in obtaining an approximate value for the concentration of a chemical compound needed to 

extinguish a flame was based on prior measurement of the compound itself or the known efficiency of 

analogous compounds. In general, candidate compounds belonged to one of three categories: 

• Physically active molecules. These compounds absorb heat into their vibration-rotational

modes. In some cases, the breakage of chemical bonds (without immediate recombination)

may result in additional endothermicity. The heat absorption removes enthalpy from the 

flame and its environment, cooling the flame. The effectiveness of such compounds typically 

increases with an increasing number of atoms, and the more effective compounds tend to

have higher boiling points. (See below.) Pitts6

 had performed a thorough study of these

chemicals (Chapter 7), and his compilation became the basis for assessing any further 

candidates of this type. 

• Reactive molecules. There are some atoms, such as fluorine, that can be extracted from the

suppressant molecule by a hydrogen atom from the flame. The resulting HF bond is 

sufficiently strong that the fluorine atoms play no further role in the quenching of the flame. 

There were flame extinguishment data for some fluorocarbons and hydrofluorocarbons, and 

these values provided a reference for considering additional such compounds. 
522 Screening Tests for Candidate Fire Suppressants 

• Catalytic molecules. Certain atoms were known to participate in chain reactions that lead to

the recombination of flame-propagating free radicals. The bromine atom, present in CF3Br,

is one of these. Iodine and phosphorus atoms were known to be of high effectiveness, as

were some metal atoms. Chlorine atoms are also known to be catalytic, but with a shorter 

chain length and thus lower effectiveness than bromine atoms. The presence of one or more

of these atoms in a candidate chemical suggested the potential for flame suppression 

efficiency near that of halon 1301. NGP research and testing refined this list as the Program 

evolved. 

6.4.2 Cup Burner 

The most commonly used apparatus for examining fire suppressant chemicals is the cup burner, and it

was the workhorse of this Program. In 1961, Creitz published a bench-scale apparatus that could be used 

to measure the effectiveness of chemicals at extinguishing a small, gaseous diffusion flame that was 

housed in a vertical glass chimney.

7

 His apparatus was the predecessor of the cup burners that were to 

follow. By 1973, workers at ICI had adapted this to enable the use of a burning liquid pool.8 Today,

there are numerous cup burners in use around the world, not all of which are identical. There is a project 

underway to create an NFPA standard for the cup burner design and procedure for its use. A schematic is 

shown in Figure 6–1; close-up photographs appear in Figure 6–2. 

A similar burner was used at the Factory Mutual Research Corporation (FMRC).9

 Researchers at the

National Institute of Standards and Technology (NIST) replaced the liquid pool with a stick of 

polymethylmethacrylate1

, and Tapscott and co-workers at the New Mexico Engineering Research 

Institute (NMERI) developed a liquid cup burner of smaller diameter that would use less agent.10 In this

apparatus, the cup diameter was 13 mm and the chimney diameter was 50 mm. 

For gaseous or volatile liquid flame suppressants, the NGP used the NMERI cup burner with n-heptane as 

the fuel. The operation was straightforward.10 The pure chemical was placed in a Tedlar bag, which was

placed in a pressure chamber. To force the agent from the bag, a known flow of air was introduced into

the chamber, compressing the bag and causing a known flow of agent vapor to the base of the cup burner. 

Dilution air, monitored with a mass flowmeter, was also introduced into the burner base. 

The level of the burning liquid fuel was set to be flush with the lip of the cup. The air stream was

established flowing upward past the cup, typically at about 0.17 L/s (flow velocity about 0.08 m/s) in the 

NMERI device. After a short pre-burn period, the candidate suppressant was introduced into the air

stream at a concentration below the expected value for flame extinguishment. After determining that this

concentration did not quench the flame (within about 10 s), the suppressant flow was increased, and the

flame was observed for about 10 s again. This stepwise increase was continued until the concentration 

was sufficient to extinguish the flame. Further tests were sometimes performed to refine the

extinguishing value, generally to within about 0.5 volume %. The process typically consumed 0.03 moles 

of the chemical if the extinguishing concentration was about 5 volume %.
Fire Suppression Efficiency Screening 523

Figure 6–1. Schematic of the 

Cup Burner Apparatus. 

(Adapted from Kidde-Fenwal 

drawing, used with permission)

Figure 6–2. Photographs of a 

Standard Cup Burner.

(Kidde-Fenwal photographs, 

used with permission) 
524 Screening Tests for Candidate Fire Suppressants 

A different injection mode was used if the candidate suppressant was of lower volatility, i.e., the boiling

point was such that the equilibrium concentration in ambient air was near or below the extinguishing 

concentration.10 The liquid sample was discharged by a syringe pump at a measured rate into a nebulizer.

The sample reservoir and syringe pump were cooled to depress vaporization and minimize bubble 

formation within the pump and connective tubing. The nebulized mist was carried by a metered flow of

heated air (Figure 6–3) into a heated glass column and fully evaporated. The air-agent mixture entered

the cup burner at a temperature between 40 °C and 50 °C. The cup burner chimney was heated to

eliminate condensation of the agent on the glass surface. (See Figure 6–4.) 

Using these approaches, the apparatus was usable to address compounds with boiling points from at least

-58 °C (halon 1301) to 130 °C. In some cases, when it was not possible to obtain a sufficient mass of the

chemical for a determination of the extinguishing concentration, the value was estimated, and a single test

was performed to establish an upper bound. 

Figure 6–3. Schematic of Sample Nebulizer. 

(Chemical Development Studies, Inc. drawing, used with permission)
Fire Suppression Efficiency Screening 525

Figure 6–4. Heated Cup Burner 

for Appraising High-boiling 

Flame Suppressants. (Chemical 

Development Studies, Inc. 

photograph, used with permission) 

6.4.3 Dispersed Liquid Agent Fire Suppression Screen (DLAFSS) 

Need for a Liquid Agent Screen 

In practice, fluid suppressants are stored as liquids under pressure. This enables storing more than 

100 times the agent mass in a given volume, compared to storage as a compressed gas. Upon release, the 

fluid partially vaporizes, with the remaining fraction emerging as droplets of varying size. When they

reach the flame zone, the droplets absorb additional heat, cooling the flame as they vaporize. This heat of 

vaporization can be a significant contribution to the agent's flame suppression efficiency, especially for

water or other polar compounds. It was anticipated that some such compounds, with other desirable 

properties, would merit screening for their fire suppression effectiveness. The cup burner, dispensing only

gases, does not capture this additional benefit.

For the NGP, Yang and co-workers designed, constructed, and demonstrated a laboratory-scale apparatus

that could perform the screening of liquid agents in a well-controlled experimental setting.11 The

following describes the design and the operational procedure of the Dispersed Liquid Agent Fire

Suppression Screen (DLAFSS). Detailed component drawings are documented in the appendices to a 

more detailed report.12 

General Description 

As shown in Figure 6–5 and Figure 6–6, the apparatus consisted of: 

• a burner that generated a flame whose properties made it appropriate for examining liquid

aerosol suppressants and whose extinguishment was readily observed, 
526 Screening Tests for Candidate Fire Suppressants 

• a wind tunnel that produced a flow field in which the droplets were transported to the flame 

in a well characterized manner, and 

• a liquid droplet generator that generated small amounts of liquid aerosol agent of controlled

and known dimension. 

The device could also be used to screen gaseous fire suppressants. In principle, the apparatus could be 

employed to screen powders by incorporating a powder delivery system in lieu of a liquid droplet 

generator; however, that was not pursued. 

Test section

Thermal insulation

Contraction section

Transition flange

Settling chamber

Screen + holder

Honeycomb + holder

Diffuser

Transition duct

X-Y traverse

Flexible

aluminum ductwork

Screw

jack

Aluminum plate

Aluminum plate

T-slotted aluminum

structural extrusion

Burner

Droplet generation

device

From blower

Gaseous agent

(optional)

Figure 6–5. Schematic of the DLAFSS Wind Tunnel.12
Fire Suppression Efficiency Screening 527

Figure 6–6. Photograph of the DLAFSS Wind Tunnel.12

The burner was designed to be durable, easily built, installed, operated, and able to generate reliable 

screen test data. It is shown schematically in Figure 6–7 and photographically in Figure 6–8. The burner

was a replaceable, porous (20 μm typical pore diameter) sintered stainless steel, standard 1⁄2” UNFthreaded cup filter, with a length of 3.18 cm, an inner diameter of 1.12 cm, and an outer diameter of 1.58 

cm. The burner was screwed onto an extended water-cooled insert through which fuel was injected. The 

water was used to cool the burner (to prevent damage to the porous surface structure) and the fuel (to

prevent fuel pyrolysis prior to its ejection through the porous surface). A water-cooled, cylindrical brass

rod, of the same diameter as the burner, extended the burner cylinder across the test section. The

downstream hemicylinder of the burner surface was coated with a thin layer of high-temperature-resistant

black paint in order to prevent fuel ejection into the wake region. The high pressure drop across the

porous sintered surface assured a very uniform fuel flow over the burner surface.
528 Screening Tests for Candidate Fire Suppressants 

3.18 cm

Fuel (propane)

1.58 cm Cooling water

Fuel Brass insert

Sintered stainless steel filter

High-temperature paint

Cooling water inlet

(Outlet on other side

of insert)

Figure 6–7. Cut-away View of the DLAFSS Burner Insert.12

Figure 6–8. Photograph of the DLAFSS Burner Assembly.

12
Fire Suppression Efficiency Screening 529

Propane, which is moderately sooting, was used as the fuel. Its flow was regulated by a mass-flow 

controller (with a range of 0 L/min to 4 L/min), which was controlled by a personal computer using a data 

acquisition board and software. A bubble flow meter was used to calibrate the mass-flow controller. The 

fuel was ejected uniformly from the cylinder surface, and a counterflow diffusion flame was formed in the

forward stagnation region of the burner (Figure 6–9a). 

This type of burner, devised by Tsuji13,14,15,16 and commonly referred to as a "Tsuji burner," has been

extensively used to study flame structure and flame extinction using inert gases,17 halons,18 and 

powders.18 There are many advantages to the use of a counterflow cylindrical burner. The fuel and the

oxidizer flows can be independently adjusted. The flame is laminar, two-dimensional, and very stable in

the forward stagnation region. The geometry of the burner and the flow field allow for relatively simple 

analysis of the forward stagnation region.19,20,21,22,23,24,25 Both enveloped (Figure 6–9a) and wake (Figure

6–9b) flames can be easily maintained over a wide range of fuel and oxidizer flows. The flame is easily

observed, and critical stages such as the blow-off limit (abrupt transition from an enveloped flame to a

wake flame) can be ascertained with ease and high reproducibility. The flame front can be easily accessed 

by intrusive

14, 15 or non-intrusive20, 25 probing techniques, thus enabling detailed studies of flame structure, if

desired. 

(a) (b)

Figure 6–9. An Enveloped Flame (a) and a Wake Flame (b).12

The advantage of the current burner design over those used in the past was that burner replacement could

be easily performed when clogging of the porous burner surface occurred due to the deposition of soot

particles or residue from liquid agents containing dissolved solids. 18 

The wind tunnel provided a uniform, low turbulence intensity oxidizer flow to the porous cylindrical 

burner. As shown in Figure 6–5, the air flow in the wind tunnel was oriented vertically upwards. There 

were five major components in the wind tunnel. The tunnel, except the test section, was made of clear 

polycarbonate and polymethylmethacrylate for visual observation of droplet transport toward the burner. 

• The variable speed blower introduced the air flow into the tunnel through an expandable,

flexible 10 cm diameter aluminum ductwork. A coupling converted this circular cross

section to the square shape of the remainder of the tunnel. A slide gate damper at the blower

inlet enabled control of the air intake. 
530 Screening Tests for Candidate Fire Suppressants 

• The 30 cm long diffuser had a 10 cm x 10 cm inlet cross section and an expansion ratio 

(based on area) of 1:9. 

• The flow straightener section, which served to insure that the flow to the test section is 

laminar and uniform over the entire cross-section, consisted of a polycarbonate honeycomb

and a screen. The honeycomb cells had a nominal diameter of 3.2 mm. A 50-mesh center-tocenter stainless steel screen with 30 % open area and with wire diameter of 0.23 mm was

used. 

• The 30 cm long contraction section had a contraction ratio (based on area) of 9:1, with an

inlet cross sectional area of 30 cm x 30 cm. A smooth-surfaced transition flange was placed

between the outlet of the contraction section and the inlet of the test section, along with a 

thermal insulation gasket. 

• The test section had a cross sectional area of 10 cm x 10 cm and a length of 20 cm. It was

made of black-anodized aluminum with three Pyrex observation windows (12 cm x 7 cm x

0.64 cm) mounted flush against the three walls and sealed using high-temperature silicone.

The burner was inserted through the fourth wall. The combustion products from the burner 

were vented to an exhaust hood. 

A three-dimensional traverse mechanism was used to position the entire wind tunnel with respect to

stationary optical instrumentation for droplet characterization at various locations near the burner. The

wind tunnel was mounted on an aluminum base plate, which was placed on four worm-gear screw jacks 

coupled together via flexible shafts. The jacks were mounted on a second aluminum plate and were

driven by a stepping motor and a computer-controlled microstepping indexer drive to raise and lower the 

tunnel. The horizontal movement of the wind tunnel was provided by mounting the whole wind tunnel 

assembly on a milling machine index table. 

Two droplet generation techniques were developed: 

• A piezoelectric droplet generator that created uniform liquid droplets of diameter near

100 μm.26 The operating principle of the droplet generator is based on the break-up of a

nitrogen-pressurized jet ejecting from an orifice as a result of controlled vibration from a 

piezoelectric transducer driven at a fixed frequency. The droplet generator consisted of a 

liquid chamber which was connected to a 40 mL stainless steel reservoir, a bleed port (for 

eliminating any air bubbles trapped inside the chamber during priming), an orifice plate 

containing a sapphire orifice, and a piezoelectric transducer. The transducer was bonded with 

conductive epoxy to a circular disc stamped from flat stainless steel shim stock (0.38 mm 

thick). 

Although distilled and de-ionized water and a few very dilute aqueous solutions have been 

successfully tested with this droplet generator,27 clogging of the orifice opening constantly

plagued the continuous operation of the piezoelectric droplet generator, aggravated by liquids

with high loading of dissolved salts. This approach was subsequently not considered for

further development. 

• A small glass nebulizer was of the type used in inductively-coupled plasma atomic emission

spectroscopy and was commercially available. The nebulizer was mounted at the same 

location (in the settling chamber of the wind tunnel) as the piezoelectric droplet generator. A 

schematic of the nebulizer is shown in Figure 6–10. 
Fire Suppression Efficiency Screening 531

The aerodynamic break-up of a liquid stream issued from the capillary by high-velocity air

caused the formation of a fine mist of droplets. Because of the differences in the droplet 

formation mechanisms, a relatively large opening (diameter ≈ 100 μm) of the capillary in the 

nebulizer, compared to the sapphire orifice (diameter ≈ 30 μm) above, could be used with a 

wide range of liquids, including those with a relatively high salt concentration. The large 

capillary opening made the nebulizer less prone to clogging. Fluid was fed to the nebulizer 

by a small, programmable syringe pump. Air was supplied to the shell of the nebulizer by a 

mass-flow controller. The resulting mist was entrained upwards toward the flame by the air 

flowing in the tunnel. The atomizing air flow was set at 0.25 L/min, which was the highest 

flow that could be used without disturbing the flame at the burner. Because of this limit, the 

atomization efficiency of the nebulizer dropped when the liquid delivery rate was increased

beyond 1.3 mL/min; that is, larger droplets were generated that may not be entrained upward

by the air flow in the tunnel. 

The droplet generator was located in the settling chamber and was approximately 42 cm upstream of the 

burner. The presence of the droplet generator in the wind tunnel did not create any significant

perturbation or blockage effect on the oxidizer flow field near the burner because the flame characteristics

did not change with or without the presence of the droplet generator in the flow stream. By adjusting the

location of the droplet generator with respect to the burner, droplet loss to the wind tunnel walls was 

minimized because the resulting dispersed droplet cloud was confined to a very narrow region near the 

burner. 

Capillary

Liquid (sample)

input

Air input (0.25 l/min)

(sidearm)

40 mm

25 mm

Figure 6–10. Schematic of the DLAFSS 

Nebulizer. 26
532 Screening Tests for Candidate Fire Suppressants 

Measurements of droplet size using a two-component Phase Doppler Particle Analyzer with a Doppler

Signal Analyzer showed that the Sauter mean diameter is 20 μm to 30 μm for fluids having a range of 

colligative properties. About 90 % of the droplets fell within ± 5 μm of the mean value. 

Operation 

There were two ways to perform the screen experiments: (1) increasing the air flow at a fixed liquid agent 

application rate until blow-off occurs, and (2) increasing liquid agent application rate at a fixed air flow 

until blow-off occurs. The former was selected because the procedure requires less agent, and there was 

no need to correct for the lag time from changing the syringe pump setting (to increase the liquid flow) to 

attaining a steady liquid delivery rate. 

For the rapid, relative indicator of the performance of candidate liquid suppressants, the fuel flow was 

fixed at 2 L/min, which corresponds to an ejection velocity of 4.2 cm/s. This value virtually eliminated

an effect of fuel flow on blow-off velocity. Once a stable flame has been established, a fixed 

concentration of agent aerosol was added to the air stream; the concentration was low enough that the

stable enveloped flame was maintained. The air velocity was then increased until there was a transition 

from a stable enveloped flame to a wake flame. This was generally a sharp change. The "figure of merit"

for the agent, at that mass fraction in the air stream, was the value of the air velocity at blow-off: the 

higher the air blow-off velocity, the less effective the fire suppressant. 

The following procedure was then used to determine a "standard" mass fraction of agent at flame

extinction. Since there were many liquid agent delivery rates that one can use in the screening procedure, 

a reference delivery rate was needed to compare and interpret the fire suppression effectiveness of various 

liquid agents in a consistent and meaningful way. 

Nitrogen was selected as a reference gas due to the availability of suppression data for both the cylindrical 

and cup burners. The mass fraction of nitrogen to extinguish a propane cup burner flame is 0.32.28 

DLAFSS experiments12 have shown that the nitrogen mass fraction at blow-off, equivalent to the cup

burner value, corresponded to a reference blow-off velocity of ≈ 30 cm/s.iii At this velocity and a propane

flow of 2 L/min, an enveloped flame could not be stabilized. Therefore, to compare the results obtained 

from the cylindrical burner to conditions commensurate with cup-burner results, extrapolation to a lower 

air velocity was required. The procedure was as follows: 

• A blow-off air velocity without fluid application (but with air flowing in the nebulizer) was

obtained, followed by a blow-off experiment with a fixed fluid application rate. 

• The fluid delivery rate at an air velocity of 30 cm/s was then deduced by linear extrapolation. 

Based on experience, an application rate between 0.6 mL/min and 1 mL/min appeared to be

appropriate, which was a compromise between minimizing the fluid consumption for a test 

and attaining a blow-off velocity close to the reference blow-off velocity of 30 cm/s. 

Once the application rate corresponding to the reference blow-off velocity was deduced, the reference 

mass flow of the liquid agent, m& agent,ref , was calculated using the liquid density. The reference mass 

fraction of the liquid agent in the air stream was then 

 

iii It is expected that similar reference blow-off velocities will be obtained when the cup burner results for other gases are used 

because at the same low global strain rate in a counterflow flat-flame burner, the agent extinction concentrations agree well

with the measurements obtained from a cup burner. 
Fire Suppression Efficiency Screening 533

Yagent,ref = m& agent,ref/( m& agent,ref 

+ m& air,ref) 

(6-1)

where m& air,ref is the mass flow of air, calculated from the cross-sectional area of the test section and an air 

velocity of 30 cm/s. 

Sample Results with Aqueous Suppressants 

Several test fluids (water, skim milk, 30 % sodium iodide, and 30 % and 60 % potassium lactate) were

used to evaluate the performance of the screening apparatus. 11,29 Milk is known to be a fire suppressant,

30

sodium iodide was selected because it may be more effective than sodium bromide31, and potassium

lactate has been demonstrated to be more effective than water. 31 Figure 6–11 shows the screening results

for these test fluids. In each case, only 10 mL of sample was needed to perform a rapid screen with one

repeat. For a given fluid, increasing the liquid application rate decreased the blow-off velocity. As

expected, a mass fraction of 60 % potassium lactate was more effective than 30 % potassium lactate. Water

was the least effective when compared to skim milk, 30 % sodium iodide, and 60 % potassium acetate.

Based on this set of data, the coefficient of variation from run-to-run using the liquid screening apparatus 

was estimated to be better than 20 %.

Agent Flow (mL/min)

0.0 0.2 0.4 0.6 0.8 1.0

Blow-off velocity (cm/s)

0

20

40

60

80

100

120

140

160

180

200

220

240

No agent

Water

60 % K-acetate

30 % NaI

Skim milk 

30 % K-lactate 

60 % K-lactate 

Figure 6–11. Screening Results for Various Aqueous Fluids. 12
534 Screening Tests for Candidate Fire Suppressants 

Table 6–1 summarizes the calculations of the reference agent mass fraction in air using the screening

results from Figure 6–11 and the approach described above. Average values of the blow-off velocities 

were used in the extrapolation. For cases where blow-off velocities at more than one liquid application

rate were available, linear regressions were used to extrapolate the reference blow-off velocities. 

Table 6–1. Calculated Nominal Agent Mass Fractions at Reference Blow-off Air Velocity

of 30 cm/s. 12

Agent 

Vagent,ref

(mL/min) 

Agent density 

(g/ mL) @ 20 °C 

Magent,ref 

(g/s) 

Nominal agent

mass % (%) 

mwater,ref / 

magent,ref

Water 4.62 1.00 0.08 2.6 1.0

60 % K-acetate 0.99 1.34 0.02 0.8 4.0

30 % NaI 1.76 1.29 0.04 1.3 2.0 

Skim milk 2.78 1.01 0.05 1.6 1.6 

30 % K-lactate 1.74 1.15 0.04 1.2 2.0 

60 % K-lactate 0.71 1.33 0.02 0.6 4.0 

The last column of Table 6–1 lists the ranking indices relative to water. For example, the 60 % K-acetate 

and 60 % K-lactate solutions were considered to be four times more effective than water at the reference 

blow-off velocity. If the droplets were not homogeneously dispersed across the total cross-sectional area, 

the calculated agent mass fraction would be underestimated. The effective area was considered as the

effective coverage area of the mist in the test section. Depending on the effective coverage area, a

difference of a factor of two to three in the calculated liquid mass fraction resulted. By placing a sheet of

filter paper over the exit of the test section for a short duration with the wind tunnel operating (without the 

burner) and the nebulizer atomizing water with a dye added, the droplet-impact (color) pattern on the 

filter paper was used as an indicator to determine the mist coverage area in the test section. The color

pattern, which was approximately circular, indicated that the mist from the nebulizer completely covered

the burner and its vicinity. The mist coverage area was estimated to be about 40 % of the total crosssectional area of the test section for all the conditions encountered in these screening tests.

Water and the aqueous agents studied were found to be more effective than CF3Br, compared to the

propane cup burner value (mass fraction of 0.17 for CF3Br).28 The computational study by Lentati and 

Chelliah32 also demonstrated that 20 μm water droplets were more effective at extinguishing an opposedflow methane diffusion flame than CF3Br (mass fraction of 4.24 % vs. 5.9 %) at an extinction strain rate 

of 176 s-1. Although the ratio of the DLAFSS nominal water mass fraction to the cup burner value for 

CF3Br using propane was smaller, the two studies were in qualitative agreement in terms of the 

suppression effectiveness of water droplets. 

Care should be exercised when extrapolating the screening results in Table 6–1. These were obtained 

using an idealized laboratory flame and a droplet delivery system such that the transport of fine liquid 

droplets to the flames was not a factor in determining the suppression effectiveness. In real fires, droplet 

entrainment and transport to the fire can significantly affect the liquid agent mass concentration required 

to suppress a fire, especially in highly obstructed enclosure fires. 
Fire Suppression Efficiency Screening 535

6.4.4 Transient Application, Recirculating Pool Fire Apparatus (TARPF) 

Need for the Apparatus

Both the cup burner and the DLAFSS determine the minimum concentration of agent needed to suppress 

a flame by increasing the agent flow slowly until a critical volume fraction is achieved in the oxidizer 

stream and flame extinction is observed. In practice, however, agents designed to replace CF3Br are 

discharged rapidly, not steadily. Solid propellant gas generators (SPGGs), for example, typically

discharge their effluent in 10 ms to 500 ms. A robust and repeatable means to evaluate the effectiveness 

of different formulations at these fast application rates requires a non-conventional screening device. 

NIST research staff developed a Transient Application, Recirculating Pool Fire (TARPF) suppression 

facility to screen different agents and prototype systems as an indicator of full-scale performance.33 The

TARPF agent suppression screen was designed to reproduce the most difficult fire situation and to allow 

control of critical agent discharge parameters, including agent discharge rate and duration, air flow, and 

obstacle geometry. The performance of gaseous agents, aerosols and SPGGs was examined. 

The apparatus derived from prior work by Hirst and Sutton,

34 who developed a wind tunnel to explore the

impact of step height, air flow, and pressure on the blow-out of a jet fuel pool fire stabilized behind a 

backward facing step. Hirst et al.

35 studied the suppression of these types of fires using various halons, and 

concluded that a liquid pool burning in a flow behind an obstacle is the most difficult fire to extinguish. This 

was borne out in full-scale tests done later.36 Experiments by Hamins et al.,37 in cooperation with Walter

Kidde Aerospace, were conducted in a wind tunnel scaled down from the earlier work by Hirst to examine 

the performance of HFC-125 and HFC-227ea. Investigations at the Air Force Research Laboratory38 as part

of the NGP sought to determine the detailed structure during suppression of a non-premixed methane/air 

flame stabilized behind a step. The changing character of the flame with step height and air velocity was 

examined, along with the amount of halon 1301 required to suppress the flame as a function of the flow

parameters and injection interval. 

A turbulent spray burner had been designed in earlier work at NIST39 to simulate an engine nacelle spray 

fire resulting from a ruptured fuel or hydraulic fluid line. Flames were suppressed using gaseous and 

powdered agents. Hamins et al. 37 redesigned the burner to include a heated disk in the center of the flow

downstream of the fuel nozzle. They showed that the concentration of nitrogen necessary to extinguish a 

turbulent propane flame increased substantially with surface temperature. The same trend, but to a lesser

degree, was observed with hydrofluorocarbons. 

Previous studies had demonstrated the effectiveness of SPGGs and their hybrids in full-scale fire

suppression experiments.40,41 Solid propellant gas generators undergo rapid solid-phase reactions

producing inert gases, principally carbon dioxide, water vapor, and nitrogen, as well as particulates 

composed of inorganic salts. Each component individually behaves as a fire suppressant. Hybrid 

generators are particularly attractive for some special situations such as inhabited spaces or cold

temperature applications.42 The hybrids use the hot inert SPGG exhaust to vaporize and expel a secondary 

suppressant, typically a liquid, such as water or a low boiling point halocarbon, through a nozzle.43
536 Screening Tests for Candidate Fire Suppressants 

±

A stream of JP-8 droplets provided a source of re-ignition for the propane flame as they were directed

directly onto the heated obstruction after flame suppression. The droplet array was generated from a 

0.14 mm sapphire orifice that was pressure fit into a 3.1 mm tube. The tube was inserted into the wind 

tunnel passage from the top wall at a position 25 mm upstream of the flame stabilizing obstacle. The end 

3.8 mm of the tube was bent 90° into the direction of the air stream, and was centered between the

passage top wall and face of the ramp. The JP-8 was forced through the injector with nitrogen back 

pressure at 170 kPa. The low back pressure and several sintered filters (with a pore size of 7 μm and

2 μm) were used to help prevent clogging of the injector. Initially, piezo-electric crystals were used to

initiate droplet breakup, but impingement of the fuel on the heated obstruction was difficult due to 

entrainment of the individual droplets into the high-velocity air flow. Impingement was achieved

successfully by directing the non-atomized fuel stream on the heated obstruction.

To investigate different re-ignition scenarios, a hot surface obstruction was set up immediately

downstream of the propane porous burner. (See Figure 6–12.) The v-shaped obstruction was 25 mm in 

height, the base was 25 mm wide, and was centered across 50 mm of the wind tunnel passage. An S-type 

thermocouple was place on the external upstream surface of the obstruction. A high-resistance, high 

temperature ceramic adhesive was used to adhere the thermocouple to the Inconel surface, 15.5 mm thick.

An ethane torch was supported underneath the obstruction to heat the inner surface to over 1100 °C. A

mass flow controller was used to regulate the obstruction surface temperature. The fire was initiated by a 

spark across two electrodes protruding from the side wall of the test section 20 mm above the surface of 

the burner and 20 mm downstream of the step. Heat release rates were up to 10 kW. A 30 Hz video

camera could view the flame and the suppression process from above and the side through 6 mm thick

Pyrex windows. For some experiments, a 1000 Hz digital camera was used to investigate the flame 

dynamics. A photograph of a typical flame taken through the window is shown in Figure 6–14. The

inclined mirror above the apparatus reflects a top view of the flame.

As shown in Figure 6–12 and Figure 6–13, the TARPF was a small wind tunnel consisting of a number of

sections. The main portion of the tunnel was 2.5 m long with a square cross section 92 mm on a side. 

(Detailed mechanical drawings of key components of the facility can be found in reference 44.) Air, 

supplied by a compressor rated for a maximum flow of 180 g/s at 1.0 MPa, could be delivered to the 

tunnel at speeds, averaged over the 92 mm square cross section, up to 17 m/s. Flow was monitored using

a calibrated sonic orifice and a piezoelectric pressure transducer. The inlet air temperature could be 

heated to above 200 °C. A honeycomb flow straightener and mixing screens were located 1 m upstream 

of the test section. The burner consisted of a sintered bronze plate, 92 mm wide by 190 mm long. 

Propane, the primary fuel, could be supplemented with a JP-8 spray. The expanded relative uncertaintyiv,45

in the flows of fuel and air were 5 % of the measured value, based upon the manufacturers'

specifications for the metering orifice and mass flow controllers. Stainless steel baffles between 10 mm 

and 55 mm high were located upstream of the burner. A ramp could also be inserted to form a 25 mm

high backward-facing step.

iv The expanded relative uncertainty on all dimensions and independent parameters described in this chapter is ± 50 % of the 

highest significant place reported, with a coverage factor of 2, unless otherwise stated.

Hardware 

Experimental Facility
ire Suppression Efficiency Screening 537

spark igniter 

92 

air 

suppressant gas 

190 

h 

2500 

reignition surface 

backward step

flow straightener diffuse

92

propane

Figure 6–12. Schematic of Step-stabilized Pool Fire Apparatus. 44 Dimensions are in millimeters. 

F
538 Screening Tests for Candidate Fire Suppressants 

Figure 6–13. 

Photograph of the 

TARPF. (NIST photo)

Figure 6–14. 

Photograph of a 

Baffle-stabilized 

Propane Flame in the 

TARPF. (NIST photo)
Fire Suppression Efficiency Screening 539

The JP-8 injection time (controlled by a solenoid valve), and flow were varied in order to optimize the 

impingement process. The impingement of the fuel onto the obstruction surface resulted in a significant

decrease in the surface temperature, and care was taken to compensate for this effect during experiments. 

The fire suppressing agent was injected downstream of the air metering orifice. Since the air flow was 

choked by the metering orifice, the introduction of the agent was accomplished without altering the total 

air flow. 

Volatile Agents 

Gaseous agents (nitrogen, CF3Br, and HFC-125) were stored in 1 L and 2 L stainless steel vessels with

the pressure monitored by a 1 ms response piezoelectric transducer, and the temperature measured with a 

chromel-alumel thermocouple. An electronic timer controlled the interval (10 ms to 1000 ms) that a 

solenoid valve on the agent vessel remains open. The agent passed through a 6 mm diameter orifice 

before it was injected through two opposed radial ports into the air passage upstream of the diffuser. A 

computer monitored the flow controllers, pressure transducers, and thermocouples, and sent a signal to 

the electronic timer to open and close the solenoid valve while releasing the flow of suppressant. The

mass of the gaseous agent released was determined from the change in pressure and temperature in the 

storage vessels. The expanded uncertainty in the calculated mass was ± 2 %, with a minimum absolute 

uncertainty of 0.12 g attributable to the resolution of the pressure transducer. The discharge rate and 

duration were controlled by the initial agent pressure and an electronically actuated solenoid valve. The 

piezoelectric pressure transducer was able to follow the change within about 5 kPa, but the thermocouple 

response was too slow. To determine the instantaneous mass discharge, the nitrogen was assumed to be an 

ideal gas with the expansion inside the bottle occurring isentropically. The uncertainty in dm/dt was 

estimated to be ± 2 g/s. 

Aerosol Agents 

Aerosol agents required a totally different injection system than what has been described above or used 

previously. The flows and injection durations of commercial fuel injectors were not adjustable over the

range and with the precision necessary for the current application. 

The liquid dispensing system (Figure 6–15) consisted of a liquid reservoir, a compressed air cylinder, two 

computer-controlled solenoids, and a hollow-cone atomizer. The atomizer was positioned equidistant

vertically between the ramp and the upper wall of the wind tunnel, and at discrete locations along the air

passage from 65.5 cm upstream of the propane porous burner to a location downstream near the ramp. 

Several different atomizers were used while trying to obtain a well-dispersed spray of droplets in the air 

passage. The atomizers were commercially manufactured to the following: (1) a nominal spray angle of

70° when operated at 1.03 MPa with a flow of 1.9 mL/s, and (2) a nominal spray angle of 60° when

operated at 0.69 MPa with a flow of 1.1 mL/s. Operating these atomizers at nominal pressures normally

resulted in a fully developed spray, and thus an increase above the nominal condition had a small effect 

on reducing droplet size. When positioned at upstream locations, the spray appeared to fill the entire air 

passage cross section. To initiate a mist for a fixed duration, the data acquisition system activated the

solenoid that was connected to the reservoir. At the end of the discharge, the solenoid was deactivated to

terminate the flow to the atomizer. The second solenoid leading to a drain was simultaneously activated 

to prevent any residual flow to the nozzle and dripping from the nozzle. 
540 Screening Tests for Candidate Fire Suppressants 

Liquid reservoir

to drain

Ramp

Hot surface for re-ignition

Solenoids

from compressed air cylinder

70 cm

Porous burner

(Propane 33 mL/s)

65.5 cm

Mist nozzle

Air 

Wind tunnel

Ethane torch

Figure 6–15. Liquid Aerosol Agent Injection System. 12

SPGG Facility and Operation 

Gas generators are manufactured in discrete units using specific chemical formulations and orifice sizes 

that are designed based on the particular application. The TARPF operator can control neither the total 

mass discharged nor the injection time interval of the SPGG. To accommodate this limitation, a custom 

discharge chamber was designed to allow the operator to select the fraction of the SPGG discharge that is

injected into the flame zone. The hardware was designed to allow standard size gas generators (which

contain significantly more material than is required for suppression in the TARPF) to be evaluated by 

repeating the test sequence with identical gas generators and incrementally increasing the fraction of

SPGG effluent allowed to flow into the TARPF air stream. The SPGG effluent passed through a metering 

orifice and was injected into the air stream using the same manifold as for the compressed gaseous agents. 

The flow through the bypass port was discarded into a laboratory exhaust hood. The pressure in the 

discharge chamber was monitored by a 1 ms response piezoelectric transducer and the temperature was 

measured with a 76 μm type K thermocouple. Figure 6–16 is a schematic diagram of the SPGG injection

system; and Figure 6–17 is a close-up photograph. 

The housing, enclosing the entire injection system, was made of 6 mm thick stainless steel as a precaution

against a premature or explosive discharge. The internal volume was approximately 200 mL. There were

four main ports on the discharge chamber as seen in Figure 6–17: 

A 3⁄4 NPT female thread connecting to the gas generator cartridge holder, 

A variable area metering orifice (1.6 mm to 6.4 mm diameter) to limit the flow into the TARPF, 

A bypass port tapped for a 2 NPT nipple, and 

A 19 mm port for mounting a pressure relief blow-out diaphragm. 
Fire Suppression Efficiency Screening 541

Blow-Out Disk

Explosion 

Protection 

Housing

Discharge 

Chamber 

Air

Bypass 

Exhaust

SPGG Cartridge 

Holder 

SPGG Ignition Wire 

Agent Injector Section 

Burner

Figure 6–16. Schematic Diagram of the SPGG Injection System. 44

The fraction of the SPGG effluent injected into the TARPF was varied by selecting the size of Ports 3 and

4. In many of the experiments, Port 4 was left open to maximize the bypass area.

 2

Discharge Chamber

1

 Cartridge Holder

3

4

Protective

Housing

Figure 6–17. Photograph of 

TARPF Injection System with 

Housing Cover Removed. The 

four ports in the discharge 

chamber are: (1) SPGG outlet, 

(2) metering orifice, (3) bypass, 

and (4) blow-out. 
542 Screening Tests for Candidate Fire Suppressants 

The mass of the SPGG injected into the TARPF was determined from the total mass discharged and the

ratio of the area of the metering orifice to the combined area of the metering orifice and the bypass area.

The expanded uncertainty in the calculated mass was ± 2 %, with a minimum absolute uncertainty of

0.12 g attributable to the resolution of the pressure transducer. From the temperature and pressure 

measurements, the rate of suppressant addition to the incoming air, dm/dt, was estimated within an 

expanded uncertainty of ± 2 g/s. The concentration of the SPGG in the air flow was determined from the 

rate of suppressant addition (dm/dt) and the mass flow of air. As in the gaseous agent experiments, the air 

flow was invariant during the agent discharge due to the use of a sonic orifice. 

For the tests performed under the NGP, identical commercial air-bag hybrid gas generators were used, 

one in each of fifty experiments. Each generator released 20.7 g ± 0.1 g. The discharged mass was 

specified by the manufacturer and confirmed experimentally by weighing the generators before and after

each discharge. The agent was composed of 20 g of compressed argon gas and 0.7 g of a solid propellant, 

which at equilibrium converts to KCl (s), H2O, N2 and a small amount of gaseous CO2.

46

The gas generator was discharged after steady-state fire conditions were achieved in the TARPF. The

discharge was controlled by engaging an electronic switch on a control box that completed a circuit 

leading from a 12 V battery to the electrical connector located on the gas generator. One ampere was 

required to fire the 40 mg squib, which was an intrinsic part of the gas generator. The squib ignites the 

solid propellant, which rapidly discharges. The combustion products of the solid propellant propelled the 

gaseous argon from the generator casing, located within the cartridge holder, into the discharge chamber.

SPGG cartridges were changed and prepared for the next run in less than 4 min. 

Numerical Modeling and Analysis

To better understand the fluid dynamics of the suppression event, the first publicly released version of the

Fire Dynamics Simulator (FDS)47, a computational fluid dynamics model, was used to simulate the baffle

or step-stabilized flame in the TARPF facility. The sub-grid scale turbulence model was not used, but 

rather the coefficients of viscosity, thermal conductivity and mass diffusivity were derived from kinetic 

theory and empirical extrapolation.48 Thus, the calculations directly simulated the fluid motion, but not

the combustion. Both two-dimensional (higher resolution) and three-dimensional (with turbulent flame

structure) simulations were performed. Details of the calculations are presented in References 33 and 44.

Presented in Figure 6–18 are sequences of images separated in time by 0.01 s that are taken from 

simulations that illustrate the dynamics of the suppression event. Figure 6–18a is a 3-D simulation of a 

flame stabilized on a 25 mm baffle that is undergoing a successful suppression. Figure 6–18b is a 2-D 

simulation of flow (moving left to right) over a 25 mm step. The darkness of the image reflects the extent

of local heat release. In both Figures, the top images show the flame just prior to discharge of nitrogen 

into the air stream. Upon injection, the flame was disturbed by a large vortex generated by the pressure 

pulse. Due to the low Mach number approximation, the gas upstream of the step was essentially 

incompressible, and the velocity jump from 2.1 m/s to 5.7 m/s was conveyed to all points in the flow

domain in 0.02 s, the time of the ramp-up from the base velocity to the injection velocity. Thus, even

before the agent arrived at the step, the flame had already been dramatically transformed from its original

state. The generation of the large vortex at the step produced a pathway by which the agent could 

penetrate the region just behind the step, mixing with the gases, cooling and diluting the fuel and oxygen. 
Fire Suppression Efficiency Screening 543

(a) 

(b) 

Figure 6–18. Disruption of a Stabilized Flame by the Injection of Nitrogen Upstream of 

25 mm High Obstacle. 

(a) 3-D Simulation of Flow over Baffle; (b) 2-D Simulation of Flow over Step. 

The frames are separated by 0.01 s. 12
544 Screening Tests for Candidate Fire Suppressants 

Flow Characterization 

The TARPF facility was operated over a range of propane and air flows to examine the flame behavior. 

Flame blowout, with no added fire extinguishing agent, was achieved either by increasing the air flow or 

decreasing the propane flow. At low air velocities, a fluctuating laminar flame was anchored on the top 

downstream edge of the step or baffle and extended well downstream of the porous plate. As the velocity 

increases, the flame became turbulent and less luminous. Near blowout, the orange color disappeared and 

the visible blue flame shrunk. With the backward-facing step installed, an average air velocity above the 

step of over 23 m/s was necessary to blow out the flame if the propane flow was greater than 33 mL/s, 

corresponding to a velocity of 1.9 mm/s. 

Two air flows were chosen to evaluate the ability of the agents to suppress the propane pool fire stabilized

by the backward-facing step: 

Mean Air Velocity Propane Flow Regime38

2.1 m/s ± 0.2 m/s 33 mL/s ± 2 mL/s Regime I (rim-stabilized flame) 

5.4 m/s ± 0.2 m/s 85 mL/s ± 2 mL/s Between Regime I and II 

(intermittent turbulent flame) 

The velocity distribution of the air 76 mm upstream of the burner was measured with a 3 mm diameter

pitot tube at seven locations across the duct. The velocity profiles were flat within 5 % over the central

three-fourths of the duct. The boundary layer above the step appeared to be less than 7 mm thick. The 

presence of the flame tended to increase the pitot probe signal, which was likely due to a combination of 

preheating the air upstream by the flame, acceleration in the flow due to partial blockage of the duct 

caused by the expanding combustion gas, and a possible shift in electrical output due to heating of the

pitot probe and transducer. 

The mass of suppressant necessary to extinguish a fire in the TARPF depends upon the fuel and air flows 

chosen to challenge the suppressant. Figure 6–19 shows how the mass of nitrogen necessary to 

extinguish a 25 mm baffle-stabilized flame varied with the flow of air and fuel. The uncertainty in any 

given value was estimated to be 0.2 g. When the air speed was less than 5 m/s and the fuel flow was

fixed at 45 mL/s 2 mL/s, decreasing the speed reduced the amount of nitrogen necessary to extinguish

the flame. No flame extinction occurred between 5 m/s and 15 m/s because the mass of nitrogen 

necessary exceeded the maximum amount contained in the storage vessel. Above 16 m/s, the strain on

the flame was sufficient at times to extinguish the flame without the need for any nitrogen. (The dashed

lines are included in the figure to assist the eye in identifying the extinction boundaries.) 

±

±

The propane flow had less than a 5 % effect on the mass of N2 needed to extinguish the flame, at fixed 

injection interval and air flow. There was a lower limit for the propane flow (< 12 mL/s) that led to

extinction due to heat loss to the burner, even with no N2 dilution. The upper flow limit (120 mL/s) was 

dictated by the maximum safe operating temperature of the burner. Since the mass of N2 needed to 

suppress the flame did not appear to increase, there was no need to operate the burner at higher fuel flows. 

The total mass of N2 required for suppression also depended on the injection interval. This relationship is 

plotted in Figure 6–20 for a 3.9 m/s ± 0.2 m/s air flow, 45 mL/s ± 2 mL/s propane flow flame stabilized

on the 25 mm baffle. The total number of experiments was more than ten times the number of data points 
Fire Suppression Efficiency Screening 545

plotted in this and the following curves; for clarity, only those conditions close to the extinction boundary

are included. (For all these figures, the open circles represent the largest mass of N2 that did not result in

extinction; the filled circles are the minimum mass of agent that successfully extinguished the flames.) 

As the injection interval was increased from 100 ms to 500 ms, the minimum mass required increased

over three-fold. The rate of mass addition (total mass/estimated injection interval) decreased with

increasing injection interval, as shown in the right-hand figure. 

Nominal Air Speed, m/s

0 5 10 15 20

Mass of Nitrogen, g

0

2

4

6

8

10

12

Extinction

No Extinction

Extinction

Figure 6–19. Impact of 

Air Speed on

Extinction of a 25 mm 

Baffle-stabilized 

Flame. 44

Propane Flow: 45

mL/s; N2 Injection 

Time: 312 ms. 

Circles: Flame 

Extinction; Crosses: 

No Extinction 

Injection Time, ms

100 200 300 400

Mass, g

0

2

4

6

8

10

Injection Time, ms

100 200 300 400

Rate of Mass Addition, g/s

0

10

20

30

40

50

25 mm Step 25 mm Step

Figure 6–20. Mass and Rate of Nitrogen Addition to Extinguish a 45 mL/s Propane Flame 

in 3.9 m/s Air Flow.

 44
546 Screening Tests for Candidate Fire Suppressants 

Next, there was an effect of the geometry of the obstruction on the mass of N2 needed for flame

suppression. The data plotted with squares in Figure 6–21 were taken with the ramp placed in front of the 

25 mm baffle to form a backward step-stabilized flame, rather than the simple baffle-stabilized flame

represented in Figure 6–20. The air flow was the same in these two cases but the propane flow was 

higher, 85 mL/s, in Figure 6–21. The addition of the ramp and increase in propane flow did not have 

much influence on the mass of N2 required for suppression. For both the baffle and backward step, just 

under 6 g of N2 are required when the injection interval is 200 ms ± 10 ms. The data plotted as circles in 

Figure 6–21 were taken with the nominal air speed reduced to about 1.5 m/s and the propane flow 

proportionately to 33 mL/s. The squares represent experiments conducted at the high air and propane

flows, and circles represent experiments conducted at the lower flows. Less than 4 g ± 0.2 g of N2 were

needed to extinguish this flame if injected over a 200 ms ± 10 ms interval. The differences in rates of

mass addition to suppress the high flow and low flow flames can also be seen at the right in Figure 6–21. 

Injection Time, ms

100 200 300 400 500

Mass, g

0

2

4

6

8

10

Extinction

Injection Time, ms

100 200 300 400 500

Rate of Mass Addition, g/s

0

10

20

30

40

50

Extinction

No Extinction

No Extinction

25 mm Baffle 25 mm Baffle

Figure 6–21. Mass and Rate of Nitrogen Addition Required to Extinguish High Flow

(squares) and Low Flow (circles) Propane/Air Flames Stabilized Behind a 25 mm Step. 44

Figure 6–22 and Figure 6–23 show what happens to the required nitrogen mass and addition rate if the 

baffle height is decreased to 10 mm or increased to 55 mm (blockage from 11 % to 60 %), respectively.

The symbols have the same meaning as in Figure 6–21. The short baffle produced a fire which was the 

easiest to extinguish, and the high baffle the most difficult in terms of the amount and rate of N2 addition. 
Fire Suppression Efficiency Screening 547

Injection Time, ms

100 200 300 400

M

ass, g

0

2

4

6

8

10

Injection Time, ms

100 200 300 400

R

ate of M

ass Addition, g/s

0

10

20

30

40

50

10 mm Baffle 10 mm Baffle

Figure 6–22. Mass and Rate of Nitrogen Addition Required to Extinguish High Flow

(squares) and Low Flow (circles) Propane/Air Flames Stabilized Behind a 10 mm Baffle. 44 

Injection Time, ms

100 200 300 400

Mass, g

0

2

4

6

8

10

Injection Time, ms

100 200 300 400

Rate of Mass Addition, g/s

0

10

20

30

40

50

55 mm Baffle

55 mm Baffle

Figure 6–23. Mass and Rate of Nitrogen Addition Required to Extinguish High Flow

(squares) and Low Flow (circles) Propane/Air Flames Stabilized Behind a 55 mm Baffle. 44
548 Screening Tests for Candidate Fire Suppressants 

The effect of baffle height was not large if the injection interval was at least 150 ms, as can be seen more

clearly in Figure 6–24. (Note that 6 mm has been added to the height of each obstacle to account for the 

distance between the floor of the tunnel and the recessed top surface of the burner.) The bottom curve 

delineates the minimum amount of nitrogen for suppression when the air flow was fixed at its high value 

and the agent injection interval was maintained at 175 ms ± 10 ms. The open circles represent the largest 

mass of N2 that did not result in extinction for flames stabilized on the different sized baffles; the filled

circles are the minimum mass of agent that successfully extinguished the flames. The diamonds are the 

results for the 25 mm baffle with the ramp in place (backward-facing step). Experiments were also

conducted with and without the reignition obstruction shown in Figure 6–12. The amount of N2

necessary for suppression was unchanged. 

The rate of mass addition is plotted in the upper curve of Figure 6–24 (the triangles are the backwardfacing step, and the squares are for the baffles). The data are plotted two ways: the higher value is the 

rate of nitrogen addition computed during the first 50 ms that the solenoid valve is open; the lower value 

is the average over the entire open interval measured from the pressure trace. The estimated rate of mass

addition varied substantially, especially for the 55 mm baffle, depending upon whether the averaging 

period was the first 50 ms or the entire time that the solenoid remains open. 

Figure 6–24. Impact of Obstacle Height on the Total Mass and Rate of Addition of 

Nitrogen Required to Suppress an Obstacle-stabilized Pool Fire. 44 

Obstacle Height, mm

0 10 20 30 40 50 60 70

Mass, g

0

2

4

6

8

10

12

Rate of Mass Addition, g/s

0

10

20

30

40

injection time = 175 ms

air flow = 3.88 m/s
Fire Suppression Efficiency Screening 549

Effect of Type of Gaseous Suppressant 

Figure 6–25 is a plot of the minimum agent volume fraction that extinguished the fires (X) as a function 

of the agent injection time interval (∆t) for both N2 and CF3Br. The parameter X is defined as the average

volume flow of agent during the injection interval divided by the sum of the agent and bulk air flows. 

Figure 6–25. Mole Fraction of Suppressants (N2 and CF3Br) Added to Air at Extinction 

 for High and Low Flow Conditions, as a Function of Injection Time Interval and 

The data represent experiments conduc ions including air velocities that varied 

from 2 m/s to 9 m/s, propane flows from 33 mL/s to 85 mL/s, and baffle heights between 10 mm and

Injection Time Interval, ∆t, ms

0 100 200 300 400 500 600

Mole Fraction of Agent,

Χ

0.0

0.2

0.4

0.6

0.8

1.0

circles - 55 mm baffle

triangles - 25 mm step, baffle, cavity

squares - 10 mm baffle

open symbols - low flow

filled symbols - high flow

CF3Br

N2

Boundary

Obstacle Geometry.

44

ted over a range of condit

55 mm, in addition to the 25 mm backward step. The open and closed symbols represent the low and

high air flow conditions, respectively. Figure 6–25 shows that X decreased with increasing injection time

interval for all obstacle types and both agents. The highest volume fraction requirements were

consistently for the low air flow conditions. For some experiments, the value of X was nearly 0.8 for short 

injection intervals. The most challenging geometric configuration was the 55 mm baffle, followed by the

25 mm obstacles, and the 10 mm baffles. There was little difference in X between the 25 mm step, 25 mm 
550 Screening Tests for Candidate Fire Suppressants 

cavity and the 25 mm baffle and those data are presented as one group in Figure 6–25. The effectiveness 

of CF3Br was compared to that of N2 using the 25 mm high backward step. The 1 L storage vessel was

used to accentuate the pressure change associated with the small quantities of CF3Br required for 

suppression. Only 1.6 g ± 0.2 g of CF3Br injected for 100 ms was needed to extinguish the flame under 

the high air flow conditions (corresponding to X ≈ 0.075), as compared to 3.9 g ± 0.2 g for N2 as agent 

(X ≈ 0.5) under similar conditions. These results were consistent with numerous studies that showed that 

CF3Br is a more effective suppressant than N2 for both free standing and baffle stabilized flames. 

Effect of Hot Surface 

Experiments were conducted to assess the importance of a nearby hot surface (Figure 6–12) on the 

effectiveness of a fire suppressant. An ethane-fueled torch was used to preheat the upstream surface of

wed liquid JP-8 to be sprayed over the 

propane pool and onto the heated inverted-vee surface. The fuel acted to cool the surface approximately

e propane fire, the propane fire was initiated under the operating conditions of 

an air flow velocity of 1.5 m/s and a propane flow of 33 mL/s, and the obstruction was heated to the 

perature was found to be bi-modal: when the temperature of the hot

surface (with the JP-8 and propane flowing) was below about 880 °C 

the obstacle to produce temperatures in excess of 1000 °C as measured on the exterior face. Pyrometer 

readings of the interior (i.e., fire-side) surface indicated temperatures were about 50 °C cooler.

Experiments determined that hot surface temperatures close to 1000 °C were needed to ignite the propane

pool when the air and propane were set to the low flow conditions.

A spray nozzle was located upstream of the stabilizing step that allo

150 °C below the temperature that had been obtained when only the gaseous propane fuel was flowing. 

The JP-8 was ignited and formed a flame that stabilized behind the vee when the temperature of the

surface exceeded 900 °C. 

To test for re-ignition of th

temperature desired. The flame was observed to extend past the obstruction. The agent (N2, HFC-125, or

CF3Br) was introduced to the system for three different injection times of 75 ms, 100 ms, and 150 ms, and

the occurrence of flame suppression was observed. (The agent injection time was varied to observe its

effect on re-ignition of the propane fuel.) The agent storage cylinder pressure was increased if

suppression did not occur, and the process repeated. In order to determine the efficacy of the JP-8 stream 

in the re-ignition process, experiments were conducted with the JP-8 stream directed against the heated

obstruction while the unignited propane was still flowing into the wind tunnel. If re-ignition did not

occur after 10 s, then the propane flow was terminated. A variety of re-ignition results were observed 

which depended on the injection time interval of the agent and JP-8 stream. Re-ignition was found to 

occur when the obstruction was above 900 °C (bright red), and was affected by the cooling provided by 

the agent and JP-8 liquid stream. 

The influence of the surface tem

± 10 °C, the amount of agent 

necessary for suppression was the same as when the surface was unheated; and when the temperature was 

above 890 °C ± 10 °C, reignition always occurred within about 10 s independent of higher temperatures

or the amount of N2 added. A similar finding resulted when nitrogen was replaced by CF3Br, although

the measured dividing temperature was about 50 °C higher. This difference may have been associated

with a build-up of carbon on the hot surface that insulated the fire-side surface from the ethane torch, 

making an accurate determination of the surface temperature more difficult. 
Fire Suppression Efficiency Screening 551

wer temperature. Ignition of 

the JP-8 and propane did not occur. Satcunanathan and Zaczek49 measured the ignition delay of kerosene

For the SPGG experiments, the nominal velocity of the air above the backward-facing step was 

nt at 5.4 m/s; the propane flow was 85 mL/s (at standard temperature and pressure).

The high pressures (typically 1000 kPa to 3000 kPa) produced in the SPGG discharge chamber and the 

pass port area. The bypass 

ratio (A /A ) is the area of the inlet metering orifice (port 2) divided by the total open area available for 

Figure 6–26. Injection 

terval (triangles)

and Calculated Mass 

on

A few experiments were conducted with the temperature reduced to the Leidenfrost point (290 °C for

JP-8) to determine if an increase in contact time would compensate for the lo

and diesel fuels on a hot metal surface and found that ignition could occur in less than 1 s for a 2 mm 

diameter droplet, but that the ignition time increased for temperatures between about 500 °C and 550 °C 

due to surface boiling. No ignition of the JP-8 droplets was observed in the TARPF when the vee-surface

temperature was maintained in the range between 360 °C and 440 °C. However, Jomaas et al.50 found 

that ignition of a JP-8 pool fire occurred in their step-stabilized burner for temperatures in the same range.

SPGG Results 

maintained consta

known area of the metering orifice allowed the mass flow of agent added to the air stream of the TARPF 

to be estimated by assuming that the flow through the orifice is choked. Five replicate tests produced 

virtually indistinguishable mass flow and thermocouple temperature traces. 

The SPGG discharge time was consistently 20 ms ± 1 ms, which is over three times faster than the 

shortest N2 or CF3Br injection interval, and was not much affected by the by

in tot

flow to exit the discharge chamber (ports 2, 3, and 4) The time interval, ∆t, is shown in Figure 6–26 as a 

function of the estimated discharge mass. The total mass delivered to the air stream during the discharge

process was found by integrating dm/dt over ∆t. Excluding the highest and lowest area ratios, the 

estimated mass delivered can be seen in Figure 6–26 to be linearly proportional to the area ratio; however, 

almost 50 % more mass was estimated than one would expect. The dashed line in Figure 6–26 indicates 

that a 1-to-1 relation would exist if the mass were directly proportional to the area ratio. 

(Ain/Atotal)mtot, g

0.00 0.25 0.50 0.75 1.00 1.25

MASS DELIVERED, g

0.0

0.5

1.0

1.5 in

Delivered to Flame 

(circles) as a Functi

of Area Ratio Times 

Total Mass of Gas 

Generated (20.7 g). 44

2.0

INJECTION INTERVAL, ms

0

20

40

60

80

100

0.303+1.368(Ain/Atot)mtot
552 Screening Tests for Candidate Fire Suppressants 

There are several factors that contributed to an uncertainty in the estimate of the deployed mass of agent. 

First, uncertainties in the gas composition and temperature upstream of the metering orifice affected the

estimate since the mass was proportional to the square root of the molecular weight divided by the 

temperature. A factor of two underestimate in this parameter would cause a 40 % over-estimate in mass, 

which, if corrected for, would cause the data plotted in Figure 6–26 to more closely align with the dotted 

line. A second source of uncertainty was the complexity of the flow within the discharge chamber created

by the jet emanating from the SPGG. The calculation assumed that the upstream flow was steady and 

parallel to the axis of the metering orifice, but the flow was highly transient and more perpendicular. 

Suppression of the propane pool fire with the hybrid gas generators occurred when at least 1.5 g of agent 

was injected into the fire; conversely, extinction never occurred when less than 0.7 g was added. The % 

of the fires suppressed varied when the agent mass was between these limits, as shown in Figure 6–27. 

The suppression statistics were generated by lumping the mass from thirty-three discharges into bins 0.2 g 

wide, centered about the data plotted. The solid line is a fit to the data assuming that the shape is 

sigmoidal. It is apparent from the curve that there was a 50 % chance that suppression would be

successful if the amount of agent were 0.9 g ± 0.1 g, and there was a 90 % success rate for 1.3 g ± 0.1 g

of agent. 

Figure 6–27. 

Percentage of 

Flames 

Extinguished as a 

Function of Mass 

Delivered to F

by an SPGG.

lame

Figure 6–28 is a plot of the mass of agent required for suppression versus the injection time interval. All 

44

MASS, g

0.50 0.75 1.00 1.25 1.50 1.75 2.00

% SUPPRESSED

0

20

40

60

80

100

of the SPGG data are lumped around 20 ms since the injection time was fixed. Linear fits to the nitrogen 

and halon 1301 data yielded intercepts of 1.6 g for nitrogen and 1.2 g for halon 1301. The SPGG data fall 

close to the halon results. The significance of the linear shape and the value of the intercept was unclear;

however, the superior performance of the gas generator was undeniable. 
Fire Suppression Efficiency Screening 553

The mass fraction of agent, β, is defined as the total mass injected divided by the injection duration, ∆t, 

over the sum of the mass flow of air plus the mass flow of agent. The percentage of occasions that the 

flame was extinguished is plotted in Figure 6–29 as a function of β. 

INJECTION TIME, ms

0 100 200 300 400

MASS, g

0

2

4

6

8

10

CF3Br

N2

Figure 6–28. Impact 

of Injection Time 

Interval on the Mass 

of Agent Required 

to Suppress a Stepstabilized Propane 

Pool Fire. 44

SPGG

β

0.4 0.5 0.6 0.7 0.8

% SUPP

RESSED

0

20

40

60

80

100

Figure 6–29. 

Percentage of Flames 

Extinguished as a 

Function of Estimated 

Mass Fraction of 

Agent. 44

All of the fires were extinguished when β was greater than 0.62; none for a mass fraction below 0.49. The

definition of β for the SPGG differs somewhat from the definition as applied to the gaseous agents 

because the mass discharge profiles for SPGG and the gaseous agents were different. Whereas the mass
554 Screening Tests for Candidate Fire Suppressants 

injection rate of the gaseous agents was controllable through selection of the injection hardware, the 

SPGG mass injection rate was practically dependent on the propellant effluent generation rate (i.e., the 

propellant burning rate). For the same mass, variation of the discharge profile led to variation in agent

effectiveness. Optimization of the rate of agent discharge is an area that would benefit from further study,

which could be approached from analytic, numerical, and experimental perspectives. 

Data Correlation 

The effectiveness of the gaseous agents was compared to that of the SPGG through use of a simple agent

mixing model to describe the suppression phenomena. A more detailed description of the model can be 

found in Grosshandler et al.51 and Hamins et al. 37

 A characteristic time, τ, for mixing of the agent into

the flame zone was defined in terms of the air and agent volume flows, (V'air+ V'agent), and the step height, 

h, as 

τ = γh /{(V'air+ V'agent)/[(L-h)L]} (6-2)

where L is the width of the tunnel and γ is an empirical non-dimensional parameter that relates the ratio of 

the distance that a fluid element travels within the recirculation zone to the obstacle height. Takahashi et 

al. 38 measured the characteristic mixing time in a similar facility and found γ to be around 40. Evaluating

Equation 6-2 for the range of flows and baffle heights examined in the current study and using a value of

40 for γ, τ was found to vary between 0.04 s and 0.40 s.

Hamins et al. 37

 found that for a specified injection duration it is possible to relate the volume fraction of

agent required to achieve extinction, Χ, to the characteristic mixing time, τ, according to the following

relation: 

Χ/Χ* = [1-exp(-∆t/τ)]-1 (6-3)

where Χ* can be found experimentally by flowing agent continuously into the air stream at increasing 

rates until extinction occurs. If the air flow is low enough, the value of Χ* is expected to be similar to 

the cup burner extinction requirements. For propane in a cup burner, Trees et al.52 found the value of X* 

to be 0.32 for N2, 0.41 for Ar, and 0.039 for CF3Br. Others have found similar results.53

Figure 6−30 compares the suppression results for N2, CF3Br, and SPGG through use of Equation 6-3,

where the normalized volume fraction is plotted as a function of the non-dimensional injection interval. 

For the SPGG results, Χ* is assumed to equal the Χ* value for argon. While the data do not fall exactly on 

the model, the trend of the results are well represented by the single curve when one considers run-to-run 

variations due to the statistics associated with suppression of a turbulent flame. The results show that the 

effectiveness of the SPGG hybrid significantly exceeds the model predictions. 

For laminar diffusion flames strained at intermediate rates, Trees et al. 52 showed that the minimum 

extinction volume fraction of agent in a counterflow flame decreases from the cup burner value when the 

strain rate is 50 s-1 to much smaller values for a strain rate of 400 s-1. Although the flow in the 

recirculating region behind a step is much more complicated than in a counterflow flame, the strain rate in

the current study should scale with 1/τ. When the flow of air is increased sufficiently, the flame becomes

strained to the point that agent is not needed for extinguishment and the flame blows out. In other words, 
Fire Suppression Efficiency Screening 555

as the air flow in the TARPF increases, the model represented by Equation 6-3 in Figure 6–30 must be 

adjusted not only for changes in τ, but also for changes in the value of X*. 

∆t/τ

0246

Figure 6–30. Normalized Volume Fraction as a Function of Non-dimensional Injection 

Interval, Comparing N2, CF3Br, and SPGG. 44 

A number of direct numerical simulations of N2 suppressing the step-stabilized flame were conducted 

assuming a finite reaction rate with one step chemistry. The injection interval and volume fraction of

nitrogen were varied, and the flame structure was followed through the transient period. Either the flame 

was extinguished, or it was reestablished following the passage of the N2 pulse. Figure 6–31 summarizes 

the results. Open diamonds indicate that no suppression occurred; solid diamonds imply that suppression 

was successful. The gray symbols refer to the experimental N2 and CF3Br data for all geometries 

examined. The dotted line is from Equation 6-3. 

The computations were able to distinguish regimes of extinction and non-extinction in the case of

nitrogen as an agent. The numerical model could not predict what would happen when a chemically 

active agent like CF3Br is introduced into the flame. The cooling and dilution of the flame by the agent 

8

X/X*

0

1

2

3

4

SPGG

N2

CF3Br

X/X∗= [1-exp(-∆t/τ)]-1
556 Screening Tests for Candidate Fire Suppressants 

could be predicted, and hopefully, simplified combustion mechanisms for various chemically-active 

agents can be developed that will lead to a better understanding of the dynamics of fire suppression. 

Figure 6–31. Suppression Volume Fraction of Agent (N2 or CF3Br) Normalized by Cup 

Burner Values (Χ*) as a Function of Injection Time Interval Normalized by Characteristic 

Important Findings 

m volume fraction of agent for suppression, normalized by the cup burner value, 

correlated with [1 - exp(- ∆t/τ)]-1, where ∆t is the injection time interval and τ characterizes

•

u n single-step chemistry, and numerical

∆t/τ

012345

X/X*

0

1

2

3

4

5

X/X*= [1-exp(-∆t/τ)]-1

simulation, suppression

simulation, no suppression

Residence Time (τ). Gray symbols: experimental results keyed to Figure 6–25; black 

diamonds: direct numerical simulations with N2. 44

• The minimu

the mixing time behind the obstacle in the flow. 

The general character of the flame and its extinction by a thermal gaseous agent was captured 

by a direct numerical simulation of the flow based po

experiments have corroborated the simple correlation of the experimental data for N2. 
Atmospheric Lifetime Screen 557

• The measured difference between the decrease in agent storage bottle pressure and the arrival

of the agent at the fire highlighted the importance of determining the agent concentration 

locally and the difficulty in relating changes in bottle pressure to actual mixing conditions. 

• For the first time, both compressed and solid-propellant generated gases could be compared

side by side, and the effect on performance of different formulations, particle loadings and 

burning rates for various SPGG designs could be unambiguously discriminated. 

• When the temperature of a hot surface downstream of the pool was above 800 °C, the flame,

following suppression, reignited and stabilized on the hot surface. At a temperature below 

800 °C, the number of reignitions approached zero. This result was in contrast to when the 

hot surface was located between the stabilizing step and the fuel pool, in which a delayed

reignition was observed at temperatures as low as 400 °C. 

6.4.5 Summation 

Because of the expected diversity of physical and chemical properties of candidate fire suppressants, the 

NGP developed three apparatus for screening fire suppression efficiency. The cup burner was the

workhorse for examination of the chemicals, as discussed further in Chapter 7. The DLAFSS was 

available for assessing those chemicals with high boiling points and high heats of vaporization. The

TARPF was available for appraising those chemicals and delivery systems for which it was expected that 

burst delivery would significant modify the interpretation of data obtained using the cup burner. 

6.5 ATMOSPHERIC LIFETIME SCREEN 

6.5.1 Factors Affecting Atmospheric Harm 

The NGP screening protocol had its roots in the 1990 development of screening procedures for ozone 

depletion potential (ODP)v

 and global warming potential (GWP).vi, 1 It was updated and refined as the

result of a 1997 NGP workshop2

 and constraints on the agent chemistry resulting from the other 

properties being screened. 

A comprehensive discussion of atmospheric chemistry and the effects of pollutants can be found in 

reference 54. The principles that determined the NGP screening protocol were: 

• Both ODP and GWP are directly related to the lifetime of a test substance in the atmosphere.

The greater the persistence of a chemical, particularly within the troposphere, the greater the 

potential for adverse effects on the global environment. It was assumed that other, as yet 

undetermined effects would also be directly related to atmospheric lifetime. 

• The tropospheric lifetime of a chemical is determined by the rates of physical and chemical 

removal processes. The most important of these are the reactivity with respect to OH radicals

and absorption of solar radiation in the VIS-UV region of the solar spectrum. For each such

process, it must be established that the products are rapidly removed from the atmosphere,

e.g., by rainout. In general, OH reactions are important if there are abstractable H atoms

 

v

 The ozone depletion potential of a gas is defined as the change in total ozone per unit mass emission of the gas, relative to the 

change in total ozone per unit emission of CFC-11 (CFCl3). vi The global warming potential of a gas is defined as the change in irradiance at the tropopause (the boundary between the 

stratosphere and the troposphere) per mass of the gas emitted relative to the change in irradiance per mass of carbon dioxide. 
558 Screening Tests for Candidate Fire Suppressants 

available or if the test compound has unsaturated C-C bonds (aromatic or aliphatic). Fully

halogenated alkanes are unreactive with respect to OH radicals. The mechanism whereby 

most CFCs and halons are destroyed in the atmosphere is by absorption of solar radiation in

the 170 nm to 300 nm region. For a replacement chemical, photolysis in the troposphere 

could also be important if the chemical absorbs solar radiation in the VIS-UV part of the

spectrum. For example, photolysis efficiently removes organic iodine compounds within the 

troposphere. Other removal processes, including physical removal (i.e., rainout, aerosol

scavenging and solvation) are significantly slower. Methods for estimating lifetimes due to

these physical processes are discussed in Reference 55. 

• Ozone depletion potential is also directly related to the numbers and kinds of certain atoms in

a compound's chemical formula. If a test substance does not contain chlorine, bromine, 

iodine, or nitrogen, the current knowledge of stratospheric chemistry indicates that the ozone

depletion potential is zero. Each of these four atoms can have a deleterious effect on 

stratospheric ozone, and many molecules containing them will have a positive ODP value. 

The calculation of ODP requires the use of an atmospheric model with a complete input of 

chemistry, solar irradiation, and transport. Comparison of the results of various models, 

ranging from the most elaborate three dimensional models to the much simpler one 

dimensional models, indicates that in the case of the halocarbons, simple models give results 

adequate for use in developing screening tests.56 Halon 1301 has an estimated ODP value of

about 13. 

• Global warming potential is directly related to the capability of a test compound to absorb

radiation in the region of the "atmospheric window" between 7 μm and 13 μm. Test

compounds which are weak absorbers in this region are unimportant with regard to global 

warming. 

6.5.2 Prior Estimation Methods 

Stratospheric Ozone Depletion 

Skaggs and Nimitz57 proposed a method for estimating ODP values for halocarbons of containing one or

two carbon atoms and no bromine atoms. The method related the ODP to two factors: a reactivity factor, 

related to the number of chlorine atoms in the molecule, and a survival factor, i.e., the fraction of 

molecules reaching the stratosphere. The relationship is shown in Equation 6-4. 

ODP = ( / )

1 2 3 c c t c n e− (6-4)

where: 

n is the number of chlorine atoms 

t is the atmospheric lifetime 

2 c n is the reactivity factor 

c2, and c3 are parameters to be fit 

( / ) 3 c t e− is the survival factor 

c1 is a normalizing constant
Atmospheric Lifetime Screen 559

This equation gave values that were relatively close to the rigorously calculated ODPs used in fitting the

parameters. Since the rigorously calculated ODP values and atmospheric lifetimes used in the parameter

fitting have changed since the equation was originally proposed, the reported parameters are not given

here. 

Iikubo and Robin58 extended this formulation to include estimation of ODP values for bromine-containing

compounds. 

ODP = A E P [(#Cl)B + C(#Br)] D(#C-1) (6-5)

Here, P is the photolysis factor, which was set equal to 1.0 if there were no special structural features that 

made the molecule subject to tropospheric photolysis. Otherwise, P = 0.180 for a bromine geminal to a 

chlorine (e.g., Br-C-Cl), P = 0.015 for geminal bromine atoms (Br-C-Br), or P = 0.370 for vicinal 

bromine atoms (Br-C-C-Br). The fitting parameter values were: A = 0.446, B = 0.740, C = 32.000, and 

D = 1.120. The hydrogen factor, E, was set equal to 1.000 if there were no hydrogen atoms; otherwise it 

was set equal to 0.0625. #C, #Br, and #Cl are the numbers of carbon, bromine, and chlorine atoms,

respectively.

Use of Equation 6-5 gave, at best, rough approximations, even if the equation was restricted to simple

saturated halocarbons (Table 6–2). The trends, however, were preserved. It may be that a better fit of 

parameters could be obtained with a larger set of more recent data as a basis. 

Table 6–2. Estimated and Reported ODP Values (Relative to CFC-11) for Selected 

Halocarbons. 

Compound Formula Calculated ODP Reported ODP Reference 

Halon 1301 CBrF3 14.3 12 59

Halon 1211 CBrClF2 2.6 5.1 59

Halon 2402 CBrF2CBrF2 17.4 6.6 60

HBFC-22B1 CHBrF2 0.89 0.74 61

HBC-30B2 CH2Br2 0.03 0.17 62

HBC-40B1 CH3Br 0.89 0.64 59

HCFC-123 CHCl2CF3 0.050 0.014 59

Atmospheric Lifetime 

To a great extent, values of ODP and GWP are determined by the tropospheric lifetimes of the 

compounds. Regardless of the chemical, the ODP and GWP values will be negligible if tropospheric 

removal is efficient. Tropospheric removal reactions can be either bimolecular or unimolecular. 

Bimolecular reactions involve reaction with atmospheric species such as hydroxyl free radicals (•OH),

tropospheric ozone (O3), oxygen atoms (•O in 3

P, 1D electronic states), etc. Unimolecular reactions

include photolysis, thermal dissociation, and heterogeneous absorption (aqueous or particulate aerosols). 

The rate of disappearance of a species, C, due to a bimolecular reaction with an atmospheric species A

can be expressed as shown in Equation 6-6, where [C] and [A] are the concentrations of species C and A, 
560 Screening Tests for Candidate Fire Suppressants 

and k2 is a second-order reaction rate constant. Similarly, the rate of removal through unimolecular 

reactions is expressed by a first-order differential equation with the first-order reaction rate constant k1 as 

shown in Equation 6-7. Typical units for k2 and k1 are, respectively, cm3

/molec-s and s-1, with 

concentrations in molec/cm3

. The rate constants k1 and k2 are constant only under a given set of 

conditions. For example, for photolysis, k1 depends on the radiation flux and wavelength; for reaction 

with atmospheric particulate, k1 depends on the concentration (and composition and other characteristics) 

of the particulate. 

[ ][ ] [ ] k2 C A

dt

d C = − (6-6)

[ ] [ ] k1 C

dt

d C = − (6-7)

Careful consideration shows that all reactions can actually be considered “bimolecular” in form since the

concentration of particulate, for example, is incorporated into k1 for particulate removal, and the 

“concentration” (actually, the flux) of photons is incorporated into k1 for photolysis. 

The concentration of a species undergoing reaction, as shown in Equation 6-6 or Equation 6-7, decays

exponentially as given by Equation 6-8. Here [C]o is the initial concentration, [C]t is the concentration at 

any time t, and t1/e is the “e-folding” atmospheric lifetime, i.e., the time required for the concentration of 

an atmospheric species to decay to 1/e (approximately 0.369) of its initial value. The e-folding lifetime is

related to the reaction rate constant by or depending on whether the reaction is bimolecular (Equation 6-9)

or unimolecular (Equation 6-10). 

e t o t t C C e / 1/ [ ] =[ ] (6-8)

t1/e = 1/k2[A] (6-9)

t1/e = 1/k1 (6-10)

Using these equations, one can estimate the atmospheric lifetime due to various removal processes by 

using globally averaged values for concentrations of reactant species. 

A globally averaged tropospheric hydroxyl free radical concentration of 9.7 x 105

 molecules/cm3 and a 

globally averaged tropospheric ozone concentration of 5.0 x 1011 molecules/cm3

 have been used for such

estimations.63,64 In the global averaging approximation, one uses reaction rate constants for a mean 

tropospheric temperature. Values around 298 K are often used65 and were used in the NGP estimations. 

In the global average approximation, atmospheric lifetime is directly proportional to the second-order rate 

constant (kOH) for reaction with •OH, when this is the primary tropospheric removal process. Thus, an

alternative way to estimate the atmospheric lifetime for a compound B removed only by reaction with 

•OH was to use Equation 6-11 to relate the lifetime of B ( ) to that ( ) of a reference compound C. B

e t

1/

C

e t

1/

66 

Here, and are the second-order hydroxyl reaction rate constants for the two materials. Methyl

chloroform (CH

B

OH k COH k

3CCl3) is usually the reference compound. This method is equivalent to using the 

globally averaged •OH concentration when this concentration has been determined from measurements of 
Atmospheric Lifetime Screen 561

atmospheric removal of methyl chloroform, as in the case of the value used in this report. 63 Note that

with the appropriate rate constants, one could use an equation similar to Equation 6-11 when a removal 

mechanism other than reaction with a hydroxyl free radical predominates, as long as the removal 

mechanism for the compound of interest (B) is the same as that for the reference compound (C). 

( / ) 1/ 1/

B

OH

C

OH

C

e

B

e t = t k k (6-11)

Attempts to use Quantitative Structure-activity Relationships (QSARs) to predict kOH values have not 

been highly successful.67 A procedure based on the C-H bond energy (and, thus, related to the activation 

energy for hydrogen atom abstraction by •OH) gave disappointing fits to rigorously calculated

atmospheric lifetimes, though re-parameterization with more recent data might help. 57 Since many of the 

compounds of interest as halon replacements contain fluorine atoms, there was one trend of interest.

Examination of rate constants for a variety of fluorinated compounds indicated that fluorine substitution 

activated hydrogen atoms on the same carbon atom toward •OH abstraction, but deactivated hydrogen 

atoms on carbon atoms immediately adjacent. The reaction of •OH with CH3CH2F, for example, 

proceeds primarily (85 %) with hydrogen atom abstraction from the CH2F group.68

Better results may be available through quantum mechanical calculations. There was strong evidence that 

kOH values vary linearly with the calculated values for the highest occupied molecular orbital in 

hydrofluorocarbons.69

6.5.3 The Screening Protocol 

Estimation of the atmospheric impact of candidate fire suppressant considered several properties. The 

following is the sequence that was typically followed:

• A small molecule that did not contain chlorine, bromine, iodine, or nitrogen was generally

"passed." 

• For a larger molecule or one that contained one or more of the four atoms, but contained a 

feature that bespoke a high rate of reactivity in the troposphere (see below), its reaction rate 

with OH was estimated. The potential of the molecule to absorb VIS-UV radiation was also 

estimated. 

• The reaction rate was measured for a molecule that fared well in the prior step and in the 

screens for fire suppression effectiveness and volatility. 

The following sections relate the methods to be used in such appraisals. 

6.5.4 Removal by Hydroxyl Radical Reaction 

Hydroxyl radicals can transform and remove trace atmospheric substances from the troposphere by

abstracting hydrogen atoms or by addition/displacement reactions at unsaturated sites in the molecule. 

The reaction of any substance, X, with OH can be written as 

X +OH → PRODUCTS (6-12)
562 Screening Tests for Candidate Fire Suppressants 

and the rate of one of these reactions is given by: 

= k (OH)(X ) , dt

- d(X )

OH

(6-13)

where kOH is the rate constant for reaction, (X) the concentration of species X, and (OH) is the average 

tropospheric OH concentration. Since (OH) is essentially constant (concentration independent of the

presence of the trace substance X in the atmosphere), the expression can be written as: 

= k (OH)(dt) (X )

- d (X )

OH

(6-14)

or: 

= k (OH ) t , X

X 1n OH

t

O

⎟

⎟

⎠

⎞ ⎜

⎜

⎝

⎛ (6-15)

where X0 is the concentration at some zero of time, and Xt the concentration at time t. The time for the

concentration to decay from X0 to Xt is thus given by: 

. k (OH )

X

X 

t= OH

t

O ⎟

⎠

⎞ ⎜

⎝

⎛ ln (6-16)

The atmospheric half-life (where X0/Xt has been set equal to 2.718, the base of natural logarithms), or 

atmospheric lifetime, can be defined as: 

. k (OH)

1 or t= k (OH )(2.718) t= OH OH

ln (6-17)

Using an average atmospheric hydroxyl radical concentration of 106

 molec cm-3 (70), the tropospheric

lifetime becomes: 

, k

3 x 10

t (trop)= OH

-14

OH

(6-18)

where the units of tOH(trop) are years (1 year = 3 x 107

 s) and of kOH are cm3 molec

-1 s-1. For the 

identification of compounds with tropospheric lifetimes of the order of a month or less, desirable values 

of kOH are 3 x 10-13 cm3

 molec

-1 s-1 or higher. 

Values of kOH may be determined by direct laboratory measurement of the rate constant of reaction, or 

may be estimated using structure-activity relationships. The latter is preferred for screening those 

compounds for which appropriate data are available, since the measurements are quite labor intensive and 

the calculation is faster and less expensive and uses none of the compound. 
Atmospheric Lifetime Screen 563

Experimental Measurement of OH Rate Constants 

The method of choice for this determination was the flash-photolysis resonance fluorescence 

technique.71,72,73 It is a precise, reliable, and commonly used method for the measurement of OH rate

constants over the range of rate constants of about 10-14

 to 10-9 cm3 molec

-1 s-1. This method measures the 

loss of OH under conditions of excess test compound and yields an absolute measurement of the OH rate 

constant. It is mandatory that the sample of the test compound be free of impurities that are significantly

more reactive than the test substrate, e.g., unsaturated compounds or compounds with many easily

abstracted hydrogen atoms. The method requires the participation of senior scientific staff. A schematic 

of a typical apparatus is shown in Figure 6–32. 

He +H2O

Pump

Multichannel

Scaler

Multichannel

Mi Scaler cro

computer

Thermostated

Fluid

Pump

Flash 

Lamp

PM Vacuum

Jacket

Microwave

Discharge

Microwave

Discharge

Flash Photolysis / Resonance Fluorescence

Gas Handling

System

Gas Handling

System

Agent (to be studied)

(OH source)

(carrier gas) 

H2O 

Ar 

Figure 6–32. Schematic 

of Flash Photolysis 

Resonance 

Fluorescence Apparatus 

for Measuring Reactivity

of Compound with OH 

Radicals. 2

The apparatus consisted of a stainless steel or glass cell surrounded by an outer wall between which fluid 

was circulated to a temperature regulator to maintain a fixed temperature, measured by means of a

thermocouple. The cell had four window ports at 90° positions around the cell, and one additional

window port on the base of the cell. The entire cell was mounted within a vacuum housing. Gases were 

admitted to the cell through inlets connected to a gas handling system. A hydroxyl radical resonance 

lamp, consisting of a microwave powered discharge in 130 Pa of a mixture of He saturated with water 

vapor, and a pulsed N2 discharge flash lamp, which was powered by discharge capacitors and a power 

supply, were located at right angles to each other and to a photomultiplier detector. 
564 Screening Tests for Candidate Fire Suppressants 

Hydroxyl radicals were produced by the flash photolysis of H2O. The OH radical resonance lamp was

used in cw mode to excite a small fraction of the hydroxyl radicals which then emitted resonance

radiation at 308 nm which was detected by the photomultiplier. The signal output was then fed to pulse 

counting electronics, and ultimately to a computer for signal averaging and subsequent statistical analysis 

and graphical display. The measurement process was initiated by a photomultiplier (PM) which was 

triggered by the visible light from the flash lamp. A delay gate introduced a delay time of 100 μs to

300 μs to eliminate any scattered light interference from the flash lamp. Fluorescence signal from 50 to 

500 flashes were typically averaged to generate a suitable decay curve. 

In a typical experiment, reaction mixtures containing the test compound, argon, and water vapor 

(prepared manometrically in a 5 L bulb before admission to the reaction cell) were flowed slowly (at 

about 100 cm3

 s-1) through the cell. The quantity measured in this test was the rate of loss of OH radicals

in the presence of a great excess of the test compound. Under these conditions, the rate expression can be 

written as: 

= k (OH )(S) , dt

- d (OH ) (6-19)

where OH is the concentration of OH radicals, S is the concentration of the test substance, k is the rate

constant, and t is the reaction time. Under conditions where (OH) << (S), S is essentially constant, and 

the OH decay is first order, and the rate expression becomes: 

= k (S ) dt= k dt , (OH )

- d (OH ) ′

(6-20)

where k', the apparent first-order rate constant, is obtained directly from a measurement of the exponential

rate of decay of OH. The true rate constant is then: 

. (S )

k k = ′ (6-21)

However, since there are OH diffusional losses from the reaction zone, in practice the first order decay

was measured as a function of the concentration of the test compound, and from a plot of first order rate 

constant against concentration, the second order rate constant was obtained from the slope. 

Tests for impurity effects must be included as part of the screening protocol. The effect of reactive 

impurities on test results can be illustrated by considering a test compound which has a rate constant for 

reaction with OH radicals of about 1 x 10-15 cm3

 molec

-1 s-1, corresponding to an atmospheric lifetime of

30 years. If the test sample contained a 1 % impurity in the form of an alkene, which typically may have a 

rate constant for reaction with OH of about 1 x 10-12 cm3

 molec

-1 s-1, then the apparent OH rate constant 

would be about 1 x 10-14 cm3

 molec

-1 s-1, corresponding to a lifetime of 3 years. Purity was thus of great 

importance in carrying out these tests, and it was desirable for the test compounds to have impurity levels

for alkenes and other unsaturated compounds of less than about 0.1 %. If the purity was not known, then the 

following test procedure was followed: 

• Measure the rate constant. 
Atmospheric Lifetime Screen 565

• If k < 3 x 10-15 cm3 molec

-1 s-1, the test results were acceptable. 

• If k > 3 x 10-15 cm3

 molec

-1 s-1, check the test sample for impurities. This was done initially 

by carrying out a trap-to-trap distillation and then remeasuring the rate constant. If the rate

constant decreased, the sample was impure, and an analysis was necessary. Only after 

analysis and, if necessary, purification, were additional measurements be undertaken. 

6.5.5 Measurement of Photolysis Rates 

Photolysis is the process whereby a molecule is chemically transformed following the absorption of light. 

For photolysis to be important in the troposphere, the molecule must be able to absorb photons in the 

VIS-UV region of the solar spectrum (between about 290 nm and 700 nm), and the resultant energy-rich 

molecule must be chemically transformed rather than being physically quenched back to the original 

state. The rate of photolysis depends upon the flux of solar photons in the VIS-UV region, the absorption

cross section of the substance over the same region, and the quantum yield for dissociation as a function

of wavelength. The rate of photolysis is: 

(6-22)

where f is the flux of solar photons in cm-2 s-1, σ is the cross section in cm2, φ is the quantum yield for 

dissociation, kp is the effective first order rate constant in s-1, and (X) is the concentration of substance X 

in molec cm-3. Thus: 

k = f . p σφ (6-23)

The rate constant must actually be determined as the integral of this quantity over the whole range of 

atmospherically-accessible solar radiation. The process is considerably simplified by carrying out the 

calculation for discrete intervals of the solar spectrum of interest and summing to obtain the total rate

constant. Other factors including solar zenith angle, season, latitude, altitude, and surface albedo also

enter into the calculation. 

The lifetime with respect to photolysis is defined as, 

 , k

1

t (trop)= p

p

(6-24)

or, numerically, 

tp (trop) = 3 x 10-8 kp

-1 (6-25)

where the units of tp(trop) are years and the units of kp are s-1. 

To determine kp, it is necessary to know the atmospheric flux of solar photons and their wavelength

distribution, the absorption cross section of the compound over the same spectral range, and the quantum

yield for dissociation, also as a function of wavelength. The first quantity is tabulated and available.54 

The second quantity is readily measured in the laboratory. The third quantity, however, is not easily

= k (X)= f (X) , dX - p σφ dt
566 Screening Tests for Candidate Fire Suppressants 

obtained. For the purpose of developing a screening test, the quantum yield was conservatively taken to

be unity at all wavelengths, and the screening test was based on measurement of the absorption cross

section. 

Then, 

(6-26)

. -1

and since f is a tabulated quantity, the test measurement involved measurement of the absorption cross 

section in the VIS-UV region of the spectrum.

The measurement can be undertaken using a variety of spectrometer configurations. Design options 

include single pass or multipass for the absorption train, and also single beam or double beam for the 

detection system. Double beam instruments consist of true double beam configurations employing two 

identical monochrometers and two detection systems, virtual double beam configurations employing a 

beam splitter at the exit of the monochrometer followed by two detection systems, and broad-band 

referencing, where the wavelength dispersed signal is referenced to the non-dispersed incident light 

source. Any of these experimental configurations are acceptable. Of particular relevance are the 

measurements of Molina and coworkers74, using a double-beam instrument to measure absorption cross

sections for several brominated methanes and ethanes, and those of Gillotay and coworkers75 who 

measured absorption cross sections for some bromine-containing compounds using both multiple 

reflection and single pass single-beam instruments. 

The choice of instrument is dictated by the requirement that it be able to measure absorption cross

sections as low as 10 cm2

 to 24 cm2. Absorption cross section is defined in terms of the fractional 

absorption of light passing through a defined path length of a given concentration of absorber. 

One can define a figure of merit for the measurement system based on the quantity σpΡ (the optical 

depth), where σ is the absorption cross section (wavelength dependent), p the pressure (or concentration) 

of the test compound, and Ρ the path length. Highest precision cross section measurements are made with

an optical depth of about one (30 % to 40 % absorption). For weak absorbers, σpΡ will be much smaller. 

In addition, for a fixed path length, absorption depends on the pressure (or concentration) of the test 

substance in the optical path. Under nominal conditions of "room" temperature (298 K), absorption may

be limited by the available vapor pressure of the test substance. 

To measure the cross section for a weak absorber requires high pressure, and/or long path length, and/or

high accuracy in measuring very small amounts of absorption. For screening purposes, the accuracy or 

reproducibility of the measurements should be about 25 %, and this should define the smallest possible 

value of σpΡ suitable for making the measurements. 

The measurement method described here was based on the use of a single beam monochrometer.

76 The

absorption cross section is defined as: 

k p ≤ fσ and t p (trop)≥ k

p
Atmospheric Lifetime Screen 567

(6-27)

where σλ is the cross section at a given wavelength, (c) the concentration of absorbing test substance in

molec cm-3, l the path length in cm, Iλ the intensity of light passing through the cell containing the

absorber, and I0 the intensity of light passing through the empty cell, both in arbitrary units. The test

measurement thus involved measuring and taking the ratio of the quantities Iλ and I0. 

A schematic of a suitable apparatus is shown in Figure 6–33. 

Figure 6–33. Apparatus for Measuring the Absorption Cross Section of Candidate Fire 

Suppressants. 1

The apparatus consisted of a 1 m 0

lines/mm grating blazed at 150 nm. The lamp shown in the figure was a Hamamatsu deuterium lamp

as observed in real time during a measurement, was 

processed and fed to a controller interface module which was part of an IBM compatible personal

 incidence McPherson vacuum monochrometer employing a 60

with a highly stabilized power supply. This provided a convenient source of radiation over the 290 nm to

400 nm range. For measurements further into the red region of the spectrum, a tungsten lamp was used. 

The detector was a Hamamatsu solar blind side-on multiplier tube. Five absorption cells having a range

of path lengths ranging from 0.2 cm to 20 cm were chosen by use of the turret assembly. The 

monochrometer wavelength drive was controlled by a stepping motor with 800 steps per revolution. The

gear train used was such that 16 pulses were equivalent to 0.1 nm wavelength change. Gas pressure in the

cell was monitored with a diaphragm type gauge. 

The signal from the photomultiplier tube, which w

1 , I

I ln

c - =

O

⎟

⎟

⎠

⎞ ⎜

⎛

⎝ ⎟

⎟

⎠

⎞ ⎜

⎜

⎛ σ(λ) λ

⎝
568 Screening Tests for Candidate Fire Suppressants 

computer system. A spectrum was first measured with no gas in the cell, then with an appropriate amount 

of gas in the cell, and finally with the cell again empty. The deuterium lamp was very stable, and this 

procedure led to a very reliable measurement of the adsorption cross section down to about 10-23 cm2

. 

The procedure was to measure the absorption cross section from 290 nm to 700 nm using the definition of

absorption cross section given above. The absorption cross section as a function of wavelength was then 

ngth intervals.

Over the same intervals, Table A7 in Reference 1 provided values of the solar flux, corresponding to a 

related to photolysis rate constant using procedures outlined by Finlayson-Pitts and Pitts. 54

The experimentally measured absorption cross section was averaged over discrete wavele

cloudless day, zenith angle of 0°, surface albedo zero, uncorrected for latitude or season. For each 

discrete spectral region the rate of absorption was calculated as the product of columns 2 and 3 or that

table, and the result entered into column 4. The summation of column 4 then gave the total rate of 

absorption. This was an upper limit to the number of photons absorbed. Following Molina and 

coworkers74, it was recommended that the maximum rate of absorption be divided by 3 to provide a more 

realistic estimate of the average atmospheric absorption rate. This absorption rate constant was an upper 

limit to the true rate constant for photolysis: 

k k . p ≤ a

(6-28)

he problem of test sample purity was of concern here as well as with respect to OH reactivity

levels of a strongly absorbing impurity will give an incorrect set of cross section results. Therefore, when 

mpurities by carrying out a trap-to-trap

ss section. If the absorption cross section

The test va

lifetime, an e.

t an analysis of the structure of the molecule and its breakdown into 

component structural sub-units. The reactivity was expressed in terms of (a) abstraction of H atoms, (b)

T . Low

the purity was not known, the following test procedure was followed: 

• Measure the absorption cross section as a function of wavelength, and calculate kp.

-9 -1 • If kp < 3 x 10 s , the test results were acceptable. 

-9 -1 • If kp > 3 x 10 s , check the test sample for i

distillation and then re-measuring the absorption cro

changed significantly, the sample was impure, and an analysis was necessary. Only after

analysis and if necessary, purification, were additional measurements undertaken. 

lue derived for lifetime with respect to photolysis provided only a lower limit to the true 

d if the quantum yield were low, the test compound could have a much longer lifetim

6.5.6 Estimation of Reactivity

The es imation method was based on

addition at double or triple C-C bonds, and (c) addition to aromatic structures. The total rate constant was 

obtained by summing the contributions of the different groups. In some cases, the reactivity and hence 

lifetimes of certain classes of compounds were estimated without going through a detailed estimation 

calculation or making laboratory measurements. For instance, the completely halogenated (with F, Cl, Br, 

or I) alkanes are essentially inert with respect to reaction with OH radicals. On the other hand, reactions 

of OH radicals at unsaturated sites (aliphatic or aromatic in character) are always very much faster than

abstraction77, and therefore, unsaturated substances have short atmospheric lifetimes. Thus, the

estimation methods were presented only with reference to saturated compounds containing at least one 

abstractable hydrogen atom. 
Atmospheric Lifetime Screen 569

The reactions of OH radicals have been the most studied of all classes of gas-phase reactions, and, as a 

result, there was a very large body of reliable kinetic data. Atkinson78 used this database to develop a group

method for estimating OH radical rate constants for hydrogen atom abstraction. He also provided a basis 

ndent on the nature of the substituents alpha and beta

to the group. In general, rate constants for the groups can be written as: 

where kp', ks', kt', and kOH' refer to the rate constant for abstraction from nominal primary, secondary, 

tertiary, and hydroxyl groups, respectively, and the factors F(X), F(Y), and F(Z) are correction factors to 

account for the effects of substituent groups. Using values for group rate constants and correction factors

f quantitative structure activity

relationships.79 Their values were based on estimates of the OH reactivity with the hydrogen atoms(s). 

dequately correlated by simple 

structure-activity relationships. In particular, when an ether linkage was introduced, even the relative

was a computational scheme for OH

reaction rate constants for those compounds for which hydrogen abstraction would be expected to be the 

molecules, seeking the lowest possible degree of 

computational difficulty. This level would then be applied to more complex molecules and, ultimately,

for estimating rate constants for attack at double and triple C-C bonds, and attack at unsaturated aromatic 

sites. Examination of his database showed that the rate constants for the latter classes of reactions were all 

very fast compared to those for abstraction reactions. 

The basis for the Atkinson method was the separation of the molecule into defined structural units. The rate 

constants for -CH3, -CH2-, and >CH- groups are depe

k (X - OH)= k F (X ) ,

k (X -CH (Z )-Y )= k F (X ) F (Y ) F (Z ) ,and

k (Y - CH - X )= k F (X ) F (Y ) ,

k (CH - X )= k F(X ),

OH

t

2 s

′

′

3 p′

tabulated by Atkinson, rate constants for test compounds were derived. 

Nimitz and Skaggs also estimated ODP values and atmospheric lifetimes of one- and two-carbon

hydrofluorocarbons and hydrochlorofluorocarbons on the basis o

They were able to predict the results one- and two-dimensional atmospheric calculations within about a 

factor of two for compounds with atmospheric lifetimes under 30 years. 

However, experimental studies had demonstrated that the reactivity of the hydroxyl radical with many,

more complex types of halogenated organic compounds was not a

order of reactivity among the various ethers could not be predicted correctly.

80 This implied that it would 

be necessary to measure rate constants for a large number of members of any new class of reactants. A

better method of screening candidate molecules was clearly needed.

Under the NGP, Huie and coworkers developed a refined method to predict atmospheric lifetimes for new 

classes of halogenated compounds.81,82,83 At the heart of the method

dominant reaction mechanism. They established accuracy of this approach utilizing a set of compounds 

for which reliable experimental data were available. Combining calculations with new measurements of 

ultraviolet absorption spectra, they then estimated atmospheric lifetimes of a set of bromine-containing 

compounds for which no data had been reported. 

At the core of the method was the establishment of a level of theory that would predict the reactivity of 

the hydroxyl radical with a series of simple
570 Screening Tests for Candidate Fire Suppressants 

applied to the new families of candidate fire suppressants. The results would be validated by a limited 

number of experimental determinations, but extensive experimentation and synthesis could be avoided. 

In the initial study, several levels of theory were explored for the reaction of OH with CH2Br2.

 81 This 

molecule was chosen both because of the importance of bromine as a fire suppressant and because the

relatively large electron system of the molecule made this reaction a serious test of the various levels of

theory. This study included the treatment of tunneling, making use of three different methods of

approximating this correction. A low level of ab initio structure calculations was found to be more than

adequate for screening purposes. Building on the results of this study, the reactions of OH with the other 

halogen-substituted methanes, up to bromine, were also calculated82, 83

 and compared with experimental

results where available. Huie and coworkers also performed measurements on some bromopropanes84

and bromofluoroalkenes.85 Table 6–3 summarizes the results of these calculations and measurements. 

Table 6–3. Atmospheric Lifetimes (years), from Theory and from OH Reactivity

Measurements, for Selected Molecules. 

Atmospheric Lifetime Atmospheric Lifetime 

(theory) (from meas.) Theory/Meas.

CH2Br2 0.1 0.25 0.5 

CH3F 6.4 2.9 2.2 

CH3Cl 1.5 1.5 1.0

CH3Br 2.5 2.0 1.3

CH2F2 3.6 5.6 0.6

CH2FCl 1.7 1.4 1.2

CH2Cl2 0.43 0.46 0.9

CH2ClBr 0.54 0.49 1.1

CH2Br2 0.57 0.42 1.4

CHF3 400 263 1.5

CHF2Cl 13.2 12.7 1.0

CHF2Br 6.0 5.6 1.1

CHFCl2 1.2 2.1 0.6

CHCl3 0.15 0.51 0.3

CH2FBr 1.9 Unknown 

CHFBr2 0.49 Unknown 

CHFClBr 0.73 Unknown 

CHCl2Br 0.16 Unknown

CHClBr2 0.12 Unknown

CH2BrCH2CH3 14 days

CH3CHBrCH3 19 days

CF2=CF2 1.1 days

CFBr=CF2 1.4 days

CHBr=CF2 2.4 days

CH2=CBr-CF3 2.8 days

CH2=CBr-CF2-CF3 3.2 days

CH2=CH-CF2-CF2Br 7.0 days
Atmospheric Lifetime Screen 571

In summary, the calculated rate constants for the reaction of OH with halomethanes were in good

agreement with their experimental counterparts, establishing the viability of ab initio calculations as the 

basis of a screening tool. Calculations for the reactions of OH with several fluoroethanes and the ethers

erived from them have reproduced the experimental trends, with predictions in absolute reactivity within 

a factor of three. Calculations are being extended to ethers with several carbons and containing fluorine

tion into atmospheric droplets and subsequent rainout. To 

investigate the likelihood of this process, Huie and coworkers calculated Henry’s Law constants (Kh) for a 

42 solvation model at the PM3 level of theory. Based on

comparisons of the results of calculations for similar alkylamines with experiment, they estimated that

ed as fire suppression agents.

They have very low volatility and are unlikely to be degraded in the atmosphere. They are, however,

this medium. As part of this 

project, Huie and coworkers measured the rate constants for their reactions with the radicals important in

d

along with one or more bromine atoms. 

6.5.7 Solution of Fluoroalkylamines 

Fluorinated alkylamines were considered as possible alternate fire suppressants. In this case, a possible 

atmospheric loss mechanism was solu

series of fluoroalkylamines, using the SM5.

these results were valid to within a factor of three. Based on these results, rainout is likely not to be a 

very effective removal mechanism for the more highly fluorinated species. 

6.5.8 Reactivity of Alkyl Phosphates towards Radicals in Water 

Some suggested fire suppressants are not volatile but their environmental fate is still of concern. Alkyl

phosphates are a class of chemicals that have been extensively investigat

soluble in water and their environmental fate is likely to involve reactions in

aqueous phase degradation processes: the carbonate radical, CO3

–

, and the dichloride radical, Cl2

–

. The

reactivity towards the sulfate radical, SO4

–

, was also measured for comparison. The procedure used was 

pulse radiolysis, in which a high-energy electron pulse is utilized to generate the radicals, whose temporal

history is monitored by the use of optical absorption. The results of this study are presented in Table 6–4 

(in units of L mol

 1 s 1). 

Table 6–4. Reactions of Phosphates. 

Compound SO4

- Cl2- CO3-

Triethyl phosphate 1.6. x 106 < 7 x 104 < 1 x 104

Diethyl Phosphate 6 4 8.7 x 10 < 1 x 10

Trimethyl Phosphate 1.3 x 105 < 4 x 104 < 3 x 103

Dimethyl Phosphate 7 x 104

A e radical reacte asurable rate with ompounds, only its could

be obtained for the reactions of the two other radicals. Th results indicated th ydrogen

a ron transfer took place for the very strong oxidant S that the

r f CO3

–

 and Cl not high enough to react with these phosphates and neither of

ese radicals had a strong propensity for hydrogen abstraction. The reactions of the two radicals, CO3

– 

lthough the sulfat d at a me all the c upper lim

e at both h

bstraction and elect reactions O4

–

, but

eduction potentials o 2

– were

th

and Cl2

–

, were too slow for this degradation mechanism to be of importance in the environment. 
572 Screening Tests for Candidate Fire Suppressants 

6.6 VOLATILITY SCREEN 

6.6.1 Extinguishant Volatility

For a chemical to be successful as a total flooding agent, it must evaporate and fill a space to the desired 

e for extinguishment. As determined by Pitts et al.86, and noted

 the fluid is below the ambient temperature, the dispensed 

Estimation of whether a given compound can achieve the required extinguishment concentration upon 

arithm of the ratio of the vapor pressure (p) at two 

2 1 mated from Equation 6-29, the Clausius-

/T

ombining the two equations for a boiling point, Tb (K), at a pressure, Pb, Equation 6-31 is obtained. 

routon’s constant, tc (J/K-mol), is a varia r a particular family of compound

mple, if tc were equal to 88 J/K-m y pounds, then the estimated vapor 

ressure at 25 °C for a compoun at 101 kPa (1 atm) is 42 kPa

apor pressure values, the maximum possible volume % were estimated. These values are listed in Table 

Equation 6-32 calculates values of Tb for 

temperatures in K, Pb in kPa, and tc in J/K-mol. The calculated estimates in Table 6−6 are for a Trouton’s

concentration within the time availabl

earlier in this chapter, if the boiling point of

fluid will flash and disperse rapidly. For fluids of higher boiling point, estimation of the capability to

flood a volume entailed two determinations: (1) the equilibrium vapor pressure at the ambient temperature 

in the fire compartment and (2) the rate of evaporation to achieve that vapor pressure. 

6.6.2 Extinguishant Evaporation Equilibrium 

discharge entailed a thermodynamic analysis.87 The log

different temperatures, T and T (in Kelvins), can be esti

Clapeyron equation, where R is the ideal gas constant and ∆Hvap is the heat of vaporization. This equation 

is based on the assumptions that (1) ∆Hvap is temperature independent and (2) the vapor is an ideal gas. 

The ratio of ∆Hvap/Tb, where Tb is the boiling point temperature, is equal to the molar entropy of

vaporization ∆Svap at the boiling point. For closely related compounds, ∆Svap at the boiling point is nearly 

constant (Equation 6-30). Thus, by selecting T1 as the boiling point (Tb) at p1 = 101 kPa (1 atm), one can 

estimate the vapor pressure at other temperatures. 

log10(p2/p1) = (∆Hvap (T2-T1)/2.303 RT2T1) (6-29)

∆Hvap b = ∆Svap ≈ tc (6-30)

C

T ble to be determined fo s.

 p2 = Pb10**(0.052) tc [1-(Tb/T2)] (6-31)

For exa

p

ol, which holds for man com

d with a boiling point of Tb = 50 °C . From

v

6–5. Similar tables can be prepared for other pressures. 

The maximum allowable boiling point (Tb) able to achieve a desired volume % of a chemical in air at a

given ambient temperature, T2, was then estimated.

constant of 88 J/K-mol. The results are shown graphically in Figure 6–34. 

 Tb = [1-(log(101* C/0.13 Pb)/0.052 tc)]T2 (6-32)
Volatility Screen 573

Table 6–5. Estimated ) as a Function of Bo

Point and Ambient Temperature. [Trouton’s Constant = 88 J/K-mol; P = 101 kPa] 

Equilibrium Vapor Pressure (Volume %

 87

iling

Ambient Temperature, °C 

Tb, °C -60 -50 -40 -30 -20 -10 0 10 20 25 30 40 50 

-60 100 

-50 61 100 

-40 37 62 100 

-30 23 39 64 100 

-20 14 24 40 65 100

-10 8 15 26 42 66 100

0 5 9 16 27 43 67 100

10 3 6 10 18 29 45 68 100

20 2 4 7 11 19 30 46 69 100

30 1 2 4 7 12 20 31 47 70 84 100 

40 1 1 3 5 8 13 21 33 s49 59 71 100

50 0 1 2 3 5 9 14 22 34 41 50 71 100

Table 6–6. tim d M imu Boil g Po t Th Can chi a G n V me rc

of Chemical in Air. 87

Es ate ax m in in at A eve ive olu Pe ent

Ambient Temperature, °C 

Volume % -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 

1 33 47 61 76 90 105 119 133 148 162 176 191

2 19 33 46 60 74 87 101 115 128 142 156 169 

3 11 24 37 51 64 77 91 104 117 130 144 157

4 5 18 31 44 57 70 83 96 109 122 135 148

5 0 13 26 39 52 65 77 90 103 116 129 142

Figure 6–34.

aximum

s

nction

Estimated 

M

Boiling Points 

That Can 

Achieve 

Selected Value

of Volume % in

Air as a Fu

of Ambient

Temperature. 87 

Top to Bottom: 

1 %, 3 %, 5 %,

7 %, 10 %, 15 %,

20 %

-30

160

170

180

190

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50

Ambient Temperature, °C 

 Boiling 

C

200

-20

-10

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

MaximumPoint, °
574 Screening Tests for Candidate Fire Suppressants 

6.6.3 Extinguishant Evaporation Rate

Estimation of evaporation rates is much more difficult than the calculation of equilibrium agent 

concentrations. The following is an overview of some of the factors that must be considered. 87

In the simplest case, a spherical droplet of fire suppressant agent of radius, r, is ejected into stagnant, 

completely unsaturated air at a velocity, U0. The rate of evaporation as a function of the droplet radius 

and the time for complete evaporation was of interest. It was assumed that the temperature of the droplet

was the same as its surroundings, and that the temperature at the surface of the droplet did not change as 

the droplet evaporated. While neither of these conditions is generally true, considering temperature 

changes was too complex for a simple analysis. The following uses cgs units. 

Fick’s first law for one-dimensional diffusion relates the flux (J in g/s-cm2

) of a vapor through a gaseous 

medium to the concentration gradient dc/dx, where c is the agent concentration (g/cm3

) and x is the 

direction of diffusion (Equation 6-33). 

dr

dc J = −D (6-33)

D, the diffusion coefficient of the vapor in cm2/sec, is a function of the agent and the ambient conditions. 

Kinetic theory shows that for ideal gases of density, ρ, of rigid spherical molecules having a molecular 

mass, m, and a diameter, σ, the diffusion coefficient is given by Equation 6-34, where k is the Boltzmann

constant and T is the absolute temperature.88

πσ ρ

π = 2

1 / 2 ( mkT)

8

3 D (6-34)

Setting the density ρ = Pm/kT, one obtains Equation 6-35, which shows that the diffusion coefficient is

proportional to T3/2

 and inversely proportional to the pressure P for this idealized gas composed of hardspheres. It is also inversely proportional to the square root of the molecular mass and to the square of the

molecular diameter. Real gases, of course, show significant deviations from this relationship, which may

be modified to take into account actual molecular interactions. 

1/ 2 2 1/ 2

3 / 2

Pm

(kT)

8

3 D

π σ = (6-35)

Continuing to use the hard-sphere model, one can relate the diffusion coefficient to the coefficient of

viscosity (η, g/cm-s), the heat capacity per molecule (cv, g-cm2

/s2-°C, = Cv/No, where No is Avogadro’s 

number), and the coefficient of thermal conductivity (λ, g-cm/s3

-°C) (Equations 6-36 and 6-37). 

v 25 c

12 m D

ρ

λ = (6-36)

ρ

η

5

6 D = (6-37)

Equation 6-33 was rewritten to give the diffusion mass transport through a spherical surface of radius r

(Equation 6-38). 
Volatility Screen 575

D

dr

dc 4 r

dt

dm 2 = − π (6-38)

o o ∞

Equation 6-38 was used to determine the mass transport from the liquid/gas interface surrounding a 

spherical drop of radius r with a concentration of c to a region far from the drop (r ) with a vapor

concentration c∞. Since dm/dt, the mass transport, is independent of r, integration of Equation 6-38 gives

Equation 6-39. If r∞ is very large and if c∞ is taken as zero, Equation 6-40 results. 

∫ ∫

∞ ∞

= − π

c

c

4 D dc

r

r r

dr

dt

dm

o o

2 (6-39)

dt (1/ r 1/ r ) o − ∞

dm c c 4 D o − ∞ = π (6-40)

), the rate of evaporation approaches zero.

If r4 is taken as sufficiently large, Equation 6-40 can be rewritten to give Equation 6-41. Note that as c∞

approaches co (the air becomes saturated

4 Dr (c c ) dt

dm = π o o − ∞ (6-41)

Equation 6-41 can be rewritten to give Equation 6-42. 

RT

4 rDM(p p )

dt

dm o π − ∞

= (6-42)

where M = 

su

molecular weight of the vapor, and po and p4 are the partial pressures of the vapor at the

rface of the droplet and at a distance infinitely far from the droplet. In the case of a fire supp

agent, it was assumed that p∞ is initially zero, since no agent has yet been released into the compartment. 

If c is zero, one obtains Equation 6-43.

ressant

∞

o o 4 Dr c

dt

dm = π (6-43)

Equation 6-43 shows that the transport rate increases with drop size and with the concentration of agent at

the liquid/vapor interface. 

Performing estimates using this equation posed two problems. First, one ust determine D for a s

(rather than for the hard-sphere idealized gas discussed earlier). Second, and more difficult, one must 

s was to

perform estimates based on available equations. Reference 89 lists several correlations for diffusivity. 

he lowest error was estimated to be from the Fuller-Chapm n-Giddings equation, Equation 6-44

m real ga

determine the concentration of agent at the interface. 

Since measured values of diffusivity were often not available, the only way to obtain value

T a .

∑ ∑+ 1 / 3 2

2

1/ 3

1 P[( v ) ( v ) ]

Here, T is the tem

+

=

0.5

1 2

1.75 ) M

1

M

1 0.1013T (

D 

perature of gases in K, M1 is the molecular weight of the gas diffusing into the air, M2 is 

the molecular weight of the air, P is the pressure of the gases, and v1 and v2 are atomic diffusion volumes

(6-44)
576 Screening Tests for Candidate Fire Suppressants 

which are given in tables. Unfortunately, atomic diffusion volumes are not available for the ma f

compounds and must be estimated for many of the compounds of interest in fire suppression. 

ion of the droplets also 

istorts the droplets; however, one usually assumes spherical droplets. 

The problem quickly escalates from a simple diffusion mechanism a heat and mass transf

echanism; it reduces to the calculation of the rate of evaporation or heat transfer from a spherical body

a moving gas stream (in the case of interest, the droplet is in motion within stagnant air, but the 

mathematics are the same). Solutions to such problems rely on the use of dimensionless numbers such as

t and mass transfer are 

radically different in laminar and turbulent flow. 

A simple correction to Equation 6-43 to allow for the increase in evaporation rate due to motion through 

w around the droplet is equal to 0.276, Equation 

jority o

The question of motionless droplets is academic at best. Droplets of fire suppressant chemicals will be 

rapidly expelled through the nozzle to ensure complete coverage of the protected area or the fire as well

as providing mechanical breakup to smaller droplet sizes. Any realistic evaluation of evaporation must

include inertia, gravity, and other forces acting on the droplets. The rapid accelerat

d

 to er 

m

in

Reynolds, Nusselt, Prandtl, and Schmidt numbers. The problem is complicated by the fact that flow may

be either laminar or turbulent, depending on the value of the Reynolds number; hea

air is where dm/dt equals the evaporation rate with flo

6-45. 

4r D(c c )(1 Sc Re ) I f

dt

dm o 1/ 3 1 = π o − ∞ + β =

or larger droplets in moving streams, f can be rather

rge, reflecting the higher heat and mass transfer rates at higher velocity. 

e clear that the estimation of evaporation rates was far beyond the capability

 fundamental properties, and literature values were 

pplier

easured the boiling point. 

For those instances where a measurement was needed, it was generally necessary that the meas t

use very little sample. Available techniques for determinations volving higher boiling compounds

92

/ 2 (6-45)

(measured for a sphere), Re is the Reynolds number characterizing the flow of the air around the drop, 

and Sc is the Schmidt number. The term f is usually called the wind or ventilation factor. It is reported 

that Equation 6-45 accurately represents the evaporation rate of droplets in moving air.90 For very small 

droplets falling in air, f is small, approaching 1; f

la

6.6.4 Boiling Temperature 

From this analysis, it becam

for screening candidate fire suppressants. Thus, the NGP utilized a compound's boiling point as the

screen for volatility. 

A chemical's boiling temperature is one of the most

available for many of the compounds under consideration by the NGP. Yang and coworkers reported on

the use of a software package to estimate boiling points.91 

For those compounds that were custom synthesized for the NGP (see Chapter 7), frequently the su

m

uremen

in

include a microscale method , based on determination of the temperature at which the pressure above the

boiling liquid is large enough to overcome its vapor pressure in an inverted capillary tube. This method 
Toxicity Screen 577

typically employs a melting point apparatus and melting point capillary tubes. Boiling point

determination methods applicable to low boiling compounds include the vapor pressure based

isoteniscope method.93 Both methods are applicable to very small samples. 

n an accidental discharge of a fire suppressant, higher 

nder the EPA SNAP Program,

4

 cardiac sensitization values helped define the use restrictions placed on

ical. For example, when using halon substitutes as total floodin ag e 

following conditions, which were adopted from an OSHA safety and health standard (29 CFR 1910 

n 30 s but less than 1 min, the

employer shall not use the agent in a concentration greater than its LOAEL.ix

reater than the LOAEL are only permitted in areas not normally

occupied by employees provided that any employee in the area can escape within 30 s. The

tory guidelines for conducting a cardiac sensitization study. However, the procedure 

most often followed for evaluating exposure to agents that might potentially evoke a cardiac sensitization 

ctrocardiographic changes indicative of the 

appearance of ventricular fibrillation. The doses were increased from 1 μg per kg of body weight toward 

. At 5 min into the exposure, the dog was injected with an epinephrine dose 

6.7 TOXICITY SCREEN 

6.7.1 Hazards and Regulation 

There are a variety of acute toxic effects that can result from exposure to a fire suppressant chemical. 

These include sensory irritation, incapacitation, or even death. The NGP was concerned with the full 

range of such effects. 

It has been recognized for decades that a principal toxic effect of halocarbon agents is cardiac

sensitization.94,vii This is the sudden onset of cardiac arrhythmia caused by a chemical sensitization of the

heart to epinephrine (adrenaline). Cardiac sensitization is a particularly important concern in fires 

because under the stress of the fire event, or eve

levels of epinephrine are secreted in the body, increasing the possibility of sensitization. Since the current 

firefighting halons and most replacement chemicals were halocarbons, most (but not all) of the NGP 

screening effort focused on this toxic effect. 

U

a replacement chem g ents, th

Subpart L), generally applied: 

• Where egress from a normally occupied area cannot be accomplished within 1 min, the

employer shall not use the agent in concentrations exceeding its NOAEL.viii

• Where egress from a normally occupied area takes longer tha

• Agent concentrations g

employer shall assure that no unprotected employees enter the area during agent discharge. 

There were no regula

response was the test performed using beagle dogs.95 In this procedure, an intravenous dose of 

epinephrine was administered to each dog to establish its cardiac response to an epinephrine challenge 

alone. The dogs were monitored continually for ele

12 μg/kg in steps of 0.1 ml/kg. After a recovery period, the dog was exposed to a selected concentration 

of the candidate suppressant

 

vii See section 6.7.8 for screening for other acute toxic effects. viii NOAEL, the "no observed acute exposure level," is the highest concentration at which no adverse toxicological or

physiological effect was observed. ix LOAEL, the "lowest observed acute exposure level, is the lowest concentration at which an adverse toxicological or 

physiological effect was observed. 
578 Screening Tests for Candidate Fire Suppressants 

just below the value that caused cardiac arrhythmia by itself. It was noted whether the dog experiences a 

response or not. Exposures at different suppressant concentrations were performed until the onset of an 

effect was bracketed to the desired precision. The results of this test are expressed in terms of the

LOAEL and the NOAEL. The separation of these values was a function of the step size in suppressant

ensitization test. The expense of the test often prevented a small

ased cardiac sensitization testing are very conservative and, perhaps 

luate the health effects of new agents. The progression of 

steps they endorsed was: 

ical tests, and 

The ste

extensive re

A proto

workers.96

reproductiv at

Hence, the NGP conducted an extensive review to identify methods to aid in the selection of compounds 

 While any

rts, it was

o

concentration used for the cardiac s

increment from being used. 

LOAEL data obtained from the dog-b

as a direct result, very rarely have people died as a result of exposure to the halons. The dog-based 

standard seems to be achieving exactly what it was designed to achieve – ensuring a wide margin of 

safety to a broad spectrum of end users. 

6.7.2 Screening Process 

The 1997 NGP Workshop on Fire Suppressant Compatibility with People, Materials, and the

Environment developed a preferred protocol for obtaining indicative data regarding the various toxic

possible effects of candidate agents. 2 The participants noted that there was no checklist to be used in a

"cookbook" fashion. Expert judgment was needed to interpret the results of each test and decide what, if

any, additional testing was necessary to eva

1. use published values, 

2. derive values from quantitative structure-activity relationships (QSARs), 

3. conduct non-animal biolog

4. conduct live animal tests. 

ps involve successively more time, cost, and agent. The report on the Workshop contains 

ferences to resources for performing the steps. 

col containing an even more comprehensive set of toxicity tests was developed by Dodd and coThis included tests for genotoxicity, subchronic toxicity, carcinogenicity, development and

e toxicity, among others. However, the NGP identified no compounds that progressed to th

level of testing. 

With regard to cardiotoxicity, performing the dog exposure tests was not feasible for the large number of 

chemicals under consideration by the NGP. Only a few laboratories in the world have the equipment and

the expertise. Completing the testing takes about two months. The testing cost per chemical is currently

approaching $100 k. The test requires about 30 moles of the test chemical, which generally would have 

needed to be custom-synthesized. Nonetheless, for the safety of personnel and for obtaining regulatory 

approval, estimating the cardiac sensitization LOAEL for a candidate fire suppressant was important. 

with a higher probability of acceptable LOAEL and NOAEL values when actually tested. 

estimation method enabling a ranking of compounds would facilitate compound acquisition effo

felt that the accuracy of the absolute LOAEL values needed to be within 2 volume % for the method to be 

f much use in final compound selection. 
Toxicity Screen 579

Halogenated compounds have found a variety of uses beyond fire suppression. Cardiac sensitization has 

surfaced as a consideration in the development of propellants for medical inhalers, it has been observed as

a cause of cardiac arrest in substance abuse by inhalation of solvents used in spray paints and hairsprays, 

and it has occurred in the use of some pharmaceuticals and surgical anesthetics. Thus, there was

significant background material to be searched for the current purpose. 

At the time of the completion of the NGP, no accurate screening procedure existed to obviate the need for

essing the toxic properties of a candidate fire suppressant was a 

compendium of health and safety or toxicological data on chemicals, such as Reference 97. A number of 

. 

of Medicine site called MEDLARS (http://toxnet.nlm.nih.gov). 

try of Toxic Effects 

B), the Integrated

 Toxicology (Mutagenicity) Database (GENEesearch Information System (CCRIS). 

• The National Library of Medicine (NLM) also had a number of other citation databases such 

ases could be accessed free via the Internet

idered to be potentially all known chemicals that can elicit abnormal biological effects. 

the standard dog exposure test. In fact, such testing of two otherwise promising chemicals generated

negative results (Chapter 7). Sections 6.7.3 through 6.7.6 summarize the NGP investigation process. 

6.7.3 Literature Values 

A useful starting place for ass

databases also existed, both computerized and non-computerized, which were helpful in determining 

known toxic properties of a chemical, a set of chemicals, or even a chemical class: 

• The Chemical Abstract Services (CAS) ONLINE (http://info.cas.org/) database was a payfor-services, informational compendium of thousands of substances, which included

information on chemical and physical properties, structures, synonyms, and in some cases,

toxicity values

• The National Library

MEDLARS contained a number of specific databanks including the Regis

of Chemical Substances (RTECS), Hazardous Substance Databank (HSD

Risk Information System (IRIS), the Genetic

TOX), and the Chemical Carcinogenesis R

as TOXLINE, MEDLINE, or CANCERLIT containing abstracted literature information on 

various aspects of toxicology. The NLM datab

(http://igm-01.nlm.nih.gov/index.html). 

• RTECS was a comprehensive online database developed by the National Institute of

Occupational Safety and Health (NIOSH) for the purpose of collecting, collating, and 

disseminating toxic effects information for “all known toxic substances,” which were

cons

• The Merck Index (http://www.dupontmerck.com) contained physical and toxicity data from 

published literature sources. The Index was free, but limited to only a select few single 

substances. 

• The Environmental Protection Agency’s Office of Pollution Prevention and Toxics holds the 

Toxic Release Inventory (http://www.epa.gov/opptintr/tri/), which was a valuable free source 

of information about toxic chemicals that were being used, manufactured, treated,

transported, or released into the environment. 

• The EPA’s GENE-TOX data bank compiled information on mutagenicity and carcinogenicity

for several thousand chemicals (found through the MEDLARS online site) and the

International Agency for Research on Cancer (IARC) (http://www.iarc.fr/pub/publist.htm)

published for-fee monographs reviewing the carcinogenic potential of certain chemicals. 
580 Screening Tests for Candidate Fire Suppressants 

• The National Technical Information Service (NTIS) covered government-sponsored research 

and development efforts and some work may contain toxicological information. Searches for 

reports could be performed free of charge, but the actual documents could only be obtained

for a fee (http://www.ntis.gov/search.htm). 

For screening purposes, the literature examination was directed at finding any severe toxic effects that

EL and LOAEL values for members of different

data enabled some preliminary insights: 

wide range of 

• m appeared to lead to a low LOAEL value. 

•

Table 6

occurred at exposures of up to concentrations of the order of 5 volume % for several minutes. If there 

was insufficient information regarding the specific chemical under consideration, chemically similar 

compounds were examined. 

Table 6–7, Table 6–8, and Table 6–9 list published NOA

families of halocarbons. These

• There were a wide range of values among and within the chemical families. 

• The limited data for partially fluorinated methanes (Table 6–7, compounds 6 and 7)

demonstrated a relatively low tendency to induce cardiac sensitization. 

• The three hydrofluoropropanes (Table 6–7, compounds 3 through 5) showed a 

LOAEL values with no obvious correlation with the degree of fluorination. There may be a

relationship with the location of the hydrogen atoms, both with this family and the 

hydrofluoroethanes (compounds 8 through 11). 

In Table 6–8, an iodine ato

Many CFCs and HCFCs, which are closely related to the halons, were cardiotoxins.

98

Table 6–9 gives little or no indication of relationships between degree of fluorination or

chlorination (or hydrogenation) and LOAEL values. While the highly fluorinated compound,

CF3CF2Cl, was notable for its unusually high LOAEL of 15 volume % most values ranged

from a low of 0.5 volume % to a more nominal high of 5 volume %.

–7. Cardiac Sensitization Values for Fluorocarbons and Hydrofluorocarbons. 

# Chemical Formula NOAEL (vol. %) LOAEL (vol. %) Reference 

1 CF3CF2CF2CF3 40 >40 99

2 CF3CF2CF3 30 >30 62

3 CF3CHFCHF2 2.5 3.5 100

4 CF3CHFCF3 9 10.5 99

5 CF3CH2CF3 10 15 99

6 CH2F2 20 25 61

7 CHF3 30 >50 99

8 CH2FCF3 4 8 61 

9 CH3CF3 4 8 62

10 CHF2CF3 7.5 10 99

11 CH3CF2H 5 15 95
Toxicity Screen 581

Table 6–8. Cardiac Sensitization Values for Br-, Cl-, and I-containing Alkanes. 

# Chemical Formula NOAEL (vol. %) LOAEL (vol. %) Reference 

1 CF3I 0.2 0.4 101

2 CF3Br 5.0 7.5 99 

3 CF2HBr 2 3.9 99 

4 CF2BrCl 0.5 1.0 102

5 CF2BrCF2Br - 0.1 103

6 CBrClFCBrF2 0.5 103

7 CF3CF2CF2I - 0.1 104

Table 6–9. Cardiac Sensitization Values for CFCs and HFCs. 

# Chemical Formula NOAEL (vol. %) LOAEL (vol. %) Reference 

1 CF2ClH 2.5 5 61 

2 CF2Cl2 2.5 5 95

3 CFCl3 - 0.5 105

4 CH2=CClH 2.5 5 95

5 CCl3CH3 0.25 0.5 95

6 CF2ClCH3 2.5 5 95 

7 CFCl2CH3 - 0.5 106

8 CF3CF2Cl - 15 95 

9 CF3CCl2H 1 2 61 

10 CFCl2CH3 - 1 106

11 CF2ClCF2Cl - 2.5 95

12 CClF2CFCl2 0.25 0.5 95

13 CF2ClCF2CFHCl - 2.0 107

6.7

Quan tive structu hips (QSAR based on scientifi gment by experienced

scientists, may be used as an integral part of health hazard characterization. This approach relies on the

toxicologist or chem to fit the new chem into a category of existing chemicals because of 

similarities in molecular structure or chemical functionality. In order for this approach to be of value, the

existing chemicals c lose structural analog st have its own robust toxicology database and 

the uncertainty of read-acr the close structural analogs or the category to the new chem must

be re nized and, i ble, defined.

Skaggs et al. perform tensive review of toxici eening approaches, identifying 

chem l attributes d in the development QSARs, predictor f relationships between

biological activity and chemical properties.108 The descriptor types included: 

• Hydrophobicity, a measure of water insolubility. Neutral, unreactive chemicals exhibit a 

strong relationship between the hydrophobicity of the compound and its LC50. 

Hydrophobicity is equated to the octanol-water partition coefficient and reflects the ability of 

a chemical to cross a biological membrane and correlates with toxicity because diffusion 

through cell walls allows chemicals to influence internal cellular processes and biochemistry. 

Both the Hammett Constant and the Taft Polar Substituent Constants are measures of

.4 Quantitative Structure Activity Relationships 

tita re-activity relations s), c jud

ist being able ical

ategory or

oss from

c mu

ical

cog f possi

ed an ex ty prediction and scr

ica employe of s o
582 Screening Tests for Candidate Fire Suppressants 

substituent polarity. Steric properties relate to molecular structure and affect the binding of

molecu es.

• Molecular connectivity descriptors describe the number of atoms in a compound and their 

formall d and spatial relationship olecular Connecti Indices (MCI) ciate

numeric with structure of the mo ule. Many propert re directly related to the

number nections of atoms in lecules.109 Molecular connectivity describes

structural information and relates to physical properties. 

• Linear Solvation Energy Relationship (LSER) analysis is based on the correlation of diverse 

chemical properties, including toxicity and solvent-solute interactions. The analysis is based

on four molecular parameters: intrinsic molecular volume, polarity, and two measures of 

abilit vides the most 

accura the wid ical c ters e

on only ount of chemicals.

• Molecu ectivity QSARs require that indices based on structure be determined, and

generall ount of data needed is less than that for q itative propert ivity 

relationships (QPAR). In both cases, sufficient toxicity data mus available with which to 

develop correlations capable of providing toxicity estimates of unknown substances. These 

data are le in the required abundance to perform highly liable assessments across 

a broad range of compounds. 

Skaggs et al. reported that the key to using any of these techniques was to determine a f y of 

chem s for which biological indices (for example, LD50), as well as physical/chemical properties,

molecular structure, or both, are known. 108

 In actual QSAR modeling, these of data are referred to as

“training sets.” By statistical techniques such as multiple regression analysis (MRA), a 

relati ip between ndices and prop molecular structure is determined based on the

“training set” data. The better or more extensive the set the more the predictions are apt to be

accurate.111 

Vinegar112 has extracted some general, but not universal, toxicity trends that have been used to provide a

ains are more toxic than branched. 

•

o Iodine presence is more potent than bromine presence. 

les to biological sit

y-bonde s. M vity asso

al values lec ies a

and con mo

y to hydrogen bond as an acceptor or donor. While this measure pro

te QSARs covering est range of chem

110

lasses, the parame are availabl

a finite am

lar conn

y the am uant y-act

t be

not availab re

amil

ical

sets

the use of

onsh the biological i erty/

training 

relative toxicity ranking for the candidate compounds and compound groups: 

• Toxicity Ranking Criteria 

o Aromatics are more toxic than aliphatics. 

o Asymmetric molecules are more toxic than symmetric molecules. 

o Straight (carbon) ch

o Ethers are more toxic than alkanes. 

o Short chain ethers are more toxic than long chain ethers. 

o Vinyl ethers are more toxic than saturated ethers. 

o Polyoxyethers are more toxic than monoethers. 

Cardiac Sensitization Ranking Criteria 

o Halogen presence is more potent than hydrogen. 

o Bromine presence is more potent than fluorine presence. 
Toxicity Screen 583

QSAR ana

mutagenicit y be useful with appropriate recognition of the limitations of 

these progra

subject o a

structural an

NCP Chem

precedents in order to identify areas of health

hazard c c

the detail d

Multiple inv

in Table 6–

Table 1 2

lysis has been formalized and computerized for some health endpoints (e.g., cancer,

y, and teratogenicity) and ma

ms. Evaluation of the computerized QSAR programs available commercially has been the

f review.

113 The U.S. EPA has grouped chemical substances with similar physical-chemical,

d toxicological properties into working categories. These groupings enable the user of the 

ical Categories guidance document to benefit from the accumulated data and decisional 

within EPA’s new chemicals review process since 1987,

on ern. There were 51 chemical categories listed in the table of contents of the EPA document,

e summaries of which may be found at: http://www.epa.gov/opptintr/newchms/chemcat.htm. 

estigators have developed numerous QSARs to predict the toxicity of chemicals, as indicated 

10, taken from Reference 2. 

6– 0. Availability of QSARs for Various Chemical Classes and Toxic Endpoints.

Chemical Class Functional Group Possible Toxicity Endpoints References 

Alcohols -C-OH Irritation 114, 115, 116, 117

Aldehydes a

Acetals 

nd > C = O Irritation, Sensitization, Anesthesia, 

Mutagencity and Carcinogenicity 

114, 118

Allyl compo

rotoxicity, Sensitization, Carcinogencity 

unds H2C=CH-R Liver Toxicity, Kidney Toxicity, 

Neu

119

Amides -CONH2 Irritation, and liver, kidney and brain toxicity 120, 121, 122, 123, 124, 

125, 126, 127, 128

Amines Cholinesterase Inhibition, Carcinogenicity 114, 129, 130, 131, 132, 

133, 134, 135, 136

Azides -N3 Cardiovascular actions and enzyme inhibition 137

Bromides Inorganic and 

organic bromides 

Neurotoxicity, Irritation, Hepatotoxicity, 

Cardiotoxicity 

138, 139, 142 

Chlorinated 

Hydrocarbons

Cl-C-R Carcinogencity, Anesthesia, Hepatotoxicity 140, 141, 142, 143, 144, 

145, 146, 147, 148

Epoxy Compounds Irritation, Mutagenicity, Neurotoxicity, 

Hepatotoxicity, Kidney Toxicity, 

119,

Embryotoxicity

149, 150, 151, 152, 

Esters RCOOR' Asphyxiants, Narcotics, Irritants 138, 153, 154, 155, 156

Ethers , Irritation, 157, 139, 158, 159, 160, 

161

R-C-O-R' Anesthesia, Cardiotoxicity

Carcinogenicity 

Ketones 18, 123, 155, 162, 

163, 164

RCOR' Anesthesia 114, 1

Nitro, Nitrate, an

Nitrite diotoxicity, Mutagenicity, 

d -NOx Irritation, Neuromuscular Dysfuction, 139, 165, 166

Hepatotoxicity, Car

Carcinogenicity 

Nitroso compounds C-N=O or N-N=O Carcinogenicity, Mutagenicity, Teratogenicity 167

Phosphorus 

compounds 

on, GI 

icity, Kidney

genicity, 

xicity

Anesthesia, Cholinesterase Inhibiti 129, 153, 168

Dysfunction, Irritation, Neurotox

Toxicity, Hepatotoxicity, Terato

Reproductive To

Sulfur Compounds Irritation, Corrosivity, Cardiotoxicity, 138, 124, 126, 139, 128

Neurotoxicity 
584 Screening Tests for Candidate Fire Suppressants 

Because the predictive capabilities of QSARs are based on the quality of the data used in their 

development and an understanding of the mechanisms of toxicity, these techniques can sometimes be

unreliable and should not be the sole criterion for screening decisions. The basic principles behind QSAR 

technology are well documented and can apply to any class of chemical and any measurable end point.169

Several texts describe the basics of QSAR techniques,170 but briefly, QSARs involve taking a sufficiently

large, consistent data set and correlating it with known descriptors of the chemicals within the data set.

The correlation techniques are principally multivariate regression and discriminant analysis for weighting 

descriptors.171 This leads to a mathematical expression, whereby endpoint data can be predicted for

chemicals with known descriptors. For example, one can use a data set of lethal concentration values

of halogens, vapor pressures, and/or octanol-water partition

coefficients. The resulting mathematical equation would allow one to calculate the lethal concentration 

f r

ch f ave be r clean fire su ,173

Te

The Case for Octanol-Water Parti

There is some evidence to support a relationship between the tendency of a c ardiac

tion and its relative solubility eve

is not yet fully understood.

ported tha chers had concluded that cardiac sensitization is a direct result of a

physical-chemical i ent with heart cell membrane structures.175 This

ypothe derived f c

ar iac sensitization response and the ely r t

cause death, oved. Below that critical

concentration, the effect does not occu

nd Tinston had reported a possi g poten

some non-halogenated chemicals with chemical properties.176 Based on the data in Table 

ey suggest the effec a chemical (reactivity based) a

generalized physical property effect that just happened to cause cardiac arrh hen sufficient 

d tant fractio absorbed into heart nerve and 

cells membranes. For the teste % of chemical that has

e such as

less so ble in a polar fl d from the

(e.g., LC50)

x

 for halogenated hydrocarbons and correlate these values with certain physico-chemical 

descriptors such as numbers or type

or a new halogenated hydrocarbon knowing only the physico-chemical descriptors for that particula

emical. Several o these approaches h en employed in the search fo ppressants.

172

6.7.5 Non-animal Laboratory sting

tion Coefficients 

hemical toward c

sensitiza in polar and non-polar liquids.174 How r, the actual mechanism

Frazier re t some resear

nteraction of the sensitizing ag

mechanistic h

needed to elici

sis was rom the observation that a critical blood oncentration of agent is

t a c eversible, if they a

sufficiently severe

d

to

effects are immediat

when the sensitizing agent is rem

r.

re no

Clark a ble correlation of cardiac sensitizin

selected physio

tials of halogenated and

6–11, th ed that t was not toxicity but instead

ythmia w

chemical “occupie

muscle

a cons n of he critical biophase,” i.e., was

d chemicals in the table, the EC50 (volume

an effect, E, on half of the test ani als) and vapor p

hypothesis suggest

m

d that if a chemi

ressure at 37 °C data appear to correlate. This

cal were preferentially soluble in a barely polar liquid,

octanol, and to be flushe

tissue environment. 

lu uid, such as water, it would be less likely

x

 LC50 (or similar terms indicating various toxi ts) denotes the concentration of a toxic gas statistically

calculated from concentration-response data to produce lethality in 50 % of the test animals within a specified exposure and

post-exposure time

cological endpoin
Toxicity Screen 585

Tanner et al.177 provided support for this polarity effect. Using differential scanning calorimetry, they

showed that the inhalation anesthetic agent halothane (CF3CHClBr) destabilized some proteins in aqueous 

solution with respect to thermal unfolding, presumably due to halothane binding to the native folded state.

Low (millimolar) concentrations of halothane produced significant destabilization of proteins, and they

concluded that destabilization of proteins by halothane could be attributed to mainly hydrophobic 

of current and former anesthesia agents to induce cardiac arrhythmia as well as comparison molecular

y ls for agent uptake to target tissues. Reviews covering cell

rane ion gate channel effects an s of molecular structure were found in studies of 

179, 180 relate uptake of anesthesia agents

ing) of the anesthesia agent based on relative 

interactions of halothane with the proteins. 

There was further information on the phenomenon of cardiac sensitization from the fields of surgical 

anesthesia agents and pharmaceutical development. While the published literature was abundant in the 

former area, not surprisingly, little was found describing the approaches and methods employed by major

pharmaceutical companies in their drug development research related to avoiding cardiac sensitizing or 

arrhythmia inducing drugs. 

Anesthesia was an active area of research, with considerable published data on the influence of various 

compounds on human systems.178 These studies included data on the relative tendencies of a wide range

propert data, toxicity information, and mode

memb d the effect

pharmacokinetics and molecular physiology. The data presented

and physical properties, such as blood gas ratios (partition

solubility in the two media. 

Table 6–11. Air Concentration Inducing Cardiac Sensitization in 50 % of Animals. 176

Volume % at Partial Pressure Vapor Pressure at Relative Saturation for

Compound EC50 at EC50, kPa 37 °C, kPa Cardiac Sensitization 

CCl4 0.53 0.53 25 0.02

CH3CCl3 0.75 0.76 28 0.03

CCl2FCF2Cl 1.0 1.0 70 0.02

CFCl3 1.3 1.3 158 0.02

CF3CHClBr 2.0 2.0 64 0.01 

CH2Cl2 2.4 2.4 88 0.03 

CHFCl2 2.5 2.5 273 0.03

CH2=CHCl 5.1 5.1 560 0.01

CF2Cl2 8.1 8.2 900 0.01

CF2ClCF2Cl 10.0 10.1 307 0.01

CH3CH2CH3 20 20 1270 0.03

CF3Br 20 20 2030 0.01

CF3Cl 80 81 5410 0.01

Evidence for some of the effects of general (volatile) anesthetics on membranes and membrane ion

channels were described in terms of a protein binding mechanism.181 However, this interpretation did not

d S) of

is

ein sites, yielding marked differences in the relative abilities of the R and S forms of

account for the lack of demonstrated differential anesthetic effects of the two sterioisomers (R an

oflurane. The R and S sterioisomers of isoflurane would be predicted to bind quite differently to

stereospecific prot
586 Screening Tests for Candidate Fire Suppressants 

isoflurane to induce anesthesia. Investigation of the effects of volatile anesthesia agent on voltage gated 

ion channels was also ongoing and provided evidence for their inhibition of voltage-gated ion channels 

and to a greater extent on ligand-gated ion channels. 181

 These same researchers reported that “we can

summarize these findings with the generalization that volatile anesthetics had high-efficacy on ligandgated ion channels, whereas they had relatively low potency and high efficacy effects on voltage-gated 

ion channels.” While these observations did not conclusively or perhaps significantly further the

identification of one particular mode of interaction they related to the search for cardiac sensitization

inated and chlorinated hydrofluoroalkanes and ethers. Some are chlorinated

fluorocarbons. Of potential use were studies of the minimum alveolar concentration (MAC) values 

cients have also been used in QSAR analysis and rational drug design as a 

measure of molecular hydrophobicity and are key parameters in studies of the environmental fate of 

che ug

absorption, bioavailabi bic nt sm d

toxic

Given the compositional an lecular simil between anesthesia agents and many he current

halon replacement compounds of interest as the reports reviewed above linkin ompounds 

hy toxicity, cardiac arrhythmia, protein interaction effects measures of h obicity

seemed a clear strong choice for further evaluation and study as potential screening methods for cardiac 

sensitization. The possibility near term scr ng method based the comparative h hobicities

of the candidate compounds seemed worthy of further investigation.

OW) and Arrh mia Propertie Selected Anest s.

184,185 

screening methods, they did provide insight to both the status of current investigations into the interaction 

of volatile halocarbons and cell membranes. 

Anesthesia agents are not unlike halons in elemental composition. Anesthesia agents are represented by

lower molecular weight brom

determined for anesthesia agents181,182 and clinical observations of the tendency of volatile anesthetics to 

induce cardiac arrhythmia or premature ventricular contractions (PVCs).183 

Table 6–12 combines these physiological effects with the hypothesis of Clark and Tinston. These data 

indicated a potential correlation between the tendency of a compound to induce PVCs and its log(KOW) 

value. 

Octanol-water partition coeffi

micals. This was because the degree of hydrophobicity of a compound was known to affect its dr

lity, hydropho drug-receptor i eractions, metaboli of a compound, an

ity. 

d mo arities of t

well as g a c

drophobicity to ydroph

of a eeni on ydrop

Table 6–12. Partitioning (K yth s of hetic

Property Halothane Enflurane Isoflurane Desflurane Sevoflurane 

Blood-gas Ratioa 2.5 1.9 1.4 0.42 0.6

Oil-water Ratiob 220 120 170 19 55

Arrhythmiac +++ + + ~ ~ 

MACd 0.74 % 1.68 % 1.15 % 6.3 % 2.0 %

a the anesthesia agent con ation in the blood over the anesthesia age ntration in the gas ph igh values

indicate more effective transport of the chemical throughout the body and thus to cardiac tissue. 

The "oil" phase is commonly, but not exclusively, octanol. 

Ratio of centr nt conce ase. H

b

c The symbols "+++," "+," and "~" reflect, in order, a decreasing tendency of the anesthetic to induce PVCs in human patients 

during anesthesia. The last of these indicates only a slight tendency. Halothane is notable for its greater tendency to induce 

PVCs than the other anesthesia agents listed. 

d Minimum Alveolar (air) Concentration (MAC) for an anesthesia agent is a measure of the anesthetic potency of the compound, 

i.e., the air concentration required to immobilize 50 % of patients such that they don't give a motor response to a pain stimulus. 
Toxicity Screen 587

Measurement of KOW

Experimental determination of the octanol-water partition coefficient, KOW, using reverse phase high 

pressure liquid chromatography, is rapid and requires only a few mg of the compound. 174

 The measured

property is the equilibrium constant for the concentrations of the chemical in the two liquids. Referencing 

the retention times on the chromatographic column of compounds for which KOW values are known 

establishes a basis for obtaining partition coefficients for candidate fire suppressants. 

Calculation of KOW

Values of KOW were estimated from the contributions of molecular fragment-based terms plus correction 

factors.186 Different algorithms can give different results, so experience was needed to select or develop 

an appropriate version for the particular chemicals under consideration. Two of the most commonly used

algorithms were from Molinspiration Cheminformatics (http://www.molinspiration.com) and Syracuse

Research Corporation (http://esc.syrres.com). 

Octanol-water partition coefficients were calculated for the difluorinated bromopropenes using these two 

molecular fragment contribution-based methods and plotted in Figure 6−35. For this family of

compounds, the two methods were in good agreement. Interestingly, the highest log(KOW) value 

calculated for this series was for CH =CHCF Br, which proved highly toxic in inhalation toxicity tests.187

e

tw d

2 2

Difluorinated Bromopropene Log(KOW)calc.

0

CH

CHF=CFC

CHF=

CFBr=CF

CHBr=C

pounds

0.5

1

1.5

2

2.5

3

Log(K

O

W)

Figure 6–35. Comparison of Two 

Difluorobromopropene Log(KOW) 

Calculation Methods. 174 

CH2=

B

CH

r=

CF2Br

CFH2

H2Br

H2

CH3

CH3

FH2

CF

CBrCF

CBr

CF2=

FC

Com

Molinspiration

Syrres

Octanol-water partition coefficients were also calculated for the trifluorinated bromopropenes using th

o methods (Figure 6–36). The plotted data points (line connecting data points from same metho

calculation) were also roughly comparable. 
588 Screening Tests for Candidate Fire Suppressants 

Predicted Log(KOW) values

1.5

2

2

F

2

3

2

H

3

H

2

2

g(K

O

3

2

2.5

W) Lo

Molinspiration

0

0.5

1 Syrres

CH2=CFCBrF

CFH=CHCBrF

CFH=CFCBrH

CF2=CFCBrH

CH2=CBrCF

CF2=CBrCFH

CHF=CBrCF2

CHBr=CHCF

CHBr=CFCF2

CBrF=CHCHF

CFBr=CFCFH

Compound

Figure 6–36. Comparison of Two Trifluorobromopropene Log(KOW) Calculation 

Methods. 174

edicted values for the two calculation methods can

OW of Selected 

An indication of the disparity as well as trends in the pr

be seen in Table 6–13. 

Table 6–13. Comparison of Calculated and Measured Values of Log K

Halocarbons. 174

Compound 

Log KOW * 

(calculated)

Log KOW ** 

(calculated)

Log KOW

(measured) 

LOAEL 

(volume %)

CF3Br (halon 1301) 1.83 1.59 1.86 7.4 

CHBrF2 1.85 0.98 NA 3.9 

CH2=CBrCF3 2.14 2.49 NA 1.0 

CF2ClBr (halon 1211) 2.17 1.90 2.1 1.0 

CF3I 2.22 2.01 NA 0.4 

C3HF7 2.35 10.5 

* Molinspiration Cheminformatics algorithm

** Syracuse Research Corporation algorithm 

is probable that the use of calculated partition coefficient values to develop predictions of trends in

OAEL performance works best when only applied to narrowly defined groups of compounds (for

Another potential shortcoming of the available calculations is that they do not recognize or differentiate 

between the cis and trans structural isomers of compounds, such as the 1-bromo-3,3,3-trifluoropropenes 

It

L

instance, bromofluoroalkenes) and even then only used as a tentative predictor of LOAEL value. This 

limitation is demonstrated by the last line of Table 6–13. The calculated log KOW value for C3HF7

suggests a LOAEL lower than that any of the other compounds, which is clearly not the case. 
Toxicity Screen 589

and the 3-bromo-1,3,3-trifluoropropenes. There were no experimental data to establish the actual 

differences between such isomers. 

6.7.6 In Vitro Tests 

A wide range of in vitro methods has been developed for specific toxic endpoints, including acute

lethality, mutagenicity, anesthesia, kidney toxicity, hepatotoxicity, and teratogenicity.2

 These methods 

commonly use perfused organ preparations, isolated tissue preparations, single cell suspensions, or tissue

culture systems. Upon exposure to the test chemical, the specific response can vary from one species to

the next. Thus, preparations should be derived from species that respond to the chemical challenge in

ways which are similar to humans. Furthermore, caution should always be exercised when utilizing in

vitro techniques during the screening process because many factors, such as absorption and transport 

processes, influence the toxicity of a chemical. Thus, it was not deemed wise to rule out further

consideration of a candidate based exclusively on the results of a single in vitro test. 

The development of an in vitro cardiac sensitization screening method requires the development and 

validation of a system that retains the essential components of the process of cardiac sensitization. 

Identified requirements175 for an in vitro system include a cardiomyocyte system that: 

1. exhibits synchronized and spontaneous contraction, 

3. exhibits sensitivity to known cardiac sensitizers, and 

d the ability to accurately rank compounds whose LOAELs differ by as little as a 

few percent. 

Frazier has reviewed a number of studies in stems.175 Most of these w med

to asses certain drugs on cardiac functions or to deter fic of c ion

itself. T egorized in Table 6–14. 

Table 6–14. In Vitro C c Cell System

2. is responsive to exogenous epinephrine, 

4. allows exposure of sensitizing agents to system in a controlled atmosphere. 

To this list must be adde

volving cardiac cell sy ere perfor

s the effects of rdiac func

hese are cat

mine speci a t

ardia s.

Model Life stage Species Reference

Myocardial cells Fetal Human 188

Cardiomyocytes Neonatal Rat 189

Ventricular Cardiomyocytes Neonatal Rat 190

Ventricular Cardiomyocytes Neonatal Dog 191

Cardiomyocytes Adult Rat 192

Cardiomyocytes Adult Rabbit 193

Cardiomyocytes Adult Dog 194

Cardiomyocytes Adult Feline 195

Ventricular Cardiomyocytes Adult Guinea Pig 196

Ventricular Cardiomyocytes Adult Rat 197

Skaggs et al. 108 concluded that, despite the variety of in vitro systems available to assess the cardiac

toxicity of certain agents, any model selected would have to be characterized with respect to the
590 Screening Tests for Candidate Fire Suppressants 

expression and response of adrenergic receptors and the ion fluxes (sodium and calcium) before cardiac 

sensitization validation studies could be initiated. At the time of completion of the NGP, there was no in 

vitro method that was established as predictive of the standard dog exposure test for cardiac sensitization. 

6.7.7 PBPK Modeling of Cardiotoxic Effects 

Figure 6–37 shows a hypothetical time dependence of a toxic response to a short exposure to a discharged

tration of the agent

at the b transport time to that site, a process that

could involve pates, the occupant breathes a lower concentration. 

Thus, th v ncentration and time) reaching the sensitive site can

be far lo r oncentration overt the exposure time). 

Time Dependence of a 

Response to

sure to a

pressant.

drawing) 

T

The Time-dependence of Toxic Hazard 

Should a suppression system discharge into an area that is either occupied intentionally (such as a ground 

vehicle crew compartment) or unintentionally (such as during maintenance of an engine nacelle), people 

could be exposed to a high concentration of the agent for a relatively short time. The methods presented 

above involve a steady exposure for a long enough time period to see an effect. This masks the kinetic 

effects of a short exposure.

fire suppressant. The concentration in the local atmosphere rises sharply following the discharge.

Depending on the airtightness of the surroundings, the agent can dissipate quickly (as in the engine 

nacelle) or moderately (as in the crew compartment). There is a time lag in the concen

ody site where the toxic effect will occur. This is due to

 multiple steps. As the suppressant dissi

e e entual dose (the integral of local agent co

we than the exposure (the integral of the environmental c

Figure 6–37. Hypothetical 

Toxic

Expo Fire

Sup (NIST 

he PBPK Model 

Under sponsorship of the EPA and the NGP, a team of toxicologists at the Air Force Research Laboratory

(AFRL) modified a physiologically-based pharmacokinetic (PBPK) model of a human to include time-
Toxicity Screen 591

dependent representation of the cardiovascular system and lungs that will allow simulation of the outcome

of short-term exposures to fire suppressant chemicals.xi This was an important improvement over

commonly used PBPK models that used steady-state descriptions to describe uptake of chemical via the

lung and subsequent transfer of chemical to arterial blood. The principal features of the model

198,199,200,201

a (possibly time-varying) exposure concentration 

following inhalation. The PBPK model is a mathematical

systems has been well represented in the scientific

this model were liver, fat, lung, gut, and slowly perfused and rapid rfused tissues. All model

sumed to occur in the liver. 

using beagle dogs, as described above. 

above physiological levels, and the LOAEL value was used

That the effect was due to the peak concentration of chemic the

curve has been established by Reinhardt et al. 95 

• Arterial concentration measured after five minutes of expo OAEL

concentration. Obtaining this information required that stu ne with beagles that

have been cannulated for the sampling of arterial blood. At least six beagles should be 

exposed, without epinephrine challenge, at three exposure concentrations for 10 min at each 

exposure. The concentrations should be the LOAEL, approximately 25 % above the LOAEL, 

and approximately 25 % below the LOAEL. Blood samples should be taken at least at 

(1, 2, 5, 7, and 10) min to check the reliability of the data obtained at 5 min. 

• Partition coefficients of the chemical in blood, liver, fat, and muscle. The most sensitive 

parameter for determination of the correct blood concentration was the blood partition 

coefficient, and there could be as much as a two-fold difference between human and rat 

herefore, since the model was being used to simulate human exposures, AFRL 

rtition coefficients. For the other partition coefficients, where much 

are summarized here. 

A PBPK model provides the quantitative link between 

and the blood levels of the chemical achieved

description of the uptake, distribution, metabolism, and elimination of a chemical in the species of

interest. The time required for an arterial concentration to reach the LOAEL target arterial concentration 

is considered the safe (i.e., protective) exposure duration. The physiological compartments that compose 

the model are based on appropriate physiological and anatomical properties for the species of interest as

well as the chemical specific properties of the test compound. The use of PBPK models for kinetic

description of chemical interaction with biological 

literature and is widely accepted by the scientific community as a tool for risk assessment. 

The human PBPK model used in this work198

 differed from the more traditional PBPK model in that it

included a respiratory-tract compartment containing a dead-space region and a pulmonary exchange area.

It was used successfully to simulate the pharmacokinetics of halothane, isoflurane, and desflurane which 

are structurally similar to many of the chemicals being considered as halon replacements. The pulmonary

exchange area had its own air space, tissue, and capillary subregions. Respiration was described on a 

breath-by-breath basis. This more detailed lung description was necessary to successfully simulate

pharmacokinetic data in the 0 min to 1 min range. 198

 Additional physiological compartments described in

ly pe

compartments were perfusion limited and metabolism, if present, was as

The model required a considerable amount of input data: 

• Cardiac sensitization LOAEL. This was determined 

The LOAEL thus determined was conservative in that the epinephrine challenge was well

without any correction factors.

al rather than the area under

sure of beagles to the L

dies be do

blood. T

measured human blood pa

 

xi This model addresses accidental exposure to the suppressant itself, such as might occur following an accidental discharge. It

does not include the contribution of any decomposition products that are formed during and following flame extinguishment. 
592 Screening Tests for Candidate Fire Suppressants 

less difference was expected, rat tissue partition coefficient data were used when human data 

were not available. Details on obtaining these data are presented in reference 201. 

• Metabolic constants. These were obtained using a gas uptake method using rats, as described

by Gargas et al.202 Fortunately, most of the proposed halon replacements were relative

inert and h

ly

ad extremely low to no measurable metabolism. Furthermore, even with moderate

metabolism, there was no noticeable effect on the outcome of short-term simulations required 

ions

of the suppressant in air. The simulation lines represent the results of 1000 Monte Carlo simulations. The

al blood concentration measured at 5 min in a

group o

concentratio

halon 1301

HFC-125. S

Table 1

for modeling of cardiac sensitization. 

• Human blood flows and tissue volumes. There is significant variation in these variables 

among people. Therefore, Monte Carlo simulations were performed as part of the simulation 

of the concentration of an inhaled fire suppressant in the bloodstream. The ranges of the 

variables were compiled in Tables 1 and 2 of Reference 201. The Monte Carlo simulations 

were performed using ACSL TOX simulation software (Pharsight Corp., Mountain View,

CA) operating under Windows 95 and Windows NT (Microsoft Corp., Redmond, WA). 

Simulations of Exposure to Constant Concentration Environments 

The combined application of cardiac sensitization data and physiologically based modeling provided a 

quantitative approach, which could facilitate the selection and effective use of halon replacement

candidates. The AFRL team exercised the model, performing the first simulations of short term (0 min to

5 min) effects. They used these results to determine short-term safe exposure times for halon 1301 and 

four potential replacement chemicals CF3I (trifluoroiodomethane), HFC-125 (pentafluoroethane), HFC227ea (1,1,1,2,3,3,3-heptafluoropropane), and HFC-236fa (1,1,1,3,3,3-hexafluoropropane). 201

 

Figure 6–38 and Figure 6–39 show the result of Monte Carlo simulations of the concentration of halon

1301 and HFC-125, respectively, in human blood during exposure to six different constant concentrat

horizontal line at 25.7 mg/L represents the lowest arteri

f 6 dogs exposed to halon 1301 at the cardiac sensitization LOAEL of 7.5 %. For the six 

ns, Table 6–15 shows the safe exposure times, the time at which the blood concentration of

reached the LOAEL for the most sensitive dog. Table 6–15 show such estimates for

imilar curves and tables were generated for the other four fire suppressants. 

6– 5. Time for Safe Human Exposure at Stated Concentrations for Halon 1301 and 

HFC-125.

199

Halon 1301 Concentration (mole %) Time (min) HFC-125 Concentration (mole %)) Time (min) 

5.5 5.00 11.5 5.00

6.0 5.00 12.0 1.13 

6.5 1.33 12.5 0.73 

7.0 0.59 13.0 0.59

7.5 0.42 13.5 0.50

8.0 0.35 11.5 5.00
Toxicity Screen 593

Figure 6–38. Monte Carlo Simulations of Humans Exposed to Halon 1301. 199 

Figure 6–39. Monte Carlo Simulations of Humans Exposed to HFC-125. 199

Halon 1301

TERI

0 0.0 1.0 2.0 3.0 4.0 5.0

MINUTES

A

R

15

AL BLOOD CON

C

20.0 25.0

5.5%

6.0%

6.5%

ENTRATI

ON (

30.0

7.0%

7.5%

8.0%

m

CONC

g/L)

35.0

EXPOSURE

. 0.0 1.0 3.0 5.0

INUTES

30.0 40.0 

ARTERIAL OO

HFC-125

2.0 4.0

M

B

L D CO

 50.0 60.0

NCE

N

TRATION (m

g/L)

11.0%

11.5%

12.0%

12.5%

13.0%

13.5%

EXPOSURE

CONC
594 Screening Tests for Candidate Fire Suppressants 

The importance of this to approval of a fire suppressant is significant. The specifications for safe 

exposure design of halon 1301 and potential replacement chemicals for use as total flooding agents in a 

fire-fighting scenario (section 6.7.1) do not take into consideration the pharmacokinetics of the agents and 

the relationship between the exposure concentration and the internal concentration actually associated 

with a cardiac event. 

The critical issue in applying any standard for safe exposure is the relationship between the effective 

concentration of agent necessary to extinguish a fire and the concentration that poses a potential threat for

cardiac sensitization. Where the extinguishing concentration approaches the cardiac sensitization

concentration, the agent may still be approved for use as a fire extinguishant but only in areas that are 

normally unoccupied. For candidate agents where the extinguishing concentration is above the cardiac 

sensitization concentration, there is reluctance by some to risk using the agent because of perceived 

potential for accidental exposure. An additional consideration is that many of these agents release halogen

acids upon contact with heat. The toxicity of these acids is of concern. Levels of release can be reduced if

the fire can be put out quickly, which can be done if higher concentrations of extinguishing agent are

used. 

A comparison was made between the concentration for safe 5 min exposure and the recommended design 

concentration of agent for extinguishing a fire. As can be seen in Table 6–15, exposure to

HFC-125 can occur safely for at least 5 min at an exposure concentration of 11.5 volume %, which is 

bove the LOAEL by 1.5 volume %. The recommended design concentration for fire extinguishment is 

10.4 vo for 5

in in an HFC-125 environment that would put out a fire. From Table 6–15, the safe 5 min exposure for

alon 130 The design 

is

not safe to stay

history

ultimately ining safe exposure 

time. 

After a range

e animal testing 

methods. 

These s as fire

suppressants. periods of time 

at concentrati

people occupy dogs has 

been applied for humans.

the biological 

 be constant 

regardless of 

a

lume % to 11.3 volume %.

99 Thus, the PBPK simulation suggested that it was safe to stay

m

h 1 occurred at 5.5 volume % which was below the LOAEL by 2.0 volume %.

concentration for fire extinguishment was 5 volume %.

203 Thus, the PBPK simulation suggested that it 

 for 5 min in a halon 1301 environment that would put out a fire. Halon 1301 has a long 

 of safe use, which points to the conservative nature of the cardiac sensitization endpoint, which 

 determines the target arterial concentration, which in turn is used for determ

 of chemical concentrations has been evaluated using the cardiac sensitization protocol, the

dose-response data can be used to establish the NOAEL and LOAEL. Utilization of th

data for assessing potential risks to humans requires development of appropriate risk assessment 

methods must address the unusual exposure circumstances involved in the use of chemical

 In particular, potential exposures would be for relatively brief, but varying, 

ons high enough to effectively extinguish fires. Egress times need to be established for 

ing a facility at the time of chemical agent discharge. The LOAEL determined in 

directly by the EPA in evaluating acceptable use and allowable exposure limits 

Establishing egress times from the 5 min exposure LOAEL requires careful consideration of the

relationship between exposure concentration, duration, and the temporal aspects of 

response. Under steady state conditions, the concentration in blood and tissues would

the exposure duration. 
Toxicity Screen 595

Simulations of Exposure to Time-varying Concentration Environments 

AFRL demonstrated how to use the method to evaluate the potential for the occurrence of a cardiacsensitizing event for time-varying concentrations. 199

 The examples chosen were exposures to halon 1301 

and halon 1211 in three aircraft for which concentration measurements had been reported. These data

were used with the PBPK model to see if any of the described scenarios could put passengers or crew at

2. A Cessna Model C-421B, a small pressurized aircraft. Halon 1211 and halon 1301 were

measured using factory field-modified Beckman Model 865 infrared gas analyzers.

3. A Cessna Model 210C, a small non-pressurized aircraft.206 Halon 1211 and halon 1301 were

measured using modified Beckman Model 865 infrared gas analyzers. Measurements were

ists. Several scenarios illustrated above showed situations where 

arterial concentrations of halon 1211 exceeded the target associated with a potential for cardiac 

sensitization. The report of an incident by Lerman et al.207 demonstrated that the risk is real if halon 1211

were used under confined situations where the exposure concentration got high enough to result in highly

elevated blood concentrations. 

potential risk for cardiac sensitization. 

1. A fleet-configured Navy E-2B “Hawkeye” airplane.204 Using a Statham Laboratories Model

GA-2A gas recorder, concentrations of halon 1301 were measured at head level in areas 

occupied by flight crewmembers and in airplane locations where fire hazard potential posed

the greatest threat. Under these conditions at floor level in the equipment area, the 

concentrations of halon 1301 in arterial blood exceeded the target concentration at about 40 s

into the exposure and remained above the target for the duration of the measurements.

Arterial concentration simulations remained below target at the other measured sites. 

205

Measurements were made at three locations: 

o The actual fire extinguisher discharge location. 

o Knee area – 0.50 m above the floor at the area of discharge. The discharge area for 

pilot’s and copilot’s seat tests was at knee level where the seat cushion meets the seat

back. 

o Nose area – 0.94 m above the floor at the area of discharge. 

The releases of halon 1301 resulted in simulated arterial concentrations well below the target

concentration. Releases of halon 1211 resulted in simulated arterial concentrations that

surpassed the target concentration of 21.0 mg/L in well under a minute. 

made at the point of release and at two other selected sites during each test. Halon 1301 

arterial concentrations remained below the target concentration, while halon 1211 arterial

concentrations exceeded the target level at all measured locations when the extinguisher was

directed under the instrument panel on the pilot’s side with overhead vents open or closed. 

The use of halon 1301 for several decades has occurred with an excellent safety record. Retrospective 

modeling of scenarios such as those illustrated here showed that generally under normal use there has 

been little to no opportunity for the occurrence of exposure situations where individuals have been put at 

potential risk of having blood levels of halon 1301 ever reach a target concentration that might predispose 

for the onset of a cardiac sensitization response. Halon 1211, when used as a streaming agent under open

conditions, likewise has posed little risk. However, when used under more confined situations, the 

potential for cardiac sensitization ex
596 Screening Tests for Candidate Fire Suppressants 

The PBPK model has been used both retrospectively to evaluate previous actual exposure scenarios and

prospectively to evaluate potential exposure scenarios. An example of a retrospective evaluation was

ion, never regained consciousness, and died as a result of the

incident

A prospecti

potential ha

F-15 aircra

exposure da

locations ar

blood conce e target

was no a

target. This

had actually s were simulated and estimated to be 100 times 

the target. T e eir exposures.

The code fo h

defensible appr

performance and personnel safety issues, to evaluate the acute toxicity potential of inhaled fire 

suppressant

6.7.8 Lim

An acute to PA’s SNAP program

requires

Directive #3

regard to th

obtain a on

single limit

The purpose of the limit test is to evaluate mammalian toxicity following a single dose or exposure of the

skin since rabbits have a well-characterized response to irritants. Standard 

given in Vinegar et al., where they evaluated an accidental exposure to halon 1211 that had occurred

during an Israeli military exercise.198 In this instance, model predictions were consistent with the

outcome where the gunner, having only brief exposure, successfully escaped without incident but the

driver, with prolonged exposure, reached levels adequate for cardiac sensitization. The driver, in fact, was 

observed to be in ventricular fibrillat

.

ve application of the model was demonstrated by Vinegar et al.208 In order to evaluate the

zard to ground crews of an accidental release of CF3I, a discharge test was conducted on an

ft to record CF3I concentration time histories at various locations near the aircraft. These

ta were used with the PBPK model to simulate the potential blood levels of workers at various 

ound the aircraft during the release. The blood levels were compared to the target arterial 

ntration associated with cardiac sensitization. Results showed that at some locations th

t re ched, but at the open nacelle the blood concentrations could potentially reach double the 

information was put in perspective with a further retrospective simulation of individuals who

 inhaled CF3I and whose blood concentration

hes individuals experienced no apparent effect of th

r t e PBPK model has been listed in a report by Vinegar and Jepson.209 It can provide a

oach and appropriate tools for decision makers, who are tasked with balancing

s.

it Tests 

xicity test is an essential component of a screening protocol, since the E

acute toxicity of a substitute chemical as part of its submission package. However, under DoD

216.1, stringent polices dictate that the most conservative approaches must be taken with

e use of laboratory animals in research programs. The need to spend time and resources to

ce traditional “LC50 value” for a halon replacement candidate is discouraged in favor of a 

test.

halon replacement candidate. A traditional limit test involves exposing several rats to a single, 

conservative dose of chemical for a fixed period of time and determining whether this dose is lethal.

210 

For gaseous chemicals, inhalation is the most important route of exposure, whereas for liquids, skin 

contact may be more important than inhalation. For volatile candidate fire suppressants, a typical

exposure is to a concentration of 10 volume % for 15 min, both of which are likely in excess of the 

exposure of people were the chemical eventually used as a fire suppressant. Following the limit test, the 

(surviving and dead) animals are typically examined for other adverse effects. 

For liquid and solid candidate suppressants, an acute irritancy test could be used to evaluate the potential 

of a chemical to cause irritation in test animals after a single dose or exposure. These tests are usually 

performed in rabbits’ eyes and
Application of Screen Tests 597

protocols for irritation testing are described in the EPA’s Office of Prevention, Pesticides, and Toxic

Substances Health Effects Test Guidelines. The guidelines for acute eye and skin irritation are 870.2400 

and 870.2500, respectively (OPPTS Series 870). The URL is: 

http://www.epa.gov/docs/OPPTS_Harmonized/870_Health_Effects_Test_Guidelines. 

6.8 APPLICATION OF SCREEN TESTS 

Table 6–16 summarizes the screening tests as described above, along with the quantities of chemicals 

. 

ee properties, a limit test would be performed. Success there would lead to a dog exposure 

test. In practice, no animal tests were performed for agents for in-flight fire suppression. 

Table 6–16. Nominal Quantities of Chemicals Needed for Screening Tests. 

needed for their use. In practice, as will be seen in Chapter 7, it was generally easy to eliminate candidate 

chemicals from further consideration based on the most basic screening procedures. 

• The NGP relied most heavily on cup burner data and the presence of one or more chemically

active atoms for estimating a chemical's fire suppression efficiency. Only compounds whose 

extinguishment concentration was measured or estimated to be below 5 volume % were

considered further. 

• Next, compounds whose boiling points were above 20 °C were eliminated from further

consideration. For fluids with slightly higher boiling points, similar fluids were considered to

see if there might be a lower boiling member of the chemical family

• Third, a chemical continued under consideration if it had a feature that rendered it

tropodegradable, i.e., likely to have a short atmospheric lifetime. 

• Most of the compounds considered were of unknown toxicity, so in relatively few cases could

a confident safe exposure value be assigned. For compounds that were successful in the first

thr

Property Screened Method Total Mass/Volume of Chemical Required 

Cup burner 750 mL (gas)

DLAFSS 10 mL (liquid) 

Fire Suppression

Efficiency 

TARPF 10 g

Volatility Boiling point 0.2 mL

Atmospheric lifetime OH reactivity 5 L (gas) 

Absorption spectrum 10 mL (gas) 

Toxicity Literature 0 g

Dog test 5 kg 
598 Screening Tests for Candidate Fire Suppressants 

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