1

ABSTRACT

Researchers from the National Institute for Occupational Safety and 

Health (NIOSH), in cooperation with mine operators, conducted a study 

of heat stress exposures among mine rescue workers in underground 

mines. Mine rescue workers face extreme heat loads as they enter hot 

and poorly ventilated environments, particularly when they are wearing 

breathing apparatuses. The burden of wearing a closed-circuit breathing apparatus (CCBA), the inability to drink fluids for extended periods 

of time, and the potential for ventilation disruptions and fires combine 

to increase the risk of heat illness during a response to an emergency. 

In this research, ambient environmental conditions and heat strain 

indicators were measured using conventional ventilation monitoring tools 

during mine rescue training exercises. Heat strain was measured with 

an ingestible temperature-sensing pill that measured core temperature 

continuously. In addition, a heart-rate monitoring chest strap was used 

to indicate physical strain resulting from physical activity and heat. Both 

core temperature and heart rate data were trans-mitted to a remote 

recorder worn by a rescuer and time-stamped. Team activities were 

also observed to determine the contribution of work rate to a person’s 

total heat load. The effectiveness of several engineering controls, such 

as cooling the air inhaled by rescuers through an apparatus, wearing 

cooling vests, and supplying water through the CCBA masks, were investigated. Administrative controls, such as limiting the duration of exposure 

and controlling the work rate through forced-rest regimens, were also 

evaluated. Results to date are discussed, and tools for evaluating team 

condition and estimating the length of time it would be safe for rescuers 

to work are proposed.

INTRODUCTION

Researchers from the Spokane Research Laboratory (SRL), 

National Institute for Occupational Safety and Health (NIOSH), in cooperation with mine operators and mine rescue associations, have conducted a study of heat stress exposure among mine rescue workers in 

underground mines. Mine rescue workers face extreme heat loads when 

they wear breathing apparatuses and enter hot and poorly ventilated environments. The burden of wearing a closed-circuit breathing apparatus 

(CCBA), the inability to drink fluids for extended periods of time, the high 

level of activity and exertion required to perform duties, and the potential 

for ventilation disruptions and fires combine to increase the risk of heat 

illness during response to an emergency (Goldstein and Kajdasz, 2000; 

Kampman and Gresser, 1999; Kampman et al., 1997; Mittal et al. 1991; 

Mine Safety and Health Administration [MSHA], 2003).

Mine rescue operations in the United States resulted in a heatrelated double fatality in October 2002 when two members of a team 

exploring an abandoned mine slope in Nevada were lost (MSHA, 2003). 

In Poland in 1998, 10 mine rescuers were overcome by heat during an 

exploration activity and subsequent rescue attempts, resulting in six 

fatalities (Goldstein and Kajdasz, 2000). While these tragedies draw 

significant attention, they are the extreme result of exposure to the heat 

stress that can occur during any mine rescue work. Thermally stressful 

environments are known to have a negative impact on workplace safety 

(Ramsey et al., 1983). For each of these fatalities, numer-ous cases of 

undocumented heat illness are also likely to have occurred that were 

reversed through first aid or termination of the activity. For each case of 

heat illness, many cases of impairments to judgment and reaction times 

are probable; such impairments can cause errors that lead to catastrophic 

results in emergency situations.

HEAT STRESS, STRAIN, AND ILLNESS

The American Conference of Government Industrial Hygienists 

(ACGIH, 2001) defines heat stress as “the net heat load to which a 

worker may be exposed...” Heat strain is defined as “the overall physiological response resulting from heat stress.” The goal of this research is 

to quantify the heat strain (in terms of net heat gain) experienced by mine 

rescuers when subjected to heat stress during their work. Heat illness is 

the result of excessive strain on the body and is a highly variable human 

response to heat stress. 

A STUDY OF HEAT STRESS EXPOSURES 

AND INTERVENTIONS FOR MINE RESCUE WORKERS

F. Varley

National Institute for Occupational Safety and Health

Spokane, WA

Table 1. Screening criteria for heat stress exposure (WBGT in Celsius)

Work demands Acclimatized Unacclimatized

Light Moderate Heavy Very heavy Light Moderate Heavy Very heavy

100% work 29.5 27.5 26 27.5 25 22.5

75% work, 25% rest 30.5 28.5 27.5 29 26.5 24.5

50% work, 50% rest 21.5 29.5 28.5 27.5 30 28 26.5 25

25% work, 75% rest 32.5 31 30 29.5 31 29 28 26.5
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ACGIH has identified the thresholds for illness in terms of core body 

temperature. While these thresholds may be exceeded by some individuals without ill effect, the application of these guidelines will ensure protection of workers and teams, as a single illness affects the safety of the 

entire rescue team. The threshold level of core temperature associated 

with loss of judgment and reaction time is 38 °C (100.4 °F). At a core temperature of 38.6 °C (101.5 °F) and above, physical heat strain has begun 

and, if not treated, will progress to acute heat illness and eventually the 

life-threatening condition of heat stroke (ACGIH, 2001).

NIOSH and ACGIH provide guidelines for work in hot environments 

that seek to control heat strain and stress by limiting the time spent working in a hot environment

Table 1 shows the current recommendations based on wet bulb 

globe temperature (WBGT) index values and level of activity (ACGIH, 

2001; International Organization for Standardization [ISO], 1989a; 

National Institute for Occupational Safety and Health [NIOSH], 1986). All 

rest is to be in cool environments

METHODOLOGY

In this research, ambient environmental conditions and heat strain 

indicators were measured on mine rescuers during mine rescue training 

exercises. The research described in this article was carried out between 

March 2002 and July 2003. Environmental monitoring was con-ducted 

using conventional ventilation monitoring tools. Heat strain was measured using an ingestible temperature-sensing pill that measured core 

temperature continuously. In addition, a heart-rate monitoring chest strap 

was used to monitor heart rate continuously, which was a surrogate for 

physical strain resulting from activity and heat. Both core temperature 

and heart rate data were transmitted to a remote recorder worn by a 

rescuer and time stamped. Activities of the teams were also observed to 

determine the contribution of work rate to a person’s total heat load. The 

focus of this research was to bridge the knowledge gap between laboratory-based studies under controlled, but assumed relevant, conditions 

and work rates encountered in underground mines.

Environmental monitoring 

Environmental conditions during mine rescue activities were 

monitored using conventional psychometric instruments, both digital and 

analog, to record ambient air temperature, natural wet bulb temperature, 

radiant (globe) temperature, and air speed. These data were used to calculate a WBGT heat index value for the exposure. WBGT for low radiant 

heat exposures such as in mines is calculated as

 WBGT = 0.7 Tnwb +0.3 T globe

where Tnwb = natural wet bulb temperature

and T globe = globe temperature 

(Goldstein and Kajdasz, 2000; Clayton and Clayton, 1991; Dinardi, 1988; 

Plog, 1996; NIOSH, 1986). 

In all cases where the activity was performed underground, the 

radiant temperature was essentially the same as the air temperature. It is 

important to use the natural wet bulb temperature and not the maximum 

wet bulb depression. Natural wet bulb temperature is the temperature 

achieved due to actual air flow and not air flow forced by swinging the 

psychrometer. Natural wet bulb temperature is therefore a reasonable 

indicator of potential evaporative cooling of an individual based on 

ambient air velocities (Brake et al., 2001). Results are grouped by WBGT 

exposure. 

Core temperature monitoring

The research utilized a relatively new wireless technology to monitor 

rescuers’ core temperatures without interrupting or otherwise impeding 

their activities (Cutchis et al., 1988; Mittal, et al., 1991; O’Brian et al., 

1997): a swallowable sensor that transmits a coded signal to an external 

receiver/recorder attached to the rescuer. The sensor was given to rescuers at least 2 hours before monitoring began to allow sufficient time for the 

sensor to move into the subject’s gut. Core temperatures were recorded 

at intervals of at least once per minute and time stamped. All recorders 

were time synchronized to correlate activities with the resulting strain.

Heart rate monitoring

The same recording device received signals from an elastic chest 

strap worn against the skin to monitor heart rate wirelessly. A commercially available wristwatch-style monitor was also provided to allow 

participants to observe their condition readily. 

In the general population, sustained heart-rate levels associated 

with excessive heat strain vary between 180 beats per minute (bpm) less 

the person’s age (ACGIH, 2001; NIOSH, 1986) to 220 bpm minus age for 

cardiovascular-conditioned persons (U.S. Army, 2000). The mine rescuers observed in this study fell somewhere in these categories. Because 

of the variety of ages and cardiovascular conditioning found in the population of mine rescuers and the variety of often physically demanding 

activities they performed, it was difficult to assess heat strain by means 

of peak heart rate alone.

Another means of assessing heat strain is through the recovery 

heart rate. ACGIH sets a limit of 110 bpm for a recovery time of 1 minute 

(ACGIH, 2001; NIOSH, 1986). Another means of assessing recovery 

is to compare recovery heart rate to fully resting heart rate. Recovery 

heart rate was taken as the heart rate 2 minutes after resting and was 

compared to a baseline resting heart rate collected while the subject was 

wearing an apparatus but before activities were begun. A rise in this resting rate indicates accumulated strain in the person.

Activity monitoring

Observations of both team and individual activities were logged by 

an observer to track the contribution of metabolic heat to a person’s total 

heat load. Activities such as walking fast, carrying loads, and working 

with hands above the torso all generate significant heat that rescuers 

must shed (ACGIH, 2001; ISO, 1990). 

Another condition observed was the amount and type of clothing 

worn by rescuers. Clothing, as an insulator, can reduce the capability of 

mine ventilation to carry away heat accumulating in the body (MurraySmith, 1987).

Observations from multiple researchers were collected first and 

then compared to ensure that surveys with only one observer would be 

consistent.

Normalizing data

When small populations are examined, an important factor is to find 

a means of drawing conclusions when comparing individual results. In 

this study, the data were first normalized individually from several different observations of the same person under varying conditions. A second 

method was to group individual results by similarities in environment, 

activity level, apparatus, and intervention. A third method was to reduce 

the raw core temperature data to a rate of temperature rise and then to a 

rate of energy absorption per unit mass and/or body size. 
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RESULTS AND DISCUSSION

Individual reactions to hot environments

Figure 1 is one individual’s absolute results along with his heart 

rate data and is an example of an individual experiencing heat illness. 

As noted on the graphic, the person voluntarily sat down at the onset of 

illness and performed no further work, but the environment, at 27 °C (82 

°F) WBGT, was still too hot to allow recovery, resulting in a continuing 

rise in core temperature until the mask was removed and active cooling 

was begun. 

Figure 2 shows the rate of temperature rise in individuals performing 

similar tasks in similar environments. While there were clear variations 

in the absolute temperature peaks observed in individuals, the rates of 

temperature and energy rise were very similar for individuals within a 

group. The two groups here can be distinguished only by age (individual 

in the lower-rate group were all over 35). This observation raises a question about the role of acclimatiza-tion.

Acclimatization is the process of developing resistance to the effects 

of exposure to heat (and other environmental factors). It is a process 

requiring 5 or more days of progressive exposure and is known to be 

reversed within 4 days (ACGIH, 2001; Brake et al., 2001; Leveritt, 1998). 

South African and Australian mines have developed well-documented 

protocols for conditioning employees for hot work in this way. The groups 

studied consisted of individuals whose typical work days encompassed 

a range of environmental exposures, from office workers with only occasional forays into a hot mine to stope miners who spent 12-hour shifts 

in the heat. 

We had assumed we would see a clear distinction between those 

workers who had had the opportunity to acclimatize and those who had 

not. However, the only significant difference between the potentially 

acclimatized individuals and others was that some, but not all, had lowerthan-average starting core temperatures. Most individuals tended to gain 

heat at the same rate. Acclimatization would then be the ability to absorb, 

rather than resist, heat. Another possibility is that the climate, often controlled, in which miners spend their nonworking hours is preventing true 

acclimatization. Compounding the effect of the nonwork exposures are 

the continuous work schedules where the miners work half the days in a 

year. These schedules, with up to 7 days off in a row and seldom requiring 

more than 4 days without a break, do not promote acclimatization.

Impact of initial core temperature

As noted, while the rate of temperature rise was consistent, individuals showed considerable variation in starting core temperatures. The 

normal range of core temperatures is between 35.5 °C (97.0 °F) and 37.5 

°C (99.6 °F) (Clayton and Clayton, 1991; Plog, 1996). The reason for this 

variation could be immediate health issues, such as a chronic problem, 

a minor ailment, medications or supplements being taken, or fatigue. 

Regardless of the cause, a person’s initial core temperature has an immediate and absolute impact on how long that person can endure operating in a hot environment. Figure 3 depicts the length of time individuals 

can operate in hot environments based on initial core temperature, work 

rate, and rest regimen. While limiting work rates can increase the time 

before an individual begins to suffer from heat strain, it does not remove 

him or her from the position of being the weakest part of a team. A single 

member on a team who has an elevated temperature has the potential 

to limit the performance of the team. A simple check of team members’ 

temperatures prior to putting on a breathing apparatus would identify an 

at-risk member.

Figure 1.—Individual response to 27 °C WBGT
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Impact of fitness

Fitness, as measured by body mass index (BMI) has a measurable 

impact on a person’s tolerance to rescue work in heat. BMI is determined 

based on a person’s weight and height. That is, BMI = weight in kilograms 

divided by height in meters squared or 704.5 times weight in pounds 

divided by height in inches squared.

BMI = weight (kg) ÷ [height (m)]2 or 

BMI = 704.5 × weight (lb) ÷ [height (in)]2.

Individual data were normalized by determining the amount of 

energy absorbed in watts. Figure 4 depicts results of this comparison 

among four groups generating varying work rates in similar environments. 

Of note is the relation-ship of work rate to energy absorption. While an 

increase in BMI results in an increase in energy absorption for the same 

work load, adequate rest can significantly reduce the degree to which a 

high BMI affects a rescuer’s heat tolerance. This result is in agreement 

with laboratory-based tests of British mine rescuers (Kampmann and 

Gresser, 1999). In the study group, the average BMI was (in keeping with 

the general population) in the overweight range. 

Impact of work rate

Throughout the study, the most significant controllable contributor 

to heat load was the work rate of the team. Mine rescue teams cannot 

choose where or under what conditions they must perform rescue operations, but they do control how they conduct these operations. The NIOSH 

Recom-mended Standard for Hot Work Environments and ACGIH have 

clear guidelines for incorporating rest into work activities (ACGIH, 2001; 

NIOSH, 1986). Work rates can be estimated using the guidelines in table 

2, which were adapted from ACGIH.

An experiment was conducted with two groups walking in a metal 

mine. WBGT conditions were 27.8 °C (82 °F) with an air velocity of >100 

m/s. One group of acclimatized rescuers traveled 2,000 ft down ramps 

and across the level in the mine, then 1,000 ft up a 10% ramp at a fast 

pace for 45 minutes without rest. They then rested for 15 minutes in a cool 

area as per the NIOSH recommendation of a regimen of 75% work/25% 

rest for those conditions. A second group covered the same course at the 

same pace with rest periods spaced throughout the activity as indicated 

using peak and resting heart rate as monitored by commercially available 

wristwatch-style monitors. The result (figure 5) clearly demonstrates that 

the team that rested periodically throughout the exercise absorbed much 

less heat while accomplishing the same amount of work. All members of 

the “rest-at-end” team experienced at least mild heat strain, and one individual had to be removed from the experiment (albeit under protest) due 

to excessive core temperature. None of the “rest-at-end” team members 

were prepared to resume after the 15 minutes of rest.

Table 2. Examples of metabolic rate categories

Categories Example activities

Rest Sitting quietly

Light Sitting with moderate arm and leg movements

Standing, working with arms in light lifting, turning

Using small power tools

Walking slowly on level carrying minimal weight

Moderate Rapid and/or forceful arm movements

Walking with moderate lifting or pushing

Walking 6 km/hr on level carrying 3 kg load

Heavy Hand sawing, Shoveling light material

Heavy whole body motions

Intermittent heavy lifting or working with hands above head

Walking slowly up steep grades

Very heavy Shoveling heavy material, near continuous heavy lifting

Walking 6 km/hr up grades and/or carrying heavy load

Figure 2.—Change in core temperature at 27 °C WBGT
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Figure 3.—Impact of starting core temperature

An analysis of the heat load sources in these tests is summarized in 

figure 6, which depicts the average energy load for each part of the activity of walking through the mine. Sixty percent of the heat load absorbed 

was due to inadequate rest, 20% came from the apparatus, and only 

20% could be attributed to the task itself. The increase in heat load due 

to wearing an apparatus, although oversha-dowed by the lack of rest, 

clearly shows that simply wearing the apparatus changes what may be a 

simple task on any other day. Wearing an apparatus also increases the 

baseline metabolic load on an individual to the point that even resting 

carries the equivalent heat load of light work.

INTERVENTIONS

The effectiveness of several engineering controls such as cooling 

the inhaled air of the apparatus, wearing frozen media cooling vests, and 

water-supplied CCBA masks were investigated. Administrative controls 

such as limiting the length of exposure and controlling work rate through 

forced rest regimens, were also evaluated. 

Limiting duration of activity based on the environment

In the 1980’s, the British Coal Board established criteria limiting the 

length of time of mine rescue activities based on environmental conditions and, after the 1998 disaster, the Polish Mining Institute developed 

independent but similar criteria (figure 7) (Goldstein and Kajdasz, 2000). 

A comparison of these criteria, which were developed from laboratory data, with our field survey results finds general agreement, but with 

several notable conditions. One is the need to determine initial resting 

body temperatures of mine rescuers. Both sets of criteria are reasonable 

for rescuers within the normal range of initial resting temperature, but 

could result in overexposures for those having elevated initial resting 

temperatures. A second, but no less signifi-cant, limitation is the need to 

control work rate. Periodic rest during the exercise will help ensure that 

a team can tolerate the planned exposure. With these conditions noted, 

the European recommendations can serve as a suitable starting point for 

emergency response planning.

Figure 4.—Fitness effect on heat retention
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Figure 5.—Effect of forced rest regimen at 27 °C WBGT

Figure 6.—Contributions to heat gain

Limiting work rate by recovery heart rate

The work rate experiment identified resting heart rate as a good 

tool for evaluating team condition and readiness to proceed. As demonstrated in figure 8, and increase in resting heart rate is proportional 

to the increase in core temperature. By limiting the rate of rise in resting 

heart rate, the rate of core temperature rise can be controlled and the 

length of operations can be maximized without risk to rescuers. Standard 

mine rescue operations require team checks at intervals not to exceed 

20 minutes. Before putting on breathing apparatuses and at each team 

check, rescuers’ resting heart rates should be checked. The rest period 

should be extended until all team members have resting heart rates no 

more than 10% greater than their previous check or 100 bpm, whichever 

is less. If any one team member can not achieve these resting heart rate 

levels within a reasonable amount of time, retreat should be required.

Impact of inhalation air temperature

One early evaluation was to determine if the use of an apparatus 

that cools the inhalation air of the CCBA provided relief from heat strain. 

Figure 9 depicts the mean temperature rise of two groups of individuals 

wearing two types of apparatus, one with and one without inhalation 

air cooling. Both apparatuses were worn in their NIOSH-approved 

configuration. No statistically significant affect on core temperature was 

observed between the two types. This was not unexpected, as the body’s 

heat exchange mechanisms rely very little on respiration for heat transfer. 

In addition, the available cooling potential of the relatively small mass of 

coolant as compared to the mass of a human body limits the ability of 

the device to affect the core temperature of the wearer within the narrow 

range of operating temperatures present in the approved apparatus. This 

investigation did not evaluate apparatuses with required cooling media 

removed. The comfort provided by this enhancement was acknowledged 

by all participants. Regardless of the reason for the cooling system, it 

should be installed if required by manufacturer (MSHA, 2003).

Active cooling through water-supplied apparatus

Adequate hydration is well known as the best active defense against 

heat illness (Clayton and Clayton, 1991; Dinardi, 1988; Plog, 1996). All of 

the body’s primary heat transfer functions require adequate fluid volumes 

for optimum performance. Heat is moved from the interior of the body 

to the outer surface through blood flow. Vasodilatation occurs as heart 

rate increases to move more heat away from sensitive organs. Without 

sufficient blood volume, collapse of the system can occur, resulting in 

inadequate blood supply to life-sustaining organs and increasing temperatures in the body core. Sweating assists in removing heat from the 

blood near the skin surface through evaporation. Sweating, at a peak of 

up to 1L/hr, can quickly deplete the body’s fluid reserves (ISO, 1989b). If 

deficient in fluids, the body will scavenge fluids from the digestive tract. 

This reverse flow of fluid through the liver can introduce enzymes into 

the bloodstream that can increase core temperature as the body defends 

itself from this toxin.
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Figure 7.—Temperature- and humidity-based time limits for mine rescue (after British Coal 1990)

Figure 8.—Observed relationship between resting heart rate and core temperature
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Figure 10.—Hydration effect in high-work-rate mine rescue

Figure 9.—Comparison of approved apparatuses

Despite the obvious need for adequate amounts of water, mine rescuers have no opportunity to replenish lost fluids while wearing CCBA’s. 

Even under cool conditions (figure 10), a heating effect can be observed 

during extended operations. This could be a symptom of dehydration. 

In hotter operating environments, sweating would increase the risk of 

dehydration-related heat illness. To assess the value of taking in fluids, 

CCBA masks were modified to provide up to 2 L of drinking water to 

rescuers through a bite valve in the breathing cup of the mask and tested 

in nonhazardous training environments.

Results of a comparison of three individuals using masks with and 

without water in thermally stressful environments are shown in figure 11. 

Their core temperatures were maintained on the days they were using 

the hydration masks as opposed to the days used as controls. Figure 

12 shows results from three other individuals who did not use hydration 
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Figure 12.—Control group, hydration effect

masks; two of the three had heat gains similar to the other group’s control 

exposure day. A third member of the control group did markedly better 

without hydration. These early results indicate that development of an 

approved water-supplied mask could be a significant tool in preventing 

heat illness. Further study of this intervention will proceed as opportunities become available.

Active cooling with cooling vests

The use of vests and other garments lined with material that cools 

by changing phase has been advanced as a means of controlling heat 

strain (U.S. Air Force, 1997; Chauhan, 1998; Doerr, 1988; Goldman and 

Kajdasz, 2000; U.S. Bureau of Mines, 1976). Phase-change vests are 

comprised of two basic elements: a material that melts at a temperature 

less than that of the human body and a method of storing the material 

as close to the wearer’s skin as tolerable. The weight and affects on the 

mobility of the wearer of these garments have been of concern to endusers.

This intervention was evaluated by use of commercially available 

vests containing 2.5 kg of material that changes phase at 16 °C (61 °F). 

One-half of the team wore vests and the other half did not. Figure 13 

Figure 11.—Hydration effect in high-work-rate mine rescue, 27 °C WBGT
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shows the rate of temperature gain in the two groups. The reduction in the 

rate of core temperature increase between the two groups was not statistically significant. This finding is in general agreement with other studies 

that have found no significant impact on core temperature changes or 

heart rate (Doerr, 1997; U.S. Air Force, 1997). The vests are generally 

accepted by the wearers because of the additional comfort they provide. 

They did not increase core temperature while the coolant remained below 

skin temperature. The studies have found that the vests do increase the 

length of time an operation can continue as defined by that point at which 

wearers voluntarily stop their exposure. This improvement in comfort 

must be used with caution to avoid overexpo-sures resulting from interference with the wearer’s ability to perceive overheating.

CONCLUSIONS AND RECOMMENDATIONS

The combination of environmental conditions and work loads 

encountered during underground mine rescue exercises has resulted in 

unacceptable physical strain on rescuers before they reach the limits of 

the breathing apparatus. The condition of rescuers and not the remaining capacity of a breathing apparatus must be the limiting factor when 

planning emergency operations in thermally stressful environments. 

Guidelines provided by European rescue agencies provide a good 

framework for planning rescue operations with fit teams and adequate 

rest regimens.

The physical condition of team members should be monitored 

before and during emergency operations to identify and exclude those 

members with an abnormally high temperature and/or resting heart rate 

so that they do not become a risk to themselves and the team. Team 

members with high BMI’s should be counseled and assisted in improving 

their fitness levels.

Team rest periods during emergency operations should be frequent 

and controlled by monitoring resting heart rates at not more than 20-minute intervals. Termination of rescue operations should be based on team 

status as measured by quantitative means and not self assessments.

The use of commercially available cooling devices should not be a 

substitute for monitoring and must be accompanied by training to reinforce the concept that comfort does not equate to safety.

Hydration of rescuers through supplying adequate water beforehand 

should become a standard practice prior to donning a breathing apparatus. Further research into the benefit of hydration-supplied apparatuses 

is needed to justify product development.

ACKNOWLEDGMENTS

The author gratefully acknowledges the work of Patrick Hintz, 

industrial hygienist, Spokane Research Laboratory, and Andrea Mahugh, 

student engineering technician, both of whom assisted with data collection and data processing. The assistance of the mine rescue teams of 

Hecla Mining Co.’s Lucky Friday Mine, Coeur Silver Valley Resources’ 

Galena Mine, and Barrick Goldstrike Co.’s Nevada operations, as well as 

the Central Mine Rescue Association, was invaluable in conducting this 

study, and the author is most grateful for their help. 

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