Robert P. Garner

Federal Aviation Administration

Civil Aerospace Medical Institute

Oklahoma City, Oklahoma 73125

Jeffrey S. Utechtt

Essex PB&R Corporation

Edwardsville, Ilinois 62025

February 2004

Final Report

This document is available to the public

through the National Technical Information

Service, Springfield, Virginia 22161.

Office of Aerospace Medicine

Washington, DC 20591

Performance Criteria for 

Development of Extended Use 

Protective Breathing Equipment

DOT/FAA/AM-04/3
NOTICE

This document is disseminated under the sponsorship of

the U.S. Department of Transportation in the interest of

information exchange. The United States Government

assumes no liability for the contents thereof.
i

Technical Report Documentation Page 

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 

DOT/FAA/AM-04/3 

4. Title and Subtitle 5. Report Date

Performance Criteria for Development of Extended Use Protective February 2004 

Breathing Equipment 6. Performing Organization Code 

7. Author(s) 8. Performing Organization Report No. 

Garner RP1, Utechtt JS2

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

1

FAA Civil Aerospace Medical Institute 

Aeromedical Research Division 

P.O. Box 25082 

Oklahoma City, OK 73125 

2

Essex PB&R Corporation 

Engineering Department 

Edwardsville, IL 62025 

11. Contract or Grant No. 

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered 

Office of Aerospace Medicine 

Federal Aviation Administration 

800 Independence Ave., S.W. 

Washington, DC 20591 14. Sponsoring Agency Code 

15. Supplemental Notes 

Work was accomplished under approved subtask AM-B-03-PRS-94

16. Abstract 

The Federal Aviation Administration (FAA) requires under Federal Aviation Regulation (FAR) 121.337 that 

crew protective breathing equipment (PBE) for smoke and fume protection is installed aboard aircraft and 

that crewmembers be trained in the proper use of PBE (FAR 121.417). A variety of designs currently exist 

that meet the requirements of these regulations. However, the threat posed by atmospheric contamination in 

an environment that cannot be quickly escaped suggests that extending the protective capabilities of PBE 

devices beyond what is mandated by the FAA may be beneficial in aviation and other arenas. These 

experiments were conducted to evaluate the use of one style of PBE in terms of potential for long-term (>20 

min) use and to identify issues critical to long-term use. A closed-circuit PBE device utilizing lithium

hydroxide (LiOH) technology for carbon dioxide (CO2

) removal was tested. The capability to supply fresh 

oxygen to the user had been incorporated into the prototype design. Breathing simulator testing clearly 

demonstrated that CO2

 levels during use were consistent with theoretical values and represented the limiting 

factor for long-term wear when coupled with the “ad libitum” use of oxygen. As expected, metabolic CO2

production rate was the primary factor limiting time of use in the prototype testing performed. From a 

practical standpoint, it was also clear that consistent and effective means of oxygen delivery and temperature 

control need to be developed. If these issues can be successfully addressed in terms of meeting a metabolic 

demand anticipated for a given operational environment, PBE capable of providing from 2 to 5 hours of 

protection to the user may be a viable possibility. 

17. Key Words 18. Distribution Statement 

Protective Breathing Equipment, Lithium Hydroxide, 

Carbon Dioxide 

Document is available to the public through 

the National Technical Information Service 

Springfield, Virginia 22161 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price 

Unclassified Unclassified 8 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized 

1

PERFORMANCE CRITERIA FOR DEVELOPMENT OF

EXTENDED USE PROTECTIVE BREATHING EQUIPMENT

INTRODUCTION

A variety of styles of protective breathing equipment 

(PBE) are available for use by passengers and crew in civil 

aviation. Historically, this type of equipment has been 

designated for relatively short-term use in emergency 

situations. PBE designed to protect the wearer from 

smoke and fumes are required on transport aircraft for 

crewmember use. These units are often referred to as 

“smoke hoods” and are required to provide protection 

for at least 15 minutes. 

There are potential circumstances where longer duration protection would be desirable in this type of PBE. 

For example, a system failure aboard an aircraft could 

result in some type of contaminant being introduced into 

the cabin. A crewmember may have to work to remedy 

the situation and be in a noxious environment for the 

duration of the flight. Isolation and administration of 

first aid to an individual with a potentially infectious 

disease is also an application. Having an isolated, sustainable atmosphere to breath in would reduce the potential 

for infection. The possibility of a chemical or biological 

agent being released into the cabin environment is also a 

possibility. Even if confined to small area, those affected 

may need treatment and tasks in a contaminated area may 

still be necessary. These situations may require the user 

of the PBE to remain inside the unit until the untoward 

condition can be resolved. 

Of the styles of PBE available, the closed circuit design seems well suited for extended wear. A closed-circuit 

design is one in which exhaled gas is processed to remove 

metabolic waste products and the oxygen consumed by 

the user is replaced. The breathable atmosphere for the 

user is isolated from the general environment. The potential of this type of device for long-term use needed to 

be examined in detail. The purpose of this study was to 

evaluate a current closed circuit design in terms of identifying modifications necessary to make it functionally 

viable for extended wear applications.

METHODS

Basic quantitative evaluations were performed to 

characterize performance. Testing included PBE activation on a metabolic simulator. The approach utilized is 

outlined below.

Equipment. The PBE utilized was a model manufactured by Essex PB & R, designated the victim rescue unit 

(VRU, Figure 1). The unit is not certified for crewmember 

use aboard aircraft because the design does not include 

features that meet the rigors of fire fighting. However, 

the materials and construction techniques are consistent 

with TSO-C116 models that have the more advanced 

features. Another difference was that the units used in 

these tests had been modified to allow additional gas 

inside the hood beyond the supply available from the 

Figure 1. VRU in use under simulated emergency conditions in 

an aviation environment.
2 

oxygen cylinder supplied. Of primary concern to this 

study was the fact that the VRU is a closed circuit device 

that uses carbon dioxide (CO2) “scrubbers” based on 

lithium hydroxide (LiOH) chemistry. Gas concentrations 

in the VRU were measured using a mass spectrometer 

(Perkin-Elmer, MGA-1100). 

The breathing simulator utilized is a model (Vacumed, 

17050/51) designed for the precise calibration of metabolic gas analyzers and related equipment. The syringe 

was plumbed into a mannequin head (Figure 2). This 

configuration permitted the unit to be activated and 

internal volume displaced consistent with a user. The 

simulator was used because it allowed very specific quantities of CO2

 to be delivered so reaction in the hood could 

be studied.

LiOH is a commonly used CO2 sorbent. It is used in 

a variety of models of PBE, aboard spacecraft, and in the 

extravehicular mobility unit worn by astronauts during 

extra vehicular activity in space. The capability of the 

scrubbers to adsorb CO2 became a limiting factor since 

the VRU was modified to allow supplemental breathing 

gas, beyond that originally contained in the unit’s oxygen 

cylinder, to be introduced into the hood.

The summary reaction of LiOH with CO2 is:

2LiOH + CO2(g) à Li2CO3(s) + H20(s) Eq. 1

A net total of 10.7 kcal of heat is released for each 

mole of LiOH utilized. The VRU has three CO2 scrubbers installed, each containing approximately 30 grams 

of LiOH. Ideally, this would react with roughly 83 grams 

of CO2 (~42 liters of gas, STPD).

Testing. Three tests were performed. The first analysis 

was to determine the relationship between theoretical 

and experimental adsorption of CO2 in the VRU. The 

simulator was run at a CO2 rate beyond the ability of the 

scrubbers to keep up with the volume of gas introduced. 

Once the CO2 level inside the hood reached approximately 

7%, the simulator was turned off and the concentration 

was allowed to return to a baseline level. Repeating this 

sequence allowed the adsorption rate over time to be 

estimated for the hood.

The second test was to let the simulator run for one 

hour at a manageable CO2 production rate for the hood. 

The value chosen was equivalent to a workload of about 50 

watts. At the end of the hour, the simulator was adjusted 

to produce a CO2 flow consistent with about 160 watts 

until the concentration in the hood exceeded 7%. During 

this trial, oxygen was added to maintain a concentration 

in the hood above 21%.

The final test consisted of letting the simulator run 

at a rate consistent with a workload of roughly 40 watts 

and observing how long it took for the CO2 level inside 

the hood to exceed 7%. Again, oxygen was added to keep 

the concentration in the hood above 21%.

Figure 2. Simulator setup and associated gases used in the 

testing.
3

RESULTS

The scrubber reaction with CO2 inside the VRU was 

not linear. It followed an exponential decay of the following form:

Y = 1.3671e-0.0267X Eq. 2

For practical applications, the reaction had stopped at 

55 minutes. This represented a CO2 volume of approximately 40 liters. Data collected after this point indicated 

that the reaction eventually would have exhausted all of 

the available LiOH, but the rate of adsorption is below 

the CO2 buffering capacity needed for continued use.

Data from the second test is presented in Figure 3. CO2

levels remained fairly steady for the first 30 minutes when 

introducing CO2 into the VRU at a rate of 0.70 l/min. After 

this there was a discernable increase in the rate of CO2

accumulation in the hood. At 60 minutes, the delivery 

rate of the simulator was increased to 2.24 l CO2/min. It 

only took a little over a minute (1:22) for the concentration to exceed 7%. These two periods represent delivery 

of 42 and 2.9 liters of CO2, respectively.

In the third test the simulator was allowed to run at 

0.56 l CO2/min (Figure 4). It took nearly 86 (85:50) 

minutes for the CO2 concentration to reach 7%. A total 

of 47 liters of CO2 was delivered to the hood during this 

time. Again, the CO2 concentration remained relatively 

constant during the first 30 minutes of the test with a 

steady increase thereafter. The rate of this increase actually 

exceeds that observed in the previous test.

DISCUSSION

Based on equation 1, 90 grams of LiOH will react 

with 82.7 grams of CO2. Theoretically this represents 

42.1 liters of CO2. The tests run are remarkably consistent with the theoretical value in light of the inherent 

variability in the experimental protocol. The amount of 

LiOH contained by any given scrubber is going to vary 

due to manufacturing tolerances. Placing the VRU on 

the mannequin, the rate and concentration of oxygen 

introduced followed general guidelines but did differ from 

test to test. All these factors could potentially influence 

the scrubber reaction.

Figure 3. Carbon dioxide concentration changes inside the VRU at a delivery rate of 0.76 l/min.
4 

The test situation was idealized. Undoubtedly, this 

contributed to the overall efficiency of the reaction in 

terms of CO2 adsorption. It is known that factors such 

as humidity levels and the way the LiOH is deposited 

on the scrubber media generally detract from the overall 

efficiency of the reaction. However, the current data still 

provides insight into long-term use potential for equipment of this type. 

In the first test, the rate of CO2 delivery was consistent 

with an estimated workload of 160 watts and the gas was 

quickly accumulating in the VRU. Therefore, it is clear 

that even moderate metabolic rates limit the application 

of the current technology in a number of situations. To 

cover higher metabolic rates and extend the useful life of 

a device at lower rates, additional LiOH could be used. 

Since the “scrubbing” is based on a relatively straightforward chemical reaction, the only potential improvement 

would be in how the LiOH is associated with the support 

matrix used for the scrubber. At best, this would only 

provide improvements in terms of adsorption rates. Total 

CO2 buffering capacity would still be limited to the mass 

of LiOH present.

At low levels of work or at rest, the technology does 

offer a significant period of protection, assuming a sufficient breathable gas supply is available. One inherent 

advantage of being able to replenish the breathing gas 

is that a variety of supplemental sources can be tailored 

to specific applications or environments. In this study, 

aviators breathing oxygen from a large cylinder was 

used because it was readily available. A smaller cylinder, 

a number of small cylinders, or an oxygen concentrating device might also meet the demand. Filtering does 

become an issue for breathing gas generation if the immediate environment is used as a source. The ability to 

design a system customized for the need anticipated is 

a valuable characteristic of this particular technological 

approach. Managing the heat inside a hood is a more 

difficult challenge. 

The reaction itself results in a net release of heat 

energy. Assuming 90 grams of LiOH, 40.7 kcal of heat 

will be released if all the reactant is consumed. The 

gas that an individual exhales is at body temperature, 

and the fact that the head is enclosed retards heat 

dissipation, adding to the heat load. Previous tests 

Figure 4. The rate of adsorption to 7% in the VRU is presented for a delivery rate of 0.56 l CO2/min.
5

have indicated that individuals using this type of device find the temperature inside the device relatively 

uncomfortable during short-term use. It is likely that 

extending the time in a device would reemphasize this 

point. It should be noted that if the alternative to wearing a device were death or injury, the comfort level 

might be less of a priority. That should not preclude 

attempts to develop equipment that actively addresses 

user comfort.

Currently, transport category aircraft are required to 

have hood type devices installed for crew use in fire fighting 

and similar emergencies. Many companies have installed 

the VRU or similar devices aboard their private aircraft 

as a means to protect passengers from any contamination situation. Based on this data, it would appear that 

development of a device that could be used for periods 

up to two hours or more is possible if proper amounts of 

LiOH are available. The next step would be to determine 

if human test data is consistent with the simulator findings in terms of CO2

 adsorption. Anticipating that no 

great discrepancies exist, it would appear that optimizing 

the reaction for both CO2 production rate and volume is 

indeed the primary limiting factor. Issues related to the 

breathable gas supply and heat buildup have to be addressed but certainly appear manageable for the protective 

needs envisioned in aviation and other environments.