Naval Submarine Medical Research

NSMRL Report 1167

Laboratory

19 April 1991

THE MEDICAL HAZARDS OF FLAME-SUPPRESSANT ATMOSPHERES

by

Douglas R. Knight

Released by:

R. G. Walter, CAPT, DC, USN

Commanding Officer

Naval Submarine Medical Research Laboratory

Approved for public release; distribution unlimited
The Medical Hazards of Flame-suppressant Atmospheres

by

Douglas R. Knight

Naval Submarine Medical Research Laboratory

Report No. 1167

Naval Medical Research and Development Command

Work Unit No. MR00001.MR000101-5103

Approved and Released by:

R. G. Walter, CAPT, DC, USN

Commanding Officer

Naval Submarine Medical Research Laboratory

Approved for public release; distribution unlimited
SUMMARY PAGE

PROBLEM

Most chamber fires can be suppressed or extinguished by reducing the concentration of oxygen in chamber air. Unfortunately, the reduction of 02 concentration may be harmful to crews living inside the chamber. Few

publications, if any, present a systematic evaluation of the medical hazards of

flame-suppressant atmospheres.

APPROACH

A significant reduction of flame-spread is defined by plotting isopleths of

combustion activity on 02% - PO2 graphs. The graphs are used to define all

possible modifications of air that suppress flames. Then, each modified atmosphere is evaluated for known medical hazards. By definition, there is no medical hazard in the atmosphere when the occupants are comfortable, able to

work productively, and safe from short-or long-term health problems.

FINDINGS

The following modifications of sea-level air will suppress combustion: 1.

SUPPLEMENTATION - the addition of an appropriate foreign gas to air. 2.

N2 PRESSURIZATION - the addition of compressed N2 to air. 3. DEPRESSURIZATION - the partial evacuation of air from the chamber. 4. N2 DILUTION - the partial replacement of 02 by N2. One or more of the following

medical problems may be induced by the modified atmospheres; barotrauma,

N2 narcosis, decompression sickness, hypoxia, and inhalation toxicity. SUPPLEMENTATION is not recommended at this time due to potential risk of

toxic inhalation. A review of experimental data suggests that N2 DILUTION

and N2 PRESSURIZATION may be combined at minimal hazard to the crews

serving aboard patrolling submarines. Novel atmospheres should be

thoroughly evaluated for unknown medical hazards.

ADMINISTRATIVE INFORMATION

This work was carried out under Naval Medical Research and Development

Command Work Unit #MR00001.MR000101-5103. It was submitted for

review on 23 October 1989; approved for publication on 19 April 1991, and assigned as NSMRL Report 1167.

ii
ABSTRACT

Flames are a potential hazard to the occupants of sealed chambers. This report

describes four modifications of air that will suppress or extinguish flames. They are:

1. SUPPLEMENTATION - the addition of an appropriate foreign gas to air. 2. N2

PRESSURIZATION - the addition of compressed N2 to air. 3. DEPRESSURIZATION - the partial evacuation of air from the chamber. 4. N2 DILUTION - the exchange of N2 for 02. The primary medical hazards of flame-suppressant

atmospheres are barotrauma, N2 narcosis, decompression sickness, hypoxia, and inhalation toxicity. Experimental evidence supports the use of N2 DILUTION to suppress flames aboard patrolling submarines. One or more of the following adjuncts

may enhance the use of N2 DILUTION without impairing human health:

Physiological adaptation to hypoxia, addition of CO 2 to the atmosphere, and N2

PRESSURIZATION.

iii
Table of Contents

FLA M E S .. . ... .... . .. . .. ... .. .. ........ .. ... .. .... 1

Flame Hazards .......................................... 1

Flamm ability Studies ................................. 2

Flam e Suppression .................................. 3

PRIMARY MEDICAL HAZARDS ........................... 5

Supplementation Hazards ............................... 5

N2 Pressurization Hazards ............................... 5

Depressurization Hazards ............................... 8

N2 Dilution H azard .................................. 10

STUDIES OF HYPOXIA AT NORMOBARIC PRESSURE .......... 10

APPLICATIONS TO SUBMARINES ........................ 12

SYM BOLS ........................................ 14

DEFINITIONS ..................................... 14

REFERENCES ..................................... 15

iv
The Medical Hazards of Flame-suppressant Atmospheres

by

Douglas R. Knight

FLAMES

Flame Hazards

Accidental ignitions of combustible

materials can quickly injure or kill the

inhabitants of sealed chambers (Matthews, 1989; Press, 1988; Galasyn &

Markham, 1969). Flames are likely to

burn trapped victims when the configuration of the cabin accelerates heating of the internal environment. Figure

1 shows two profiles of air temperature,

depending on whether the chamber is

sealed (closed-hatch condition) or open

to the outside air (open-hatch condition).

The fire was ignited in a pool of butane

and the flames spread upward to

polyvinylchloride nitrile rubber hull insulation coated with intumescent paint.

The closed-hatch condition accelerated

the rise of air temperature inside the

chamber, indicating that sealing a

chamber can reduce the effective time of

fire fighting (Alexander, Bogan, Brandow, Carhart, Eaton, et al., 1985).

The failure of fire fighters to stop the

spread of flames leads to smoke inhalation or burn injury, a fact illustrated by

disastrous fires in 02-rich atmospheres.

Figure 2 summarizes several case

reports of injuries or deaths caused by

flames in either space cabin simulators

(75-100% 02) or hyperbaric chambers

(25-50% 02). The accidents occurred in

atmospheres containing higher Po 2's

than the sea-level value of approximately 160 torr. Five of nine deaths resulted

from flashover and the few survivors

were seriously burned. Most of the fires

were ignited by malfunctioning electriFLAME HAZARD OF SEALED CHAMBERS

(AIR TEMPERATURE, 0 C)

800-

600 -

400 -

200-

CLOSED HATCH

OPEN HATCH

SKIN

I I I 2 3

3 44

INJURY

5

5

(MINUTES)

(ALEXANDER, NRL REPORT 8872)

Fig. 1. FLAME HAZARD OF SEALED CHAMBERS. Two curves show the changes of air temperature

when equal amounts of butane are burned inside a chamber that is either sealed (close hatch) or open (open

hatch). Skin-burns occur when air temperatures rise above the horizontal line.

1

• . • • .
FLAME HAZARD OF 0 2 -RICH ATMOSPHERES

U'

H-d-- - - - - - - - - - - AITR at-------------

0 -

a 1

100 I I I

500

P0 2 ., TORR

9 900 rC;LJ NONFATAL

* FATA L

Fig. 2. FLAME HAZARD OF 02-RICH ATMOSPHERES. Symbols define the Po2 and 02 concentrations

of chamber atmospheres in chich fires caused injuries or deaths.

cal equipment (Huggett, 1969; Galasyn

& Markham, 1969; Harter, 1967).

Flammability Studies

According to Cook, Meirer, & Shields

(1967), the behavior of combustion

processes in N2-02 atmospheres varies

according to the 02% and the gas pressures. The addition of 02 to a pressurizable chamber raises the 02%

without changing the PN2. The resultant atmosphere lowers the ignition

temperature and raises the burning rate

of solid materials. Chamber pressurization with a N2-02 mixture of fixed composition increases the P02 without

changing the 02%. This also lowers the

ignition temperature and raises the

burning rate of solids. Increasing the

02% without changing the P0 2 serves to

increase the burning rate of cloth (Cook,

Meirer, & Shields, 1967).

Since the behavior of combustion

processes in N2-02 atmospheres varies

according to the 02% and gas pressures,

the relative effects of 02%, PO2, and PN2

on flammability can be analyzed by plotting isopleths of combustion activity on

02%- PO2 graphs. The 02% (Y axis) is

functionally related to PO2 (X axis) by

equation 1,

02% = (100 / PB)P0 2 (1)

which conforms to the general equation for a straight line. The slope of the

line, 100/PB, intersects the X axis at 0.

The barometric pressure is the sum of

the partial pressures (equation 2).

PB = P0 2 + PN2 + PAOG (2)

The PN2 is derived in equations 3 and

4, which are used later in this paper to

analyze the effects of the hyperbaric

atmosphere on human health and performance.

2

02%

100 -

50 -

AIR 

m m * ! I

I | I
N2% = 100 - 02% - AOG% (3)

PN2 = (PB / 100)N2% (4)

The data in Figure 3 are burn-times

for strips of filter paper. The lower burntimes indicate more rapid rates of complete combustion (Simons & Archibald,

1958) and the undefined burn-times indicate incomplete combustion (Kimzey,

1986; Cook, Dorr, & Shields, 1968). The

undefined burn times (denoted by "X")

are connected by a visual best-fit curve

to identify the highest 02%'s for incomplete combustion. The range of 02%'s for

incomplete combustion decreases

toward an asymptotic value of about

14% when the P0 2 is increased above a

minimum value of 25 torr. Above this

02%

100

50

0 -

FLAMMABILITY OF PAPER STRIPS

X

- -

---- AIR ....- X- - - -

range is the zone of 02%'S which support

complete combustion. At a constant P0 2

in the zone of complete combustion, the

burn rate is reduced by lowering the

02% (Figure 3).

Flame Suppression

Air can be converted to a flame-suppressant atmosphere by lowering the

PO2, raising the PN2, or adding the appropriate foreign gas. The arrows of Figure 4 illustrate three different

modifications of air that produce the

zone of incomplete combustion as

defined in Figure 3. Arrow "A" (Figure

4) represents the process of N2 PRESSURIZATION in which the well-mixed

atmosphere is pressurized with nitrogen

until the 02 concentration is reduced to

COMPLETE COMBUSTION

D

INCOMPLETE COMBUSTION

I I I

80

S I 10

160

I0 

240

P0 2 , TORR

BURN TIME:

0 6.0 s

S7.5 s

X UNDEFINED

Fig. 3. FLAMMABILITY OF PAPER STRIPS. The position of the symbols defines the

Po2's and 02 concentrations of experimental atmospheres. Each symbol represents the

burn time of paper strips (see INSET). "Undefined" burn time refers to the incomplete

combustion of paper strips. Atmospheres on or below the curve do not support the complete

combustion of paper strips.

3

I

0

-- X . ... . ].€
POTENTIAL MODIFICATIONS OF AIR

INCOMPLETE

COMBUSTION

A. N2 PRESSURIZATION

B. N2 DILUTION

c. DEPRESSURIZATION

A.

/ I

U I

I

0 80 160

P0 2 , TORR

240

Fig. 4. POTENTIAL MODIFICATIONS OF AIR. Three solid arrows represent unique

modifications (see INSET) of sea level air that produce a flame supporessant atmosphere.

Atmospheric conditions on or below the curve do not support the complete combusion of

paper strips.

14%. The PB is increased by this procedure. Elevation of the PN2 at a constant

P0 2 interferes with flame propagation

by lowering the temperature of the

flame, reducing the gradient for diffusion of 02 into the flame, and introducing a third body into the recombination

reactions of combustion (Tatem, Gann,

& Carhart, 1974; Shilling, Werts, &

Schandelmeier, 1976).

Arrow "B" (Figure 4) illustrates the

process of N2 DILUTION in which N2 is

exchanged for 02 until the 02 concentration is reduced to approximately 14.5%

at normobaric pressure. The P0 2 is

reduced by this procedure in direct

proportion to the decrement of 02%.

Carhart (1987) believes that reducing

the 02 concentration below 19% will effectively slow the growth of fires.

Arrow "C" (Figure 4) depicts the use

of atmospheric DEPRESSURIZATION

to reduce thePo2 without changing the

02 concentration (Roth, 1964; Simons &

Archibald, 1958). The PB is decreased by

this procedure.

A fourth modification of air, called

SUPPLEMENTATION, utilizes the addition of a foreign gas to either drop the

temperature of the flame or inhibit the

recombination reactions of combustion

(Huggett, 1969). A class of halocarbon

gases extinguishes flames very effectively at the expense of decomposing into

toxic gases which are difficult to remove

from the atmosphere of a sealed chamber. Fluorocarbon gases are reported to

be non-toxic to animals and might be

useful for preventing flames if found to

be safe for human exposure (McHale,

1972).

Assuming that the incomplete combustion of paper strips predicts a significant improvement of fire safety, it is

4

02%

22 -

20 -

18 -

16 -

14 -

12- m
important to ask the question, "Can

people inhabit these modified atmospheres without experiencing discomfort,

illness, or impairment of their industrial productivity?" The remainder

of this paper will address the primary

medical guidelines for modifying the

02%, P02, and PN2.

PRIMARY MEDICAL HAZARDS

The modifications of air necessary to

suppress flames also have the potential

for injuring the human body. The injuries of primary concern are toxic inhalation, barotrauma, N2 narcosis,

decompression sickness, and hypoxia

(Table 1).

Supplementation Hazards.

Toxic inhalation is a primary hazard

of foreign gases that will not be discussed in this report.

N2 Pressurization Hazards.

The increase of PB is transmitted into

gas pockets of the sinuses and middle

ears through channels that communicate with the human airway. Blockage

of one of these channels prevents the

transmission of atmospheric pressure

into the gas pocket, thus establishing a

pressure differential between the gas

pocket and the atmosphere. For example, Figure 5A illustrates the middle

ear as a gas pocket encased by hard bone

(cross hatched area) and connected to

the main airway by the Eustachian

tube. Blockage of the Eustachian tube

obstructs venting of the middle ear to atmospheric pressure, thereby preventing

the equalization of gas pressures acting

on the inner and outer surfaces of the

flexible eardrum. The eardrum is

stretched toward the side of lower pressure in relation to the pressure differential between the atmosphere and middle

ear (Figure 5B). Ear pain begins at pressure differentials <60 torr and increases with higher pressures until

differentials of 100-500 torr rupture the

ear drum (Armstrong & Heim, 1937).

Damage of the eardrum is an example of

barotrauma, which is the injury of tissues caused by pressure differentials.

MEDICAL GUIDELINES

MODIFICA7ION PROCEDURE

N2 DiLUT101

N2 PR6SSURIZATION

Table 1.

PRESSURE CHANGE

V p02

A Pj

A PN2

eventual V PB

GUI DELI VES

Hypoxia

Barotraurna

N2 Narcosis

Barotraura,

Hypoxia

Decompression sickness,

DEPRESSUR: ZATION SP02

SPs

Hypoxia

Barotrauma, Decompression sickness

eventual A PB Barotrauma

5
Chemists refer to N2 as an inert gas,

but pressurized N2 is not inert when absorbed to excess amounts by biological

systems. Figure 6 shows isobars of PN2

that intersect compressed air at points

A-C. N2 narcosis can result from exposure to compressed air at point A,

where the PN2 = 2420 torr. Assuming

that compressed N2 is the sole cause of

N2 narcosis (p 175 of Shilling, Werts, &

Schandelmeier, 1976) and not some

other factor such as the accumulation of

C02 in tissues, then N2 pressurization of

air to point D would induce a state of

narcosis equivalent to that of exposure

to compressed air at point A. The narcosis is reversible by lowering the PN2.

Decompression sickness (DCS) occurs

when rapid depressurization of the atmosphere induces N2-rich bubbles to

grow in the tissues or blood stream

(Shilling, Werts, & Schandelmeier,

1976). The risk of DCS exists when the

ratio of tissue PN2 /ambient PB exceeds a

value of 1.0, in which case the depressurization of an occupied atmosphere

may need to be interrupted or slowed to

prevent symptoms and injury (Behnke,

1967). There are published tables of

pressure-time schedules which prescribe

an acceptable decompression from hyperbaric atmospheres (U.S. Navy

Diving Manual).

Figure 6 shows air pressures which

predispose humans to DCS at exposure

times of 0.5 h (point A), 1 h (point B),

and 48 h (point C). Weathersby, Hart,

Flynn, & Walker, (1987) suggest that

only the inert gas need be considered in

calculating human decompression

schedules which therefore permits the

A.

B.

ADAPTED FROM (BILLINGS, j973 NASA SP-O306)

Fig. 5. BAROTRAUMA OF THE MIDDLE EAR.

Bone (cross-hatching) completely surrounds

gas-filled spaces of the ear. Graph A shows the

position of the eardrum when there is no pressure

difference between the external and middle ear

cavities. Graph B shows inward distension of the

eardrum produced by higher pressure in the

external ear canal. Soft- or hard tissue blockage of

the eustachean inlet prevented the equalization of

pressures across the eardrum.

assumption that the same risk of DCS

exists anywhere along the PN2 isobars of

Figure 6 when the atmosphere is

decompressed to sea level air. Therefore,

each PN2 isobar predisposes humans to

DCS at the same exposure times used for

the intercepts with 21% 02. Note that

the isobar for PN2 = 1140 crosses the

160 torr coordinate of the X axis at point

E, where the 02% is 12.3. The PN2

isobars of Figure 6 are not true

6
threshold values for DCS and N2 narcosis, but serve to warn of the risks

(which are yet to be defined).

Eckenhoff, Osborne, Parker, & Bondi

(1986) reported no incidence of DCS

after 48 h exposures to 1064 torr N2 in

compressed air suggesting that limiting

the N2 pressurization to PN2'S < 1064

torr avoids the risk of DCS upon returning to sea-level air. This PN2 would cross

the 160 torr coordinate of the X axis at

an 02% of 13.1 (Figure 6). Unfortunately, the variability of human responses to

hyperbaric environments may mean

that some people are at risk for

symptoms of DCS after 48 h exposures to

PN2 < 1064 torr.

02%

25-

20

15-

10-

5

0 I

02 is absorbed by body fluids and

binds to heme pigments in all cells. 02

transport to the cells is driven by

gradients of Po 2 that exist between the

atmosphere and the cellular fluids.

Oxygen may damage the lung when the

PO2 exceeds 380 torr (Shilling, Werts, &

Schandelmeier, 1976), so it is important

to emphasize that N2 pressurization

does not change the P0 2 in a sealed

chamber. Therefore, pulmonary oxygen

toxicity is not a primary medical concern of N2 pressurization.

Flame extinction by rapid N2 PRESSURIZATION has been successfully

tested on laboratory animals (Dressler,

Robinson, Gann, Stone, Willimas, et al.,

1977). N2 PRESSURIZATION to 6% 02

EFFECTS OF N2 PRESSURIZATION

C. B. A.

E.

D.

0

0

200

A. PROMPT NARCOSIS

B. >1 H PREDISPOSES TO "DCS"

c. 48 H PREDISPOSES TO "DCS"

T - - - I 1

400 600 800

P02, TORR

Fig. 6. EFFECTS OF N2 PRESSURIZATION. A solid arrow represents the N2

pressurization of sea level air. Three solid curves are N2 isobars that produce narcosis or

predispose humans to decompression sickness (see INSET) in hyperbaric atmospheres.

Specific points on the graph are discussed in the text.

7

I W
EFFECTS OF DEPRESSURIZATION

25 --

A.

-- - - AIR--------------

X

'=r

--. I

I--

-"4

0.

LJJ

"U,-

LJ

,,--

I I I I I I I I I

0 40 80

C0

U.'

C9_

I 1 I

120 160

P02 , TORR

Fig. 7. EFFECTS OF DEPRESSURIZATION. A solid arrow represents the

depressurization of sea level air to an isobar of hypobaric pressure. Three vertical isobars

of 02 represent thresholds to progressively severe hypoxia in unacclimatized humans.

creates an atmospheric zone of noncombustion (Cook, Dorr, & Shields, 1968),

but human use of the zone is infrequent.

There is a case report of unpleasant

symptoms in humans exposed to 3% 02

by N2 PRESSURIZATION, but the

cause of symptoms in this extreme environment has not been determined

(Moeller, 1975).

Rapid decompression of the pressurized atmosphere introduces the risk

of barotrauma to the lung. Overinflation

of lung tissue occurs when obstruction of

an airway prevents alveolar gas from escaping to the relatively lower pressure

of the atmosphere. The rupture of alveoli is likely to occur when intrapulmonary pressure exceeds atmospheric

pressure by 80 torr (Shilling, Werts, &

Schandelmeier, 1976; Billings, 1973A).

The subsequent leakage of air into tissues causes at lease one of the following

medical disorders; mediastinal emphysema, pulmonary interstitial

emphysema, air embolism, and

pneumothorax. Life is threatened by

these conditions when there is reduction

of the cardiac output, collapse of the

lung, or flow of bubbles into coronary

arteries and vessels of the brain (Waite,

Mazzone, Greenwood, & Larsen, 1967).

Depressurization Hazards

The physiological effects of depressurization are documented in numerous

studies of mountaineers and aviators.

One effect, hypoxia, is known to impair

the individual's physical and mental

function due to reduction of the P0 2.

Points A-C of Figure 7 represent the

threshold Po2's between four phases of

acute hypoxia as defined by the investigative work of Strughold (Randel,

1971). Point A illustrates the transition

from an "indifferent" phase of hypoxia

to "compensated" hypoxia when the PO2

8

20 -

15 -

10-

5-

0-

02%

I

I
STAGES OF CEREBRAL HYPOXIA

SAO2,

(McFARLAND. ANN NY ACAD SCI, 1970)

Fig. 8. STAGES OF CEREBRAL HYPOXIA.

scatterplot of data published by McFarland

is below 110 torr. According to

Strughold's classification, the "indifferent" phase of depressurization does

not impair human function. "Compensated" hypoxia stimulates the cardiorespiratory system, thus assisting the

transport of 02 in resting individuals.

However, decompensation occurs when

there is an additional exposure to heat

or the performance of exercise. The transition from "compensated" to "decompensated" hypoxia occurs at the P0 2 of 90

torr (point B, Figure 7). During "decompensated" hypoxia (also referred to as

"incomplete compensation") the cardiorespiratory system does not prevent

deterioration of physical and mental performance. A "critical" phase of hypoxia

develops when the PO2 falls below 65

torr (point C, Figure 7). "Critical"

hypoxia can severely degrade human

performance and cause unconsciousness. A lethal threshold for hypoxia is

believed to exist near point D of Figure 7

(Randel, 1971).

iUU

90

80

70

60

NO DECREMENT

MINIMAL

SENSORY

IMPAIRMENT

MINIMAL

MENTAL

IMPAIRMENT

GREATER

IMPAIRMENT

POTENTIAL

COLLAPSE

130 160

A visual best-fit curve is drawn through the

(1970). Percentage saturation of arterial

Strughold's thresholds (ie, points A-C

Figure 7) warn of the hazard of hypoxia,

but it is wrong to interpret the

thresholds as well defined changes in

physiological function that equally

apply to all persons. People differ in

their tolerances of hypoxia, as indicated

by published frequency distributions of

Po 2's known to impair the health of exposed populations (Randel, 1971).

There are conflicting opinions about

the minimum PO2 at which healthy persons can sustain their work performance. Residents live and work in cities

at low altitude where task performance

is apparently not impaired by hypoxia.

For example, air traffic controllers successfully manage the congested air traffic at Denver's airport (Po2 = 130 torr)

without breathing supplemental

oxygen. As residents, they have adapted

to low altitude by increasing their red

blood cell volumes (Knight, 1985). McFarland (1970) studied human cerebral

9

I I I I

75 90 110

P0 2 , TORR
function at altitude and in sea-level atmospheres of lowered 02%. His findings,

and those of other investigators, are

summarized in a plot of the SaO2 as a

function of altitude. Figure 8 is an adaptation of McFarland's plot in which the

X axis is converted from units of altitude to units of Po2. The curve is intersected by a zone of minimal sensory

impairment at Po 2's of 100-130 torr.

The existence of this zone is supported

by published reports of impaired visual

adaptation to the dark and induced body

sway at Po 2's of 120-130 torr (McFarland, 1971; Fraser, Eastman, Paul,

& Porlier, 1987; Billings, 1973B; Randel, 1971). Whether or not the zone of

minimal sensory impairment overlaps

with degradation of task performance is

a matter of controversy since investigators do not agree on the decrement of

psychomotor performance at Po 2's of

120-130 torr (Denison, Ledwith, & Poulton, 1966; Fowler, Paul, & Taysor,

1985). Figure 8 shows a zone of minimal

impairment in mental function at PO2 'S

of 100-85 torr and greater impairment

at Po 2's of 85-70 torr. Exposure to P0 2

< 70 torr may cause the individual to

collapse (McFarland, 1970). Based on

Figure 8, it is reasonable to postulate

that the minimum PO2 for unacclimatized humans is 130 torr, below

which the risk for hypoxic impairment

of health or performance increases with

the reduction of ambient PO2.

Figure 8 does not identify the

domains of Po 2 causing insomnia (Pappenheimer, 1984), impairment of industrial productivity (Tichauer, 1963),

reduction of maximum aerobic power

(Billings, 1973B), symptoms of acute

mountain sickness (Billings, 1973B;

Johnson & Rock, 1988), or significant interactions of hTpoxia with atmospheric

contaminants . It is therefore important to consider effects other than

cerebral hypoxia when attempting to

define the minimum PO2 for safe occupation of chambers.

Depressurization of the atmosphere

may cause barotrauma and DCS, both of

which were discussed as potential complications of N2 PRESSURIZATION.

DCS is known to occur when air is

depressurized to PB = 282 torr (point D,

Figure 7) and may occur at even less

severe decreases of PB (U.S. Naval

Flight Surgeon's Manual).

N2 Dilution Hazard.

With this approach, there is no

change of PB that could cause

barotrauma, nor does the slight increase

of PN2 produce narcosis. The effects of

hypoxia from N2 DILUTION are

presumably equivalent to the effects of

hypoxia from DEPRESSURIZATION

[1] There are literally hundreds of inorganic and organic vapors that contaminate submarine atmospheres. The prevalent contaminant, C02, exists at molar concentrations of approximately 1%.

10
ACUTE HYPOXIA IN SEA-LEVEL SUBJECTS (MEAN & SD)

SAO2, %

100 -

75 90

I Ij II

110 130

MENT OF COORDINATION *

ED SA0 2

NESS *

MOUNTAIN SICKNESS *

MENT OF MENTAL TASK *

RED NIGHT VISION

S I I I 60

160

P02 , TORR (* PC02 = 6.8 TORR)

Fig. 9. ACUTE HYPOXIA IN SEA-LEVEL SUBJECTS. A visual best-fit curve is drawn through the

mean values of experimental Po2's studied by the author and colleagues. The INSET describes significant

medical and physiological conditions encountered

legend for figure 8.

when both atmospheres contain

equivalent reductions of the P02.

STUDIES OF HYPOXIA AT NORMOBARIC PRESSURE

Six experiments were performed by

military scientists (see ACKNOWLEDGEMENTS) in which separate

groups of volunteers 2 breathed 10%,

12%, 13%, or 17% 02 without knowing

the composition of the gas. The objective

was to determine a minimum atmospheric PO2 that permits safe occupancy

of chambers for periods of days. All subat the specific isobars of 02. Axes were defined in the

jects were trained to perform one or

more tasks of mental function before the

experiment began and all completed a

control experiment in 21% 02. Measurements were made of the SaO2, moods,

symptoms of acute mountain sickness,

mental function, muscular coordination,

night vision, visual contrast sensitivity,

and speech discrimination. The details

of experimental design and data

analysis have been reported by the

various investigators (Knight, Luria,

Socks, & Rogers, 1987; Luria & Knight,

1987; Marshall, 1987; Schlichting,

Knight, & Cymerman, 1988; Shukitt,

[2] The nature and purpose of the studies and the risks involved were explained verbally and given

on a written form to each subject prior to his voluntary consent to participate. The protocol and

procedures for the studies were approved by the Committees for Protection of Human Subjects at

the Naval Submarine Medical Research Laboratory, New London, CT and the U.S. Army

Research Insitute of Environmental Medicine, Natick, MA.

11

-----------------------------

I A. DECREI

A.

B.- LOWER

DIZZI

e. ACUTE

DECREI

C. c. IMPAII

I ________________

90 --1

80

70

60
Burse, Banderet, Knight, & Cymerman,

1988; Luria & Morris, 1988).

Figure 9 illustrates the following:

The standard deviation bars illustrate the variability in SaO2'S

of unacclimatized subjects exposed to various PO2.

129 TORR 02 (17% 02):

The mean Sa02 (97%) did not significantly differ from a mean control value of 98% and there were

no changes of moods or symptoms

of acute mountain sickness. The

mental and visual performance

tests were not degraded by 4-h exposures to uncontaminated gas

mixtures. The 2.6-d exposure to

129 torr 02 was purposely contaminated with 6.8 torr C02 in

order to simulate the atmospheres existing aboard submarines[11 . One task of muscular

coordination was slightly

degraded during the first 4 h of

the contaminated exposure, but

this impairment of coordination

did not degrade the subjects' performance of other tasks. The impairment disappeared with

continuation of the exposure

(Schlichting, Knight, & Cymerman, 1988).

99 TORR 02 (13% 02)-6.8 torr

C02-balance N2:

There were transient bouts of impaired coordination and impaired

mental function during the first 4

h of the 2.6 d exposure. These

were followed later by the onset of

dizziness. Forty percent of the

subjects developed symptoms of

acute mountain sickness by the

11th h. The incidence of mountain sickness and dizziness

diminished in d 2 and all

symptoms disappeared by d 2 of

the recovery in 21% 02 (Shukitt,

Burse, Banderet et al., 1988;

Schlichting Knight, & Cymerman, 1988).

87-90 TORR 02 (12% 02):

A 2 h exposure reduced the mean

Sa02 to 80% without causing

symptoms of hypoxia in resting

subjects. This degree of hypoxia

was not associated with changes

of the visual function tests,

speech discrimination, or the performance of an arithmetic task.

76 TORR 02 (10% 02):

A 10 min exposure reduced the

mean SaO2 to 77%, induced dizziness, and degraded the night

vision. This was the only exposure to change any of the visual

function tests (Luria & Knight,

1987).

The results indicate that muscle coordination and mental performance quickly acclimatize to hypoxia after reduction

of the atmospheric P02 to 130 torr.

Therefore, N2 dilution may be used to

reduce the 02 concentration to 17%.

Further reduction of the P02 to 100 torr

induces symptoms of acute mountain

sickness in unacclimatized people. Additional experimental work is needed to

determine if exposures to 110-120 torr

02 produce symptoms of acute mountain

sickness or transiently impair

psychomotor performance.

12
APPLICATIONS TO SUBMARINES.

Adjunct strategies may enhance the

physiological tolerance of reduced 02

concentrations produced by N2 dilution.

Physiological adaptation to hypoxia will

abolish the discomfort of multiday exposures to 13% 02(PO2 99 torr), as

reported by Shukitt, Burse, Banderet, et

al., (1989). Gradual use of N2 DILUTION to reduce the 02 concentration

below 17% may avoid the discomfort of

abrupt, multiday exposures to 13% 02.

A second adjunct is the addition of a

respiratory stimulant, C02, to the atmosphere. C02 is known to enhance 02

delivery to the tissues by stimulating

ventilation, improving the release of 02

from hemoglobin, and dilating blood vessels. Preliminary studies indicate that

13% 02 - 3% C02 - balance N2 does not

degrade mental performance in resting

subjects (Knight, Luria, Socks, et al.,

1987). A third adjunct is the use of N2

PRESSURIZATION to supplement N2

DILUTION, as discussed in the following concluding paragraphs.

suppression (Huggett, 1969). The subsequently proposed use of 130 torr 02-

balance 870 torr N2 (Knight, 1985) took

into account the hazard of DCS in flamesuppressant atmospheres. More modest

N2 pressurization may provide safe atmospheres for a variety of human activities, particularly if DCS can be

completely avoided by limiting the exposure to a maximum atmospheric PN2

of 760 torr (the tissue PN2 would not exceed 760 torr under these conditions).

This is based on the theory that no risk

for DCS exists when the ratio of tissue

PN2 to ambient PB < 1 (Behnke, 1967).

In a binary gas mixture of N2 and 02, a

range of safe maximum PB'S can be computed as a function of the atmospheric

P0 2. A sample range is shown in Table 2.

N2 PRESSURIZATION OF 17% 02

21

12 -

I

0

--AIR------------

-PN2 760 TORR

80

I I1 1 1

(130) 160

P02 , TORR

Huggett suggested that an atmosphere of 128 torr 02-balance 722 torr N2

reduces the flame hazard of combustible

materials by raising the heat capacity of

the atmosphere. His reason for raising

the PN2 to 722 torr was based on flame

Fig. 10. N2 PRESSURIZATION OF 17% 02. Sea

level air is initially modified by N2 dilution (open

aarrow) and further processed by N2 pressurization

(solid arrow). The solid curve is a N2 isobar for 760

torr. Intercepts of the dashed lines are discussed in

the test for Table 2.

13

TABLE 2

Safe max PB

P02 torr (torr) %02

120 880 13.6

130 890 14.6

140 900 15.5

17
Figure 10 presents a scheme for avoiding hypoxia and DCS during the occupation of a flame-suppressant atmosphere.

The open arrow depicts the use of N2

DILUTION to lower the 02% to 17,

where the P0 2 is 130 torr. Additional N2

PRESSURIZATION is used for two

reasons; (a) the 02% is lowered to 14.6

without changing the PO2 and (b), the atmosphere can be decompressed to PB -

760 torr without exposing crews to the

hazard of DCS. The incidence of

barotrauma can be minimized by

using very slow rates of compression

and decompression. Experimental support for the habitability of such an

atmosphere comes from studies showing

that subjects can safely breathe either

11%or 13%O2 at PO2 = 130 torr for

4 h without hypoxemia or decrement

of psychomotor performance (Knight,

Luria, Socks, et al., 1987).

ACKNOWLEDGEMENTS

I am especially grateful for the advice

and help of my colleagues and collaborators, who are: Dr's. S. Luria, C.

Schlichting, W. Rogers, J. Socks, L.

Marshall, A. Messier, and D. Tappan (deceased) of the US Naval Submarine Medical Resarch Laboratory.

Dr's. A. Cymerman, R. Burse, L.

Banderet, P. Rock; Mr's. C. Fulco and J.

Devine; and, Ms. B. Shukitt of the US

Army Research Institute of Environmental Medicine.

DISCLAIMER

The views expressed in this article

are those of the author and do not reflect

the official policy or position of the

Department of the Navy, Department of

Defense, or the U.S. Government.

SYMBOLS

d day.

h hour.

02 molecular oxygen.

02% The molar concentration of

oxygen, expressed as a percentage of the gas mixture.

N2 molecular nitrogen.

PAOG The combined partial pressures of gases other than

oxygen and nitrogen in the

atmosphere, expressed in

units of torr.

PB The barometric press u r e, expressed in units of

torr.

PN2 The partial pressure of

nitrogen, expressed in units of

torr.

P02 The partial pressure of

oxygen, expressed in units of

torr.

s seconds.

Sa02 The 02 saturation of arterial

hemoglobin, expressed as a

percentage of complete saturation.

14
DEFINITIONS

Air: 78% nitrogen, 20.9% oxygen, 0.9%

argon, and additional gases in

trace concentrations.

Air embolism: Bubbles of air that flow

in the bloodstream until blocked by

vessels of smaller caliber than the

bubble diameters.

Decompression sickness (DCS): A

group of illnesses characterized by

formation of gas bubbles in blood or

tissues, caused by release of physically dissolved gases when environmental pressure is decreased at

sufficient rate and magnitude (Bartels, Dejours, Kellogg, & Mead; 1973).

Flashover: The sudden spread of flame

over an area when it becomes heated

to the flash point.

Hypoxemia: A state in which the

oxygen pressure and/or concentration

in arterial and/or venous blood is

lower than its normal value at sea

level (Bartels, et al.; 1973).

Hypoxia: A physiological state in which

the amount of 02 in the lung, blood,

and/or tissues is abnormally low compared to that of normal resting man

breathing air at sea level (Bartels, et

al.; 1973).

Hemoglobin: The protein in red blood

cells that transports 99% of the 02

carried by the blood stream.

Incomplete combustion: The self extinction of a flame before there is complete burning of the test material

(Cook, Meirer, & Shields, 1967).

Mediastinal emphysema: The accumulation of air in normally gasfree tissue spaces surrounding the

heart, esophagus, and large blood vessels in the chest.

N2 narcosis: Depression of brain function due to high partial pressure of

nitrogen (Bartels, et al.; 1973).

Normobaric pressure: The

barometric pressure of approximately 760 torr. The variation of normobaric pressure may be defined by

the range of barometric pressures

measured at sea-level.

Oxygen saturation (SaO2): The amount

of oxygen combined with

hemoglobin, expressed as a percentage of the 02 capacity of that

hemoglobin (Bartels, et al.; 1973).

Pneumothorax: The escape of air into

the pleural space of the lung.

Pulmonary oxygen toxicity: The injury

to lung tissue caused by exposure to

Po 2 > torr. The degree of injury can

progress to an irreversible stage

depending on the level of Po 2 and

duration of exposure (Shilling,

Werts, & Schandelmeier, 1976).

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18
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11. TITLE (Include Security Classification)

The Medical Hazards of Flame-suppressant Atmospheres

12. PERSONAL AUTHOR(S)

Douglas R. Knight

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16. SUPPLEMENTARY NOTATION

17. COSATI CODES 18. SUBJECT TERMS (Continue on reverse if necessary and identify by block number)

FIELD GROUP SUB-GROUP Barotrauma, N2 Narcosis, Decompression sickness, Hypoxia,

Chamber fires, Submarine medicine

19. ABSTRACT (Continue on reverse if necessary and identify by block number)

Flames are a potential hazard to the occupants of sealed chambers. This report describes

four modifications of air that will suppress or extinguish flames. They are: 1.

SUPPLEMENTATION - the addition of an appropriate foreign gas to air. 2. N2 PRESSURIZATION

- the addition of compressed N2 to air. 3. DEPRESSURIZATION - the partial evacuation

of air from the chamber. 4. N2 DILUTION - the exchange of N2 for 02. The primary medical

hazards of flame-suppressant atmospheres are barotrauma, N2 narcosis, decompression

sickness, hypoxia and inhalation toxicity. Experimental evidence supports the use of

N2 DILUTION to suppress flames aboard patrolling submarines. One or more of the following

adjuncts may enhance the use of N2 DILUTION without impairing human health: Physiological

adaptation to hypoxia, addition of C02 to the atmosphere and N2 PRESSURIZATION.

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