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Significant New Alternatives Policy Program

Fire Extinguishing and Explosion Prevention Sector

Risk Screen on Substitutes for Halon 1301

Total Flooding Systems in Normally Occupied or Unoccupied Spaces

Substitute: N2 Towers Inert Gas Generator Fire Suppression System

This risk screen does not contain Clean Air Act (CAA) Confidential Business 

Information (CBI) and, therefore, may be disclosed to the public.

1. Introduction

Ozone-depleting substances (ODS) are being phased out of production in response to a series of diplomatic 

and legislative efforts that have taken place over the past decade, including the Montreal Protocol and the 

Clean Air Act Amendments of 1990 (CAAA). The U.S. Environmental Protection Agency (EPA), as 

authorized by Section 612 of the CAAA, runs the Significant New Alternatives Policy (SNAP) Program, 

which identifies acceptable and unacceptable substitutes for ODS in specific end-uses based on assessment 

of their health and environmental impacts.

EPA’s decision on the acceptability of a substitute is based largely on the findings of a screening 

assessment of potential human health and environmental risks posed by the substitute in specific 

applications. EPA has already screened a large number of substitutes in many end–use applications within 

all of the major ODS-using sectors, including refrigeration and air conditioning, solvent cleaning, foam 

blowing, aerosols, fire suppression, adhesives, coatings and inks, and sterilization. The results of these risk 

screens are presented in a series of Background Documents that are available in EPA's docket.

The purpose of this risk screen is to supplement EPA’s Background Document on the fire suppression and 

explosion protection sector (EPA 1994) (hereinafter referred to as the Background Document). This risk 

screen discusses potential human health and environmental risks posed by the N2 Towers Inert Gas 

Generator Fire Suppression System (N2 Towers System) when used as a total flooding agent in normally

occupied or unoccupied spaces as a replacement for Halon 1301.

The N2 Towers System is an inert gas system designed for total flooding applications for fires in normally

occupied or unoccupied spaces. The nitrogen gas generating propellant grains are formed into purplecolored discs, each with a diameter of 39 mm and a height of 15 mm. Each disc contains 19 perforations, 

with an inside diameter of 2.5 mm. Each ten-inch N2 generator unit contains 96 propellant grain discs in a 

total of 14 layers: seven discs per layer for the 12 middle layers and six discs per layer for the top and 

bottom layers. Depending on the fire suppression requirement, several generators may be stacked inside an 

N2 Tower in a room, or a single generator may be bracketed inside an armored vehicle.

Upon activation, the substitute undergoes the following chemical reaction: 

4NaN3 + Fe2O3 → 2Na2O.FeO + 6N2
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The nitrogen is discharged into the space and the remainder of the activation products are filtered and

contained inside the N2 generator, where moisture is absorbed from the ambient air, neutralizing the 

sodium hydroxide until all that is left inside the generator is a burnt propellant sinter. The sinter is 

contained within the filter inside the sealed N2 generator until the entire unit is disposed of at an 

appropriate disposal facility. The largest N2 generator, 24” long by 7” in diameter, can discharge up to 6.9 

pounds of nitrogen gas in approximately 100 milliseconds. The submitter will use this size for the existing 

stockpile of grains.1Each 24” long by 6” diameter generator can protect up to 400 cubic feet of volume, 

and multiple generators may be combined to protect a single space (N2 Towers Inc., 2009, 2010a, 2010b).

Table 1 provides the composition of propellant grains contained in N2 Tower Systems, prior to activation.

Table 1. Composition of N2 Towers

Component Chemical Formula CAS No. Weight Percent

Sodium Azide (s) NaN3 26628-22-8 50-70

Ferric Oxide Fe2O3 1309-37-1 20-35

Graphite C 7782-42-5 2-7

Bentonite Al2O3.4(SiO2).H2O 1302-78-9 0-5

Sodium Nitrate NaNO3 7631-9-4 0-5

Potassium Perchlorate KClO4 7778-74-7 0-3

Amorphous Silica SiO2 112926-00-8 0-2

Source: N2 Towers Inc. (2009)

Section 2 of this risk screen summarizes the results of the risk screen for the proposed substitutes listed in 

Table 1. The remainder of the risk screen is organized into the following sections:

 Section 3: Atmospheric Assessment; 

 Section 4: Occupational Exposure Assessment;

 Section 5: End-Use Exposure Assessment; 

 Section 6: General Population Exposure Assessment; 

 Section 7: Volatile Organic Compound Assessment; and

 Section 8: References.

2. Summary of Results

The N2 Towers System is recommended for SNAP approval as a total flooding device in normally

occupied or unoccupied spaces for fire suppression. EPA recommends that Section VIII of the 

Occupational Safety & Health Administration (OSHA) Technical Manual be consulted for information on 

selecting the appropriate types of Personal Protective Equipment (PPE) recommended (OSHA 1999). The 

risk screen indicates that the use of the proposed substitute in normally occupied or unoccupied spaces will 

be less harmful to the atmosphere than the continued use of Halon 1301. Additionally, the risk to the 

general population is expected to be below levels of concern for non-cancer risks. For applications of this 

 

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In evaluating the occupation exposures during manufacture, this risk screen is limited to the existing stockpile of 

propellant grains that have already been manufactured. According to the submitter, the quantity of propellant grains 

in the existing stockpile can be used to manufacture approximately 20,000 N2 generators. Any future manufacture of 

N2 Towers Systems which requires production/manufacture of propellant grains will require additional EPA 

screening for approval under the SNAP program. 
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and all other water mist/inert gas fire suppression equipment, EPA recommends that the latest editions of 

National Fire Protection Agency (NFPA) Standards 750 and 2001 be followed.

3. Atmospheric Assessment

This section presents an assessment of the potential risks to atmospheric integrity posed by the use of the 

N2 Towers System as a total flooding system in normally occupied or unoccupied areas. The constituents 

of N2 Towers Systems are solids before use and therefore have zero ozone depleting potential (ODP) and 

global warming potential (GWP) Further, as indicated in Table 2, the ODP of each of the post-activation 

constituents of N2 Towers is zero and most of the post-activation constituents also have zero GWP. The 

GWP and ODP of the compounds in N2 Towers are well below the GWPs of previously SNAP-approved 

fire-suppression agents (e.g. HFCs and other high-GWP fluids). Accordingly, use of the N2 Towers

System is not expected to pose any significant adverse atmospheric impacts.

Table 2. ODP and 100-Year GWP of the N2 Towers System Constituents

Constituent ODP 100-Year GWP

N2 0 0

CO 0 3

a

CO2 0 1

aBased on indirect radiative effects, Fuglestvedt et al. (1996)

4. Occupational Exposure Assessment

The potential for personnel exposure during manufacture, installation, and maintenance are examined in 

this section. According to information provided in the N2 Towers SNAP application, all N2 Tower

Systems will be fabricated using an existing stockpile of TRW sodium azide propellant grains. The 

stockpile of grains is stored in an airtight moisture proof bag made of aluminum foil and are not opened 

until they are placed inside the N2 generator. Thus, any exposure during manufacture will be limited to 

propellant grain loading into the N2 generator.2 According to the submitter, all loading of the grains will 

be performed using a down draft loading table. Due to the inhalation, oral and dermal toxicity of sodium 

azide, all employees involved in propellant loading should wear appropriate PPE, such as protective 

clothing, gloves, goggles, and respiratory protection, as described in the compound’s MSDS. It is 

expected that level B of the levels of protection presented in the OSHA Technical Manual (OSHA 1999) 

will afford the appropriate protection. In the case of an accidental spill, workers should immediately

vacate the premises and follow the first aid guidelines listed in the MSDS. 

Because of the minimal chance of worker exposure to the chemicals during the manufacturing process, the 

use of these exposure controls and adherence to the appropriate occupational safety guidelines as 

mentioned above, are sufficient to ensure that the manufacture of N2 Towers Systems does not pose a risk 

to human health. 

 

2 According to the submitter, all grains will be placed inside the N2 generators by UTeC technicians. Once loaded, 

end caps are sealed and screwed into place on the N2 generators. After sealing, the generator is safe for storage and 

shipping (N2 Towers, Inc. 2010b)
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During installation or maintenance of an N2 Towers System, inadvertent discharge of the substitute may 

occur. According to the submitter, during installation and maintenance, no exposures to the propellant are

expected to occur as the generator body is a sealed steel casing with propellant grains contained therein. 

However, in the event of inadvertent discharge, oxygen concentration would be expected to decline over 

time. To ensure that exposures are minimized, it is recommended that N2 Towers Systems provide safety

training for technicians to minimize the occurrence of an inadvertent discharge. Training should also be 

provided regarding egress from the space should such a discharge occur.

5. End-Use Exposure Assessment

The primary health concern for agents containing or releasing inert gases, including nitrogen gas, is 

asphyxiation due to reduced oxygen concentrations. To determine whether asphyxiation could occur at the 

end-use, the N2 Towers System design parameters were analyzed. N2 Towers Inc. conducted fire 

suppression tests to measure the oxygen, carbon monoxide, carbon dioxide and particulates concentrations 

at the time of flame suppression Table 3 presents the calculated design concentrations based on the results 

of N2 Towers’ fire testing and a conservative estimate provided by the submitter. To determine the 

minimum design concentration for the N2 Towers System, a 30 percent safety factor was applied to the 

extinguishing concentrations, in accordance with NFPA 2001 Guidelines (NFPA, 2007).

Table 3. Summary of N2 Towers Inc.’s Fire Tests and Calculated Minimum Design 

Concentrations

Test

Extinguishing Concentrationa

Design Concentration (% N2/% air 

(v/v)) % O2 /% air c,d

(v/v)

% N2 /% air 

(v/v)c

% CO/% air 

(v/v)b

1 16.2 b 22.7 215.6 29.5

3 16.0 b 23.6 288 30.7

4 15.8 b 24.6 299 32.0

5 15.8 b 24.6 339 32.0

NAe 14.5e 30.8 NA 40.0

a Source: N2 Towers Inc. (2009 and 2010b).

b O2 and CO levels are experimentally measured. 

c N2 levels are calculated based on experimentally measured O2 levels. These values are exclusive of atmospheric nitrogen, i.e. represent 

the concentration of nitrogen from the N2 Towers System only in the air.

d Design concentrations were estimated by applying a 30 percent safety factor to the extinguishing concentrations

e O2 level provided by submitter as a conservative estimate. 

According to the submitter, the N2 Towers System creates wind vorticity and lowers the oxygen levels of a 

space down to approximately 14.5 % oxygen per volume. Based on the conservative estimate of oxygen

levels, the nitrogen extinguishing concentration was calculated to be 30.8 percent and the design 

concentration to be 40.0 percent. 

Because N2 Towers Systems are designed to ensure that the oxygen concentration in any protected space 

will not fall below 12 percent over the discharge period, N2 Towers is considered safe for fires in normally

occupied or normally unoccupied spaces for inert gases. 
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6. General Population Exposure Assessment

Given the large natural quantities of nitrogen and water in the atmosphere, it is unlikely that an accidental 

release of N2 Towers will affect the average atmospheric concentrations of these gases. As such, N2

Towers is not believed to pose a toxicity threat to the general population. 

7. Volatile Organic Compound Assessment

The constituents of the N2 Towers are not considered VOCs for purposes of local air quality. 

8. References

EPA. 1994. Risk Screen on the Use of Substitutes for Class I Ozone-Depleting Substances: Fire 

Suppression and Explosion Protection (Halon Substitutes). Stratospheric Protection Division. March 

1994. 

Fuglestvedt, J.S., I.S.A. Isaksen and W.-C. Wang, 1996: Estimates of indirect global warming potentials 

for CH4, CO, and NOx. Clim. Change, 34, 405-437.

N2 Towers, Inc. 2009. SNAP Submission to EPA for N2 Towers. December 22, 2009.

N2 Towers, Inc. 2010a. http://n2towers.com/ Accessed March 22, 2010.

N2 Towers, Inc. 2010b. Response to Incomplete SNAP Submission for N2 Towers. March 10, 2010.

NFPA, 2007. NFPA 2001: Standard on Clean Agent Fire Extinguishing Systems. 2008 Edition. 

Available at: http://www.nfpa.org/freecodes/free_access_document.asp

OSHA. 1999. OSHA Technical Manual. Department of Labor. Occupational Safety & Health 

Administration. January 20, 1999. Available online at: http://www.osha.gov/dts/osta/otm/otm_toc.html