How smoke escape 

from coal mine 

F.N. Kissell and C.D. Litton 

thelevel of smoke thatminers 

might meet while trying to escapea coal minefire and describes 

how smoke would impede their safe escape. For this study, the 

authors assumed that miners exit through an escapeway 

cent to burning entry, that some air would leak from the 

burning entrytotheescapeway.Thefollowing conclusionswere 

A very low amount of air leakage to the escapeway can 

produce a critical level of visual obscurity from smoke. This 

shows that intake airways may be as susceptible to smoke 

visibility

* This critical level of smoke obscurity is reached much 

problems as return airways. 

before a critical level of carbon monoxide is reached. 

can causea severesensory irritationthatfurther 

limits vision. Thus,reliable eye protection is very important. 

These results show that methods to guide miners through 

dense smoke will contributegreatly to saving lives during mine 

fires. 

Intrsductisn 

The aim of this research was to look at the role of smoke 

in hindering escape from coal mine fires, from either lack of 

visibility or sensory irritation. To do this, the authors 

forecasted smoke levels relative to fire size and carbon 

monoxide levels using data from conveyor belt test fires. 

Smake visibility and sensory irritation 

Visibility 

For in buildings, various researchers have investigated 

the relationship between smoke density and visibility. Jin 

(1981) concluded the upper limit for adequate Visibility corresponds optical

forpersons who familiar with the escape route. These 

equate to visibilities of 13and 4 m (42 and 13ft), respectively. 

Rasbash 975) concluded the visibility limit is at a smoke 

optical density value of corresponding to a 10-m

visibility minimum. He determined visibility values using a 

black letter on a white background. Thetest used focused 

headlampsheld waist-high while thesubjectwore 

a breathing apparatus. 

Because of the short travel distance involved, Babrauskas 

(1979)used an optical density of asan obscuritycriterion 

for escape from containing burning furniture. 

For fires in mines, Heyn (1977) measured the relation 

between smoke density and visibility in the Tremonia Experimental Mine in Germany, with similar results. In his 

tests, small conveyor belt fires resulted in visibilities of afew 

decimeters. Heyn also pointed out that water vapor condensation can make the smoke thicker. 

Sensory irritants 

Smoke cloud irritants play a role in escape from fires. It 

is well known that smoke clouds contain a variety of sensory 

irritants that can make it impossible to see or breathe. For 

example, hydrochloric acid is a common combustion 

product in coal mine conveyor belt fires. While not likely to 

be as lethal as carbon monoxide, it is a severe eye, nose and 

throat imtant. Purser (1988) reported that severe sensory 

irritancy occurs at an concentration of 100 ppm. 

Tewarson and (1981) gave 50-100 ppm as the 

critical value for escape from fires. Research by Smith and 

Kuchta (1973) on burning belts showed that the production 

of HCI averages about 12% of the production. 

Rasbash (1975) reviewed the impact of smoke cloud 

irritants. He indicated that eye irritation further decreased 

visibility. Also, lab workers used as subjects (Jin, 1981) 

reported that at the end of the test (optical density 0.2 to 0.3) 

the irritation and suffocation they experienced were about 

the most they could tolerate. Because of limited visibility in 

the test room, they could see only a small floorarea around 

their feet. The smoke was from wood chips in an electric 

furnace. levels had not reached 50 

F.N. Kissell,member SME, is research supervisor and C.D. Litton is 

supervisory physical scientist, US Bureau of Mines, Pittsburgh, PA. 
KEY 

X

0 

0

0 

m 

a 

Optical density 

0 20 60 80 

TIME FROM START OF COAL 

FLAME, min 

Fig. 1-Downstream carbon monoxide and smoke density levels 

during the growth of a typical SBR test fire. 

Smoke in a single entry 

Conveyor belt fires 

For many years, the Bureauof Mines has conducted

in which various materials are burned to simulate a mine fire 

Oxygen 

I 

I 

l2 

8 

6 

4 

i 

E 

TIME FROM START OF COAL FLAME. min 

Fig. 2-Downstream oxygen level and during the growth of a 

typical SBR test fire. 

OD = 

where: OD = smoke optical d = length of light 

path in meters, =,initial transmission on light path in clear 

air, and = transmission on light path in smoke. 

KEY 

..- Visibility -

and 

and Recently,

and (1991) studied the detection of 

conveyor belt fires. For these tests, they 

continuously measured fire products in the 

air as a pile of coal under the belt first 

smoldered,broke intoflame, then setthebelt 

on fire. These tests were in a tunnel that 

simulated a single mine entry. 

Figures and 2 illustrate the conditions 

20 m (65 ft) downstream during the early 

growth stage of a typical test fire with sty­ rubber (SBR) belt. Time 

measures from the instant that flames erupt 

from the coal pile. The air velocity over the 

fire was (200 fpm) and the air 

quantity was 7.6 cfm). The 

carbon monoxide and oxygen concentrations in theair were measured with infrared 

and electrochemical sensors. The smoke 

optical density was measured with a light 

intensitymeteremploying aprescribed light 

as follows (Rasbash, 1975): 

,oo 

Direct visual 

VISIBILITY, ft 

I80 

6o 

140 

60 

4o 

20 

0 

Fig. smoke visibility and carbon monoxide levels during the growth of a typical 

SBR test fire. 
Note that the trend of the curves in Fig. indicates that the 

ratio does not vary much over the entire 90 min. At 

10rnin ,the ratio is 120 ppm x m. At 90 min, it is 93 ppm x 

m. 

Calculating visibility 

Values for visibility, or how far an individual can see in 

the smoke, can be calculated from the optical density. The 

relation between the two can be derived in the following 

manner: 

According to Rasbash an optical density value of 

0.08 corresponds to a 10m (33 ft) visibility. Assume a test 

in which measurements are conducted with a smoke density 

meter, and the path length over which measurements are 

made is m (33 ft). Assume the smoke density during the 

test is such that an optical density of 0.08 is computed, or 

0.08 = From this, = 0.8 

In accordance with Rasbash visual observations 

during the test indicated that the visibility limit was 10m (33 

ft). Thus, given a path d, where the smoke level is such that 

is equal to 0.8, then d is the visibility. 

It follows generally that OD=( So, if the optical 

density is known, the visibility can be calculated. The 

calculated Visibility values at various fire times are shown in 

Fig. 2, and the direct between CO and visibility, 

using the information in Figs. 1 and 2, is in Fig. 3. 

Direct visual observations on videotape 

The visibility values in Figure 3 correspond to surprisingly low CO values. Therefore, to gain visual proof of this 

information, the authors conducted a small test fire with 91 

(200 of coal and 0.28 (3sq ft) of belt in a x 

m x 18-ft)entry of the Experimental Mine at the Bureau’s 

Lake Lynn Laboratory. 

Downwind of the fire in the same entry at a distanceof 270 

m (900 ft), a video camera was set up with a single light next 

to it. Three cardboard signs were placed at 3,6 and 9 m (10, 

20 and 30 ft) from the camera, as well as a mannequin 4.5 m 

(15 ft) from the camera. The signs showed the distance in 

267 mm in.) black letters on a white background. This 

is similar to the approach used by Rasbash (1975) for 

visibilityexperiments. Near the signs, the COconcentration 

was measured at the roof and halfway up one rib. As the fire 

grew, the was noted at the time each of the 

signs and the mannequin disappeared from view as the 

smoke grew thicker. The CO data and the video camera 

showed no stratification, although close to the fire there was 

a thick smoke layer at the roof with clear air underneath. 

Traveling 270 m (900 ft) was enough to mix the smoke and 

CO evenly into the airstream. The results are shown in Fig. 

3 as “direct visual observations.” 

For example, the 9 m (30 sign disappeared at an 

average CO concentration of 10.4 ppm, the 6 m (20 ft) sign 

at 17.7 ppm, etc. The resulting curve confirms well the 

visibility and vs. time values from Figs. and 2. 

Analysis single airway data 

and (1981) gave some tentative critical 

values of various fire contaminants that affect escape from 

mines. For visual obscurity in smoke, their critical optical 

density value is This equates to a 3.7-m 

acceptablevisibility minimum. For CO, theircritical maximum 

ENTRY WITH FIRE ESCAPEWAY 

Fire 

Contaminants at 

concentration 

Airflow, 

Leakage, 

a 

Contaminants at 

concentration 1 

Fig. with fire and adjacent escapeway. 

value is 1500ppm. For the test fire illustrated in Figs. 1 and 2, 

the smoke density reachesthe minimum acceptablevisibility at 

13min. At this 13rnin point,the COlevel was 24 Mitchell 

1990)gave a rule of thumb that the buming of 0.45 (1 lb)of 

asan indicator of fire size for an equivalent coal fire. Given an 

airflow of 7.5 cfm), the burning rate of an 

equivalentcoal fire at 13 rnin was 0.36 (0.8 lbs per min). 

As the fire grew,the critical maximum CO value of 1500ppm 

was reached at 5 1min. At this point,the calculated burning rate 

of an equivalent coal fire was 22 (48 per min), or 60 

times greater. 

In summary, small fires in their early growth stage can 

produce minimum smoke visibility levels. 

Smoke in the adjacent escapeway 

Only miners escaping through the entry on fire would experience the contaminant levels shown in Figs. l and 2. It is more 

likely that miners would choose to escape through an entry that 

was less contaminated. So,it is betterto assume that miners will 

use an escapewaynext to the entry on fire(Fig. 4).Assumealso 

some leakage from the entry containing the fire into the 

escapeway, typically through several stoppings. The aim here 

is to determinethe visibilitylevel along with theCOand oxygen 

concentrations in the escapeway. 

Escapeway contaminant concentrations are calculated 

with a simple dilution equation. For example, if is the 

leakage from fire airway to the escapeway and the 

original amount of air flowing in the escapeway, then: 

where is the of in theescapeway and 

is the concentration of contaminant in the fire airway. For 

at the60 point, forcarbonmonoxidefrom is2700 

ppm. 
.05 0.1 

LEAKAGE, 

20 

a

* 

-

-

-

-

-

a 

LEAKAGE, cfrn 

to a wide variety of underground fires 

in their early growth stage. For example, coal fires alone give off similar amounts of smoke relative to their 

CO production. It could be argued 

that different critical values for smoke 

and CO should have been used. How- ever, when any set of 

cal values for smoke or CO (Purser, 

1988)are used, the general outcome is 

the same. Other information relating 

smoke to carbon monoxide is given in 

the appendix. 

Irritants in smoke , 

Sensory irritation from smoke may also 

hinder escape from coal mine fires.Therefore, some subjective observations of 

smoke cloud irritation were made during 

Fig. escapewaysmoke visibility, oxygen and carbon monoxidevs. a conveyor belt test tire. In this test, 910 

fire at 60 minutes. of coal and a 2.1-m (7-ft) 

This simpledilution also applies to the smoke concentration 

expressedinopticaldensity.Forexample, in 

= from Fig. at 0.94 cfm) leakage: 

Escapeway OD = Fire airway OD 

= Since visibility = then escapeway visibility 

0.812.67 = 0.3 m. 

= 29.3 

= 

The escapeway oxygen percentage is calculated the same 

way by assuming the oxygen deficiency (AO,) is the contaminant. 

Note that since the same formula is used for escapeway 

OD and CO, the ratio of CO to smoke OD is the same in both 

the escapeway and the fire airway. As a result, the data 

shown in Fig. 3 also apply to the escapeway. 

Analysis of the adjacent escapeway data 

Using these dilution equations, escapeway concentrations were calculated as a function of leakage for the min 

fire,which has an equivalent coal burning rate of 31 

(67 per These escapeway concentrations are shown 

in Fig. 5, with smoke and CO critical values from Tewarson 

and (1981). 

The implications of these data forescaping miners and for 

fire protection are startling. For the fire, the smoke 

density reaches the visibility minimum at only 0.094 

(200 cfm) of leakage. However, the CO concentration only 

approaches the critical maximum at leakages more than 9.4 

cfm). The escapeway oxygen level is adequate for the fire size and leakage range shown. With the 

critical values selected, the visibility drops below 

able minimum at a leakage rate less than of the leakage 

producing the CO critical maximum. Smoke seriously 

hindersminers before the fire grows largeenough to produce 

a carbon monoxide or oxygen deficiency 

These test results were obtained with combined SBR 

belt-coal fires. However, since the CO-to-smoke OD 

ratio for different burning materials varies within a 

limited range (see Table in the appendix), the 

authors believe that the same general results may apply 

of belt burned. The CO concentration and subjective to the smoke were noted: 

CO concentration Reaction 

up to 40 ppm Mild discomfort. Breathing labored 

and eyes mildly irritated. 

Hard to breathe. Eyes stung. 

160 ppm Very difficult to breathe. Severe 

eye irritation.Could barely see. 

It is significant that severe sensory irritation can take 

place at CO levels that do not represent an immediate 

carboxyhemoglobin danger. These results on sensory 

tion generally the work of Rasbash (1975) and Jin 

but are tentative. More research on smoke irritation 

from coal fires should be conducted. 

Conclusions 

Smoke is a key factor in escape from mine fires. In 

particular, if a fire is in the early growth stage, escaping 

miners will meet with visibility problems before any other. 

The minimum acceptable smoke visibility is reached before 

the critical maximum carbon monoxide value. This means 

that methods to guide miners through dense smoke may 

contribute greatly to saving lives during mine fires. The low 

leakage quantities necessary for poor visibility indicate that 

intake airways can be as susceptible to smoke visibility problems as return airways. 

The role of the self-contained self-rescuer in isolating the 

lungs from smoke cloud irritants is vital since unbreathable 

levels of irritants may be more common than oxygen deficiencies. Severe sensory irritation is possible at CO levels 

indicating a small fire or very low air leakage. This means 

that reliable eye protection is very important.

The authors gratefully acknowledgeRon 

Lazzara for their invaluable help with the visual observation 

and smoke irritation tests. 
References 

Babrauskas. Vytenis. 1979. 'Full-scale burning behavior of burning chairs," NBS 

TechnicalNote 1103, National Bureau of Standards, Washington DC. 

Heyn. Wolf. 1977, "Underground measurement of smoke density at the Tremonia 

Experimental Mine in order to determine visibility in gases: Proceedings, 17th 

International Conferenceof Directors of Safety in Mines Research. Varna. Bulgaria. 

Jin. T.. 1981, 'Studies emotional instability in smoke lrom tires,' Journalof Fire and 

12, April. pp. 

J.M.. al.. 1982, 'Diagnostics of SealedCoal Mine Fires: US Bureauof Mines 

Reponof Investigation8625.25 

Lazrara. C.P.. and Perzak. F.J.. "Conveyor belt flammability studies." Proceedings. 21 AnnualConferenceon MiningHealth, Safetyand Research.

VA. 

Litton. C.D., 'Product-of-combustion lire detection in mines,' chapter in 'Underground metal and mine fire protection," US Bureau of Mines 

Circular 8865. 

Litton, submicrometer particlesin sealed fire areas." US 

Appendix-Other information relating smoke 

monoxide 

Woodfires 

The findings relating smoke to in coal mine fires have 

some precedent. For wood fires in hard rock mines, Litton 

(198 found that the "critical fire size"for CO is 36 times 

greater than that for smoke. "Critical fire size" was defined 

as the size of fire beyond which escape is either impossible 

or marginal. 

Smolderingfires 

Many fires will smolder for a long time before breaking 

into flames. In general, a fire that is large enough to 

contaminate the adjacent airway has already passed from the 

smoldering to the flaming stage. Nevertheless, it is worthwhile to at the ratio of smoke to CO for smoldering fires 

also. Litton 989) gave some relative CO values for flaming 

and smoldering fires that are equivalent to a smoke optical 

density of (Table A-1). Depending on the material 

burning, the smoke-to-COratio is 1.6to 13.8 times higher for 

a smoldering fire than it is for a flaming fire. In short, for a 

given CO level smoldering fires generate more smoke than 

flaming fires. 

Table A-1-Relative values of CO at a smoke sensor alarm 

level of OD 

Fuel ppm flaming ppm CO smoldering 

Wood 11.1 7.1 

Coal 3.4 

belt 3.7 0.5 

PVC belt 8.3 0.6 

Neoprene belt 6.3 1.4 

Transformer fluid 1.5 - PVC - 2.2

Bureau of Mines of Investigation 20 pp. 

Litton, 'Relationships between smoke and monoxide and their 

implicationtowardminefire SafetyinMines 

Research Institutes. Washington DC, September. 

Litton, C.D.. and Perzak. F.. 1991. of belt fires.' US 

Bureau of Mines of to be published. 

Mitchell, D.W.. Mine Fires. Hunter. Chicago. 67. 

Purser, 1988. *Toxicity Assessment of Combustion products," chapters 1-14 in 

The Handbook of Fire Engineering, NationalFire ProtectionAssociation, MA. 

Rasbash. 1975. "Sensitivity criteria lor detectors used to life," Fire 

Vol. No.40. pp. 

Smith. A.F. and Kuchta. J.M.. 1973, 'Toxic products from burning of fire-resistant 

materials,' US Bureau of Mines TechnicalProgressReport February, 19 

Tewarson, A.. and J.S.. 1981. "An experimental investigation of the fire 

hazardsassociatedwith timber setsinmines," chapter in'Underground metaland nonmetalmine fire protection." USBureau of Mines Circular8865, pp. 

Note also from Table A- that the CO-to-smokeOD ratio 

varies within a limited range for different flaming materials. 

Observations by mine rescue teamslthe hazard 

The levels of smoke and CO used to Figs. 1-5are 

only from fires in the early growth stage. 

During the later stages of fire growth, the available fuel or 

oxygen dips below the level necessary to sustain it. Then the 

fire begins to die out, and the smoke begins to dissipate. 

Smoke dissipates because there are loss mechanisms for 

smoke particles, such as sedimentation, which will lower 

smoke levels. This is not true for CO, which remains for 

much longer periods. For example, Kuchta et al. (1982) in 

full-scale studies of sealed coal mine fires saw CO levels 

increase from ppm to ppm four days after 

sealing, while the smoke level decreased by a factor of 10. 

Other data acquired also show that in a 

sealed (or quiescent) mine, the smoke level not only decreases at the source of the fire that is dying out, but also as 

the distance from the fire source increases. 

Since mine rescue teams are not in the mine during the 

early stages of a fire, they may see conditions different from 

those in Figs. through 5. Discussionswith individuals from 

these teams have indicated they see no fixed relationship 

between the smoke visibility and the CO concentration. 

Moreover, they have measured as much as 1500ppm of CO 

without any smoke visibility 

Suppression of fires, such as with an automatic 

system, may result in higher CO levels relative to the amount 

of smoke. One important conclusionto draw from this is that 

the miner cannot use good visibility as a guaranteed indicator of low CO levels. 

The role of smoke in hindering escape has been emphasized here by pointing out that the smoke hazard arrives 

before the CO hazard. One should not, however, minimize 

the CO hazard from mine fires. The coal industry is well 

aware of the hazards presented by CO, which need no 

here.