Heat Stress and Strain 

Evaluation Among 

Aluminum Potroom 

Employees – Texas 

Bich Dang, MD

Chad H. Dowell, MS, CIH

Charles Mueller, MS

Health Hazard Evaluation Report

HETA 2006-0307-3139

August 2011

DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

Workplace

Safety and Health

 National Institute for Occupational 

Safety and Health
The employer shall post a copy of this report 

for a period of 30 calendar days at or near 

the workplace(s) of affected employees. The 

employer shall take steps to insure that the 

posted determinations are not altered, defaced, 

or covered by other material during such 

period. [37 FR 23640, November 7, 1972, as 

amended at 45 FR 2653, January 14, 1980].
Health Hazard Evaluation Report 2006-0307-3139 Page i

Appendix A

Report Abbreviations.......................................................................i

Highlights of the NIOSH Health Hazard Evaluation............ iii

Summary ............................................................................iv

Introduction..........................................................................1

Assessment.........................................................................3

Results.................................................................................4

Discussion ...........................................................................8

Conclusions.........................................................................9

Recommendations...............................................................9

References ........................................................................12

Contents

Occupational Exposure Limits and Health Effects.............23

Tables ................................................................................13

Methods.............................................................................21

Acknowledgment

Appendix B

Appendix C

Acknowledgments and Availability of Report.....................29
Page ii Health Hazard Evaluation Report 2006-0307-3139

ACGIH® American Conference of Governmental Industrial Hygienists

BEI® Biological Exposure Index

BPM Beats per minute

BUN Blood urea nitrogen

CBT Core body temperature

CDC Centers for Disease Control and Prevention

CFR Code of Federal Regulations

°F Fahrenheit

HR Heart rate

N Number of study participants

NAICS North American Industry Classification System

NIOSH National Institute for Occupational Safety and Health

OEL Occupational exposure limit

OSHA Occupational Safety and Health Administration

PEL Permissible exposure limit

PPE Personal protective equipment

REL Recommended exposure limit

TLV® Threshold limit value

TWA Time-weighted average

WBGT Wet bulb globe temperature

WEELTM Workplace environmental exposure limit

Abbreviat
Health Hazard Evaluation Report 2006-0307-3139 Page iii

What NIOSH Did

● We evaluated the smelter in August 2006 and again in July 

2007.

● We talked with employees, union officials, managers, and 

medical staff at the facility about the hot working conditions.

● We looked at the company’s heat stress program.

● We looked at logs of work-related injuries and illness.

● We reviewed medical records of employees who were 

reported to have heat-related illness.

● We measured core body temperature and heart rate of 

employees.

● We measured electrolyte levels of employees.

● We asked employees about their symptoms, medical history, 

and work history.

● We measured employees’ exposure to the heat in the forming 

department.

What NIOSH Found

● The company had a detailed heat stress management 

program, but it was not being followed at all times.

● Most of the tasks that we monitored exceeded the limits for 

working in a hot environment.

● Most employees we monitored had some heat strain.

● Of the employees we monitored, several did not drink 

enough fluids.

What Managers Can Do

● Reduce the physical demands on employees working in the 

potrooms.

● Require the use of heat reflective personal protective 

equipment.

● Install cooling recovery areas in the potrooms.

● Do not use outdoor air to cool employees when it is over 

95°F outside.

● Follow the heat stress management program.

● Stop 8-hour overtime shifts during extremely hot weather.

What Employees Can Do

● Use heat reflective personal protective equipment.

● Use the cooling recovery areas when on breaks.

● Take time to work safely.

The National Institute for 

Occupational Safety and 

Health (NIOSH) received 

a union request for a 

health hazard evaluation 

(HHE) at an aluminum 

smelter in Texas. The 

union submitted the 

HHE request because of 

concerns about the hot 

working conditions in the 

aluminum potroom and 

possible health problems 

from working mandatory 

overtime.

Highlights of 

NIOSH 

Hazard Eval
Page iv Health Hazard Evaluation Report 2006-0307-3139

On July 14, 2006, NIOSH received a request from the United 

Steelworkers to assess employee exposure to heat while working 

in the potrooms at an aluminum smelter in Texas. The union was 

concerned about heat-related illnesses during the hot summer 

months and the health implications of mandatory overtime.

On July 23–27, 2007, we conducted an in-depth heat stress 

and strain evaluation. We measured potroom employees’ HR 

and CBT throughout the shift. We also measured preshift and 

postshift serum electrolyte levels and urine specific gravity to assess 

hydration. After the shift, employees filled out a questionnaire 

about medical history, work history, and symptoms experienced 

during the shift on which they were monitored. We measured 

the WBGT and outdoor weather conditions during the shifts on 

which employees were monitored.

At the time of our evaluation we found that most monitored tasks 

exceeded the ACGIH TLV and NIOSH ceiling limit for working 

in a hot environment. The majority of the employees we evaluated 

met at least one ACGIH criterion for heat strain. Several employees 

were not sufficiently hydrated, which could have precipitated 

the onset and increased the intensity of their heat strain. Lack of 

acclimatization and lower body mass index could have contributed 

to heat strain in some employees.

This report includes recommendations to help reduce the 

potential for heat-related illnesses in potroom employees. These 

recommendations include reducing physical demands, installing 

cooling recovery areas, eliminating 8-hour overtime shifts during 

extremely hot weather, using heat-reflective PPE, and instituting 

other administrative controls.

NIOSH investigators 

found that potroom 

employees were exposed 

to heat stress in excess 

of the occupational 

screening criteria. We 

recommend reducing 

physical demands of 

the work, installing 

cooling recovery areas, 

stopping 8-hour overtime 

shifts during extremely 

hot weather, and using 

heat reflective personal 

protective equipment to 

reduce the heat stress.

Summ

NAICS 331314 (Secondary Smelting & Alloying of Aluminum), heat 

stress, heat strain, hot work environment, aluminum smelting
Health Hazard Evaluation Report 2006-0307-3139 Page 1

Introduct

On July 14, 2006, NIOSH received a request from the United 

Steelworkers to assess employee exposure to heat while working 

in the potrooms at an aluminum smelter in Texas. The union was 

concerned about heat-related illnesses during the hot summer 

months and the health implications of mandatory overtime.

We visited the aluminum smelter on August 16, 2006, and met 

with employer representatives, union representatives, and medical 

staff. After touring the potrooms, reviewing the company’s 

health and safety programs and employee medical records, and 

interviewing employees, we determined that heat stress was a 

potential health hazard among potroom employees and that 

further investigation was warranted. Because peak summer 

temperatures in Texas had already passed, we postponed a followup visit until July 23–27, 2007, to conduct an in-depth heat stress 

and strain evaluation. An interim letter with findings from our 

first site visit and preliminary recommendations was mailed to the 

employer and to union representatives on October 16, 2006.

Process Description

This was the largest primary aluminum smelting operation in the 

United States. It was capable of producing 1.67 million pounds 

of aluminum per day before closing in October 2008. This facility 

produced sheet ingots, primary ingots, and aluminum powder. It 

opened in 1952, and included a lignite mine, power plant, and 

processing plant. The lignite mine provided 6 million tons of fuel 

annually for the power plant. The electrode department produced 

carbon anodes used in the potrooms for aluminum reduction. The 

six potroom lines consisted of two rooms each.

The aluminum smelter used two pot styles for aluminum 

production. Each of the older T-51 pots contained 20 carbon 

anodes while the newer, larger P-100 pots contained 26 carbon 

anodes each. Each T-51 potroom housed 72 pots, while the P-100 

potrooms housed 80 pots per room. Each crew was responsible for 

12 T-51 pots or 13 P-100 pots per shift. All pots were approximately 

15 by 40 feet in size; the rectangular steel tanks were lined with 

refractory bricks to resist the heat and the corrosive effects of 

fluoride metal.

In aluminum smelting, alumina is reduced to nearly pure 

aluminum at an operating temperature of approximately 1,800°F. 

Alumina is mixed with molten cryolite, which dissolves up to 20% 

of alumina and lowers its melting point. Fluorspar is added to 

lower the melting point of the mixture, and aluminum fluoride 
Page 2 Health Hazard Evaluation Report 2006-0307-3139

Introducti

(continued) is added to increase current efficiency. A crust forms as cryolite

solidifies at the surface of the bath mixture and provides a layer of 

thermal insulation. Because of the high melting point of aluminum 

oxide, the alumina cannot be smelted by thermal reduction; 

reduction energy must be supplied. The carbon-lined pots act 

as the cathode, and the carbon electrodes act as the anode. The 

passage of direct electrical current reduces alumina to aluminum 

and oxygen. The resulting molten aluminum metal sinks to the 

bottom of the pot and is siphoned off periodically into crucibles.

Each carbon anode weighed approximately 300 pounds and had 

a copper rod inserted through its middle. These anodes were 

systematically replaced approximately every 18 days as the carbon 

material was gradually consumed in the aluminum smelting 

process. Approximately one ton of molten aluminum was removed 

(“tapped”) from each pot every 48 hours.

Approximately 600 employees worked in the potrooms over three 

shifts. Operations were continuous, with employees working 

8-hour shifts and rotating weekly. When a shortage of employees 

for a given shift occurred, volunteers were solicited from the prior 

shift to work an additional 8-hour shift. If too few employees 

volunteered, mandatory overtime was enforced. No employees were 

allowed to work more than 64 hours per week.

The following jobs were performed by potroom employees:

● Tapper carbon changers removed molten aluminum from the 

pots and removed spent carbon anodes. Fresh anodes were 

placed into the pots using an overhead crane.

● Pot tenders periodically broke the insulating crust that 

formed at the surface of the bath mixture, added aluminum 

ore, and aligned the anodes. They used wooden probes to 

break up air gaps in the molten material.

● Overhead crane operators removed spent rods from the 

pots, positioned new carbon anodes, transported and 

added alumina via portable bins, and moved all pot tapping 

equipment.

● Potroom helpers added ore and other materials to the pots, 

and performed miscellaneous tasks.

● Heat stress carbon setters were additional members on the 

potroom crews during the warmer summer months. They 

performed the same duties as the tapper carbon changers, 

pot tenders, and potroom helpers.
Health Hazard Evaluation Report 2006-0307-3139 Page 3

Assessment

Initial Site Visit

On August 16, 2006, we held an opening conference with union 

and employer representatives, walked through the plant, reviewed 

the OSHA Form 300 Log of Work-Related Injuries and Illnesses, 

and reviewed medical records of employees reported by the 

union or medical staff to have had heat-related illness. We also 

interviewed several potroom employees privately and reviewed the 

current heat stress management program.

Second Site Visit 

On July 23–27, 2007, we recruited employees identified by the 

employer and the union as working in the hottest jobs. We 

assumed that the hottest jobs occurred during the day shifts, so we 

selected participants from the first (8 a.m. to 4 p.m.) and second (4 

p.m. to 12 a.m.) shifts each day over a 5-day period. Participation 

was voluntary, and we obtained written informed consent from all 

participants.

Participants completed a questionnaire about their medical 

history, work history, and symptoms experienced during the shift 

on which they were monitored. Symptoms of interest included 

muscle cramps, headache, lightheadedness or dizziness, nausea 

or vomiting, racing heartbeat or palpitations, unsteady walk, 

and confusion or disorientation. Participants were considered 

unacclimatized if they had been off work or worked completely 

outside the potrooms for four or more consecutive days in the past 

2 weeks.

Continuous CBT and HR were measured throughout a work 

shift. The HR monitors were attached to participants before their 

shift and removed after their shift. Before their shift, participants 

swallowed a VitalSenseTM temperature sensor pill, and we 

attached a data logger to the participant’s waist belt. At the end 

of shift, data loggers were removed, and heat strain information 

was downloaded. The sensor pills passed naturally through the 

participant’s gastrointestinal tract.

Urine specific gravity and blood electrolytes were measured before 

and after each participant’s shift. All participants were told their 

electrolyte results at the end of their shift.

WBGTs were monitored at several locations throughout the 

potrooms, simulating the locations where employees normally 

worked. Outdoor weather conditions were monitored with a 
Page 4 Health Hazard Evaluation Report 2006-0307-3139

Assessment (continued

Results

weather station near the main gate. WBGT and outdoor weather 

data were collected to assess potential for heat stress during the 

shifts when heat strain monitoring was performed.

Additional details of the methods used for heat strain and stress 

can be found in Appendix B. Appendix C contains heat strain and 

stress evaluation criteria used for this evaluation.

Initial Site Visit 

We interviewed 21 employees. One was reported by the union 

as having been treated for heat-related symptoms, two asked 

to be interviewed, and the others were serially selected from 

the employee roster. Three employees reported they had not 

experienced symptoms of heat strain. The other employees 

reported symptoms of heat strain occurring during the summer 

months, mostly muscle cramps and lightheadedness. Ten 

employees had visited the medical department during the summer, 

and one employee had been sent to the emergency room to receive 

intravenous fluids. Two of the ten who sought medical care for 

heat-related symptoms noted that they preferred to sit outside and 

felt that cooling off too quickly worsened their muscle cramps. 

They verified that they were allowed to cool off outside or inside; 

however, the medical staff did not necessarily recommend one 

or the other. The other eight had remained indoors while in the 

medical department. The 2006 corporate heat stress protocol 

recommended that employees suspected of having heat cramps, 

heat syncope, heat exhaustion, or heat stress “should be allowed to 

rest in a cool (e.g., air-conditioned) environment.” All interviewed 

employees reported satisfaction with the medical care received.

One employee reported falling asleep while driving home after 

a 16-hour shift. Medical staff thought fatigue from overtime 

contributed to employees being given a written diagnosis of “hot 

and tired.” The interviewed employees also reported that when 

they were required to work overtime, the jobs they worked during 

the second shift did not always involve a lighter task than during 

the first shift, as required in the heat stress protocol. For example, 

it was common to work as a pot tender or carbon setter for two 

shifts in a row.

We reviewed medical records of nine employees who had entries 

on the OSHA Logs for heat-related illness, or were reported by 

coworkers, themselves, or the union to have been treated for heatrelated illness. Six employees were diagnosed by the emergency 
Health Hazard Evaluation Report 2006-0307-3139 Page 5

Results (continued) room physician with heat exhaustion, two employees were sent

to the emergency room for cardiac problems, and one employee 

was sent to the emergency room for an extremely elevated blood 

glucose (sugar) level. We noted a belief among employees that when 

an employee was sent to the emergency room it was because of heat 

stress. Our review of medical records did not support that belief.

According to the medical department logs, 158 episodes of heat 

disorders (mostly “hot and tired” or muscle cramps) and three 

episodes of employees with heat-related illness had occurred since 

March 8, 2006. Only employee diagnoses of heat-related illness 

were documented on the OSHA Logs. Six cases of heat-related 

illnesses occurred in 2002, three in 2003, and four each in 2004 

and 2005.

Second Site Visit 

The daily outdoor weather conditions are summarized in Table 

A1. Mean temperature for July from 1971–2000 was 85°F [NOAA 

2004]. The mean temperature for the 5 days of our evaluation was 

78°F.

We collected 31 WBGT measurements on July 23 and July 24, 

2007, in areas where employees performed routine job tasks. These 

included a T-51 and a P-100 pot, a pot with side covers removed, 

a tapping crucible next to a T-51 and a P-100 pot, an aisle with 

no spent anodes and one with spent anodes, and the operator’s 

enclosure on the overhead crane. Protective clothing worn by 

employees in the potrooms included a fire retardant long-sleeve 

shirt and pants, long-sleeve cotton t-shirt, steel toe boots, and a 

hardhat.

The WBGT data collected in the potrooms are presented in Table 

A2. The WBGT measurements ranged from 83°F to 120°F, with 

dry bulb air temperatures reaching 134°F and radiant temperatures 

reaching 188°F. The highest WBGT measurement in the potroom 

was collected between a T-51 pot and a tapping crucible during 

tapping. The second highest WBGT measurement was collected 

on the center of the catwalk between pots with one side open. 

This measurement was probably lower than the actual temperature 

because the WBGT monitor was not fully equilibrated because the 

high radiant heat load was starting to melt the monitor’s tripod.

Radiant heat (globe bulb temperature) measured at all except one 

location inside the potrooms equaled or exceeded 96°F. At these 

temperatures, employees absorb rather than radiate heat unless 
Page 6 Health Hazard Evaluation Report 2006-0307-3139

Results (continued) proper shielding is provided. The highest globe bulb temperature

was measured on the catwalk in the center of two pots with the 

side shields removed on one pot. The second highest globe bulb 

temperature was measured between a T-51 pot and a crucible 

during tapping.

A clothing adjustment factor was estimated at 5°F for the clothing 

worn by employees in the potrooms. Metabolic rates for employees 

in the potrooms were estimated to be in the moderate to light 

category, ranging from 115 to 360 watts. When compared to the 

WBGT measurements collected in the potrooms, portions of all 

tasks, except for the crane operator, exceeded the ACGIH TLV and 

NIOSH ceiling limit for working in a hot environment.

Of an estimated total population of 600 potroom employees, 61 

participated in the heat stress evaluation. One employee completed 

the questionnaire but was excluded from analysis because that 

individual only worked 30 minutes. Personal characteristics of 

the participants are summarized in Table A3. Participants were 

predominantly male. The average body mass index, a measure 

of body fat on the basis of height and weight, was 30.5. Values 

between 25 and 29.9 fall in the overweight category, and values 30 

or greater fall in the obese category. However, muscular individuals 

may have an increased body mass index due to high muscle mass 

rather than body fat.

The distribution of job titles during the first 8-hour shift is 

shown in Table A4. Six of the 60 study participants worked an 

overtime shift. We observed three participants perform equally 

hot job duties during their overtime shift as during their regular 

shift, which is prohibited according to the company heat stress 

management program. Two worked as a tapper carbon changer 

(considered one of the hottest jobs) for two 8-hour shifts in a row, 

and one worked as a head tapper carbon changer during the first 

shift and then as a tapper carbon changer/crane operator for 2 

overtime hours.

The prevalence of symptoms reported during the work shift 

is shown in Table A5. The most common symptoms were 

racing heartbeat or palpitations, headache, muscle cramps, and 

lightheadedness or dizziness.

Study participants’ preshift and postshift changes in blood and 

urine measurements are shown in Table A6. Postshift blood 

bicarbonate, BUN, creatinine, and urine specific gravity increased 

significantly. The BUN to creatinine ratio and potassium level 

decreased significantly over the shift. These changes suggest volume 

depletion. Volume depletion is different from pure dehydration, 
Health Hazard Evaluation Report 2006-0307-3139 Page 7

Results (continued) and occurs when loss of both water and salt/sodium results in a

reduced circulatory blood volume [Mange 1997]. Sweat contains 

water and salt, and excessive sweating can cause volume depletion.

Three participants had a marked increase in preshift to postshift 

creatinine values, which indicates acute kidney injury. Their 

preshift and postshift BUN to creatinine ratios were 14.0/1.1 to 

20.0/2.3, 11.0/1.3 to 13.0/2.4, and 12.0/1.0 to 15.0/2.6. These 

participants were advised to drink plenty of fluids and to have their 

blood creatinine level rechecked the following day either by us or 

medical staff from the facility. One of these participants returned 

the following day, and his creatinine had dropped from 2.3 to 1.4 

milligrams per deciliter. This drop in creatinine suggests that the 

acute kidney injury was reversed.

The average amount of fluids consumed during the shift was 180 

ounces. The three participants with a marked increase in preshift 

to postshift creatinine values had a higher mean fluid intake (191 

ounces) than the rest of the study participants (180 ounces). Medilyte® packets, provided by the employer to replace electrolytes 

lost from excessive sweating, contain 40 milligrams potassium 

chloride, 18 milligrams calcium phosphate, and 9 milligrams 

magnesium carbonate. Among those reporting consumption of 

Medi-lyte packets during their shifts (20%), the average number of 

packets used was 3.1. Participants who reported drinking alcoholic 

beverages during the 24 hours prior to starting work (18%) 

consumed an average of 3.5 alcoholic beverages.

The number of participants meeting the ACGIH criteria for 

heat strain is shown in Table A7. Most participants (54%) met 

at least one ACGIH criterion for heat strain, and 17 participants 

(29%) exceeded the CBT criterion for their acclimatization status. 

Participants who exceeded the CBT for their acclimatization 

status and/or exceeded the sustained peak HR criterion had a 

significantly lower average body mass index than those who did not 

exceed those ACGIH criteria. Among unacclimatized participants, 

88% exceeded their ACGIH heat strain criterion for CBT 

compared to 20% of acclimatized participants (P< 0.01).

Comparisons of the differences in preshift and postshift laboratory 

values for participants who did and did not meet ACGIH criteria 

for heat strain are shown in Tables A8 and A9. Participants who 

exceeded any of the ACGIH heat strain criteria had, on average, a 

significantly greater decrease in their BUN to creatinine ratio than 

those who did not. Participants who exceeded the CBT for their

acclimatization status also had, on average, a significantly higher 

increase in blood creatinine.
Page 8 Health Hazard Evaluation Report 2006-0307-3139

Results (continued) Unacclimatized participants were significantly more likely to exceed

the ACGIH heat strain criteria for CBT, as shown in Table A10. 

Among participants who exceeded ACGIH CBT criteria, the 

average percentage of the time period monitored above criteria 

was 22% for unacclimatized employees and 9% for acclimatized 

employees. Unacclimatized potroom participants had a significantly 

higher average HR than the acclimatized participants.

A comparison of CBT and HR values based on symptom 

presentation is shown in Table A11. The only participant who 

reported nausea or vomiting had the highest peak CBT value of 

102.83°F.

Many participants were not sufficiently hydrated. The average 

preshift urine specific gravity for the participants was 1.023, 

indicating that most were significantly volume depleted or 

dehydrated before starting their work shift [Casa et al. 2000]. The 

significant increase in preshift to postshift urine specific gravity 

indicates that participants did not drink enough fluids to keep up 

with the amount of water and salt lost through excessive sweating.

Several participants had laboratory evidence of acute kidney 

injury. Volume depletion may have played a role, causing decreased 

blood flow to the kidneys and acute injury. However, subclinical 

rhabdomyolysis, a syndrome characterized by the breakdown of 

muscle cells due to injury, can occur as a result of excess heat stress 

exposure or extreme physical activity. Acute kidney injury due to 

volume depletion or rhabdomyolysis is a reversible condition, and 

generally responds well to appropriate rehydration.

Discrepancies between the written heat stress management 

program and actual practice created the potential for increased 

heat stress and strain. For example, employees working overtime 

performed similar and not the less strenuous tasks required by 

the heat stress management program during their overtime shift. 

We also observed that crews were not all fully staffed with a heat 

stress carbon setter, so people could not rotate away from hot tasks. 

Other work crews compressed their job tasks and did not take the 

required breaks in order to have a longer break at the end of the 

shift.

Employee interviews and workplace observations during our initial 

visit in 2006 indicated that the facility had a reduced workforce. As 

a result, the company policy was to require rather than voluntarily 

offer overtime to ensure uninterrupted production. Because of 

Discussi
Health Hazard Evaluation Report 2006-0307-3139 Page 9

Discussio  (continued) this policy, many employees we interviewed worked consecuti

8-hour shifts, which potentially placed them at increased risk of 

heat-related illnesses because of the additional heat stress exposure. 

For example, employees who worked an 8-hour overtime shift (a 

total of 16 consecutive hours) had just 8 hours to drive home, 

eat, and sleep before reporting back to work the next day. Sleep 

deprivation reduces reaction time and increases perceptual and 

cognitive distortions and changes in mood [Pilcher 1996; Ferrara 

2001]. Employees and managers need to be aware of the effects of 

cumulative fatigue and sleep deprivation associated with excessive 

overtime work.

We were unable to compare differences in physiologic markers 

for heat stress and strain in potroom employees working 8-hour 

shifts and those working an additional overtime shift because 

of the small number of employees who worked overtime during 

our evaluation. Additionally, outdoor environmental conditions 

significantly differed between the time of the initial health hazard 

evaluation request and the days that we performed physiologic 

monitoring. The outdoor temperatures on the days of monitoring 

were cooler than expected during the summertime in Texas, 

and the heat strain and symptom data may have underestimated 

what potroom employees might experience during extreme heat. 

Finally, although the CBTs and HR data were gathered in real 

time, the data were not analyzed until later. As such, we were not 

able to provide direct intervention on the basis of physiologic 

data. Instead, the goal of this evaluation was to use the data to 

provide future recommendations for the prevention of heat-related 

illnesses.

The majority of the employees we studied met at least one ACGIH 

criterion for heat strain. All jobs except the crane operator 

exceeded the ACGIH TLV and NIOSH ceiling limit for working 

in a hot environment. Several employees were not sufficiently 

hydrated, which could have precipitated the onset and increased 

the intensity of their heat strain. Lack of acclimatization and lower 

body mass index were also important factors in the level of heat 

strain experienced by some employees.

On the basis of our findings, we recommend the actions listed 

below to create a more healthful workplace for workplaces similar 

to the one we evaluated. We encourage employers to use a labormanagement health and safety committee or working group to 

Conclusion

Recommendatio
Page 10 Health Hazard Evaluation Report 2006-0307-3139

Recommendatio

(continued) discuss the recommendations in this report and develop an action

plan. Those involved in the work can best set priorities and assess 

the feasibility of our recommendations for their specific situation. 

Our recommendations are based on the hierarchy of controls 

approach (Appendix C: Occupational Exposure Limits and Health 

Effects). This approach groups actions by their likely effectiveness 

in reducing or removing hazards. In most cases, the preferred 

approach is to eliminate hazardous materials or processes and 

install engineering controls to reduce exposure or shield employees. 

Until such controls are in place, or if they are not effective or 

feasible, administrative measures and/or personal protective 

equipment may be needed.

Engineering Controls 

Engineering controls reduce exposures to employees by removing 

the hazard from the process or by placing a barrier between the 

hazard and the employee. Engineering controls are very effective 

at protecting employees without placing primary responsibility of 

implementation on the employee.

1. Reduce the physical demands for employees working in the 

potrooms through process automation and/or by increasing 

the number of employees on each crew.

2. Install cooling recovery areas along the curtain walls in 

the potrooms where employees typically sit [AIHA 1975; 

ASHRAE 2007]. Cooling recovery areas should be easily 

accessible to encourage employee use. They should reduce 

the radiant and convective heat loads while increasing the 

evaporative cooling of the employees.

Administrative Controls 

Administrative controls are employer-dictated work practices and 

policies to reduce or prevent exposures to workplace hazards. 

The effectiveness of administrative changes in work practices 

for controlling workplace hazards is dependent on management 

commitment and employee acceptance. Regular monitoring and 

reinforcement are necessary to ensure that control policies and 

procedures are not circumvented in the name of convenience or 

production.

1. Do not use outdoor air to cool employees when the ambient 

outdoor air temperatures exceed 95°F.
Health Hazard Evaluation Report 2006-0307-3139 Page 11

Recommendatio

(continued) 2. Implement and enforce the company heat stress program,

with special attention to the following:

a. When employees are performing overtime work, their 

jobs during the second shift should involve a lighter task 

(section 3.4.4 of the company’s heat stress program).

b. Maintain full crews during period of extreme heat.

c. Reacclimate employees returning from a period of 3 days 

or more away from the potrooms.

3. Eliminate 8-hour overtime shifts (voluntary or mandatory) 

during periods of extreme heat. An alternative to 8-hour 

overtime shifts is to split up the overtime shifts into two 

4-hour overtime shifts and assign call duties to a rotating 

number of employees. Call duty means that an employee 

may be asked to stay for a 4-hour overtime shift or called in 

4 hours earlier to start a shift. They are notified ahead of 

time regarding their call duty schedule and may be called in 

to work on assigned dates if there are not enough employees 

for a given work task.

4. Employees should take the time to work safely. They should 

be discouraged from compressing work tasks in order to 

have a longer rest period at the end of shift.

Personal Protective Equipment

PPE is the least effective means for controlling employee exposures. 

Proper use of PPE requires a comprehensive program, and calls 

for a high level of employee involvement and commitment to be 

effective. The use of PPE requires the choice of the appropriate 

equipment to reduce the hazard and the development of 

supporting programs such as training, change-out schedules, and 

medical assessment if needed. PPE should not be relied upon as 

the sole method for limiting employee exposures. Rather, PPE 

should be used until engineering and administrative controls can 

be demonstrated to be effective in limiting exposures to acceptable 

levels.

1. Require employees to wear lightweight heat-reflective aprons 

when working around pots, or when tapping. The reflective 

aluminum-foil surface should be facing away from the 

employee and be kept clean to maintain its effectiveness. 

To aid evaporative cooling, reflective aprons should not be 

worn when employees are in the cooling recovery areas or 

break rooms.
Page 12 Health Hazard Evaluation Report 2006-0307-3139

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VA: American Industrial Hygiene Association.

ASHRAE [2007]. ASHRAE handbook: HVAC applications. 

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Casa DJ, Armstrong LE, Hillman SK, Montain SJ, Reiff RV, Rich 

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Mange K, Matsuura D, Cizman B, Soto H, Ziyadeh FN, Goldfarb 

S, Neilson EG [1997]. Language guiding therapy: the case of 

dehydration versus volume depletion. Ann Intern Med 127(9):848–

853.

NOAA [2004]. Climatography of the United States report No. 

20: College Station ETRWD AP, Texas. Asheville, NC: U.S. 

Department of Commerce, National Oceanic and Atmospheric 

Administration, National Environmental Satellite, Data, and 

Information Service. [http://cdo.ncdc.noaa.gov/climatenormals/

clim20/tx/411889.pdf]. Date accessed: August 2011.

Pilcher JJ, Huffcutt AI [1996]. Effects of sleep deprivation on 

performance: a meta-analysis. Sleep 19(4):318–326.

Referenc
Health Hazard Evaluation Report 2006-0307-3139 Page 13

Appendix A: Tab

Table A1. Outside weather conditions during the evaluation*

Date Temperature (°F)† Relative Humidity (%)† Wind Speed 

(miles per hour)†

July 23, 2008 75–90 53–94 0–6

July 24, 2008 72–87 56–93 0–5

July 25, 2008 70–85 62–67 0–10

July 26, 2008 70–85 68–96 0–8

July 27, 2008 72–86 67–98 0–6

*over a 24-hour period

†minimum–maximum
Page 14 Health Hazard Evaluation Report 2006-0307-3139

Appendix A: Tabl   (contin

Table A2. Wet bulb globe temperature measurements

Equipment Monitor Location Wet Bulb 

(°F)

Dry Bulb 

(°F)

Globe 

Bulb (°F)

WBGT* 

(°F)

T-51 Pot Edge of pot 81 95 105 88

Center of pots on catwalk 87 121 154 107

Curtain wall 78 91 95 83

Courtyard (outside) 78 91 95 83

Tapping Crucible at 

T-51 Pot Next to crucible 82 93 140 99

Between pot and crucible 92 127 186 120

Curtain wall 80 91 110 89

Courtyard (outside) 80 92 112 89

P-100 Pot Edge of pot 82 95 112 91

Center of pots on catwalk 91 134 141 106

Curtain wall 80 93 98 86

Courtyard (outside) 79 90 93 83

Tapping Crucible at a 

P-100 Pot Next to crucible 83 91 114 93

Between pot and crucible 86 107 146 104

Curtain wall 81 97 100 87

Courtyard (outside) 76 81 88 79

Open pots Edge of pot 82 93 109 90

Center of pots on catwalk 89 134 188 119

Curtain wall 79 87 96 84

Courtyard (outside) 77 83 92 81

Aisle without spent 

carbon anodes Edge of pot 80 90 96 85

Curtain wall 80 90 96 85

Edge of pot 81 93 107 89

Curtain wall 81 93 99 87

Courtyard (outside) 76 83 92 80

Aisle with spent 

carbon anodes Edge of pot 85 118 137 101

Curtain wall 81 99 118 92

Edge of pot 85 109 136 100

Curtain wall 83 101 123 95

Courtyard (outside) 76 87 97 81

Crane Behind operator seat in cabin 82 104 108 90

*No clothing adjustment factor has been applied to reported values. An adjustment of plus 5°F is required for the work 

clothing worn in the potrooms.
Health Hazard Evaluation Report 2006-0307-3139 Page 15

Appendix A: Tabl   (contin

Table A3. Personal characteristics of study participants*

Average age (range) 32 (19–50)

Male, number (%) 57 (95)

Average years working in a potroom at the facility, (range) 1.2 (0.1–6.0)

Average body mass index, (range)† 30.5 (19.4–47.3)

Average hours slept before starting work, (range) 6.6 (3.0–10.0)

Unacclimatized, number (%) 8 (13)

History of, number (%) 

 Diabetes 1 (2)

 Heart Disease 0

 High blood pressure 3 (5)

 Kidney disease 0

 Thyroid disease 1 (2)

 Other‡ 4 (7)

*N = 57–60

†Body mass index was calculated using self reported height and weight.

‡Arthritis, asthma, depression, and sarcoidoisis

Table A4. Distribution of job titles during the first 8-hour shift*

Job Title Number (%)

Tapper carbon changer 22 (37)

Pot tender 14 (23)

Head tapper carbon changer 10 (17)

Tapper carbon changer/crane operator 4 (7)

Roving head tapper carbon changer 3 (5)

Roving pot tender 2 (3)

Roving tapper carbon changer 1 (2)

Pot tender sampler 1 (2)

Crane operator 1 (2)

Potroom helper 1 (2)

Trainer (new hire orientation) 1 (2)

*N = 60
Page 16 Health Hazard Evaluation Report 2006-0307-3139

Appendix A: Tabl   (contin

Table A5. Prevalence of symptoms reported during the shift*

Symptom Number (%)

Racing heart beat or palpitations 18 (30)

Headache 12 (20)

Muscle cramps 8 (13)

Lightheadedness or dizziness 8 (13)

Confusion or disorientation 5 (8)

Unsteady walk 3 (5)

Nausea or vomiting 1 (2)

*N = 60

Table A6. Comparison of mean pre- and post-shift laboratory values

Preshift

N = 59–60

Postshift

N = 60

P

value

Blood

 Sodium (corrected)* 138.01 138.37 0.26

 Potassium 4.45 4.28 0.03

 Chloride 108.67 108.23 0.21

 Bicarbonate 23.03 23.52 0.02

 BUN 14.47 16.23 <0.01

 Creatinine 1.06 1.34 <0.01

 BUN/ creatinine ratio 14.04 12.61 <0.01

 Glucose 110.20 107.12 0.43

 Osmolality 287.31 288.48 0.07

Urine

 Specific gravity 1.023 1.028 <0.01

*Sodium is corrected for participants with glucose concentrations > 200 using the formula: corrected 

sodium = measured sodium + 0.016 (glucose – 100).
Health Hazard Evaluation Report 2006-0307-3139 Page 17

Appendix A: Tabl   (contin

Table A8. Differences in preshift and postshift lab values of participants by ACGIH CBT criteria

Core Body Temperature Greater Than Criterion*

Yes

N = 17

No

N = 41

PremeanPostmeanPremeanPostmeanP

value

Blood

 Sodium (corrected)† 137.82 137.82 138.09 138.56 0.34

 Potassium 4.49 4.36 4.42 4.22 0.76

 Chloride 108.82 108.18 108.56 108.24 0.66

 CO2 22.41 22.65 23.27 23.78 0.66

 BUN 15.35 16.29 14.15 16.32 0.11

 Creatinine 0.95 1.28 1.11 1.37 <0.05‡

 BUN/Creatinine 16.65 13.44 12.91 12.35 <0.01‡

 Glucose 112.76 106.65 107.29 107.37 0.46

 Osmolality 287.39 287.39 287.19 288.91 0.23

Urine

 Specific gravity 1.023 1.026 1.023§ 1.029 0.59

*ACGIH Heat Strain Criteria: CBT greater than 101.3oF for acclimatized employees or greater than 100.4oF for 

unacclimatized employees.

†Sodium is corrected for participants with glucose concentrations > 200 using the formula: corrected sodium = measured 

sodium + 0.016 (glucose – 100).

‡A P value ≤0.05 was considered statistically significant.

§N = 40

Table A7. Participants meeting ACGIH heat strain criteria*

ACGIH heat strain criteria Number (%)

Core body temperature greater than criterion† 17 (29)

Sustained‡ heart rate greater than (180–age) 24 (43)

Average heart rate greater than115 11 (20)

Number of ACGIH Heat Strain Criteria met:

0 26 (46)

1 12 (21)

2 15 (27)

3 3 (5)

*N = 56–58

†ACGIH Heat Strain Criteria: CBT greater than 101.3oF for acclimatized employees or greater than 100.4oF for 

unacclimatized employees.

‡Values sustained for greater than or equal to 5 continuous minutes.
Page 18 Health Hazard Evaluation Report 2006-0307-3139

Appendix A: Tabl   (continued) Table A9. Differences in preshift and postshift lab values of participants by ACGIH HR criterion Sustained* Heart Rate Greater Than (180 – age) Average Heart Rate Greater than 115 Yes N = 24 No N = 32 Yes N = 11 No N = 45 Pre- mean Post- mean Pre- mean Post- man  P value Pre- mean Post- mean Pre- mean Post- 

P

value

Blood Sodium  (corrected) † 137.98 137.33 137.91 139.06 <0.01‡ 138.36 138.45 137.84 138.29 0.66 Potassium 4.45 4.22 4.42 4.29 0.51 4.56 4.36 4.40 4.24 0.94 Chloride 109.00 107.92 108.25 108.50 0.07 109.00 109.00 108.47 108.07 0.92 CO2 22.67 22.71 23.28 24.00 0.17 23.09 22.64 23.00 23.64 0.07 BUN 15.13 16.88 14.16 16.03 0.95 17.00 18.73 13.98 15.82 0.66 Creatinine 0.99 1.32 1.11 1.32 0.24 1.04 1.44 1.06 1.29 0.07 BUN/Creatinine 15.62 13.52 13.01 12.32 0.03‡ 16.56 13.96 13.53 12.56 0.04‡ Glucose 113.96 112.17 105.53 102.66 0.90 106.00 103.73 109.91 107.47 0.99 Osmolality 287.70 286.92 286.73 289.55 <0.01‡ 288.69 289.36 286.77 288.20 0.66Urine Specific gravity 1.022 1.026 1.024§ 1.029 0.93 1.026 1.031 1.022¶ 1.027 0.82*Values sustained for greater than or equal to 5 continuous minutes.†Sodium is corrected for participants with glucose concentrations > 200 using the formula: corrected sodium = measured sodium + 0.016 (glucose – 100).‡A 

P value ≤0.05 was considered statistically significant.

§N = 31¶N = 44
Health Hazard Evaluation Report 2006-0307-3139 Page 19

Appendix A: Tabl   (contin

Table A10. Heat strain measurements in acclimatized and unacclimatized employees

Acclimatized Unacclimatized P value

Number 48–50 8

CBT, average 99.6oF 99.7oF 0.68

Peak CBT, average (range) 100.7 (99.0–102.8)oF 101.0 (99.5–102.4)oF 0.34

HR, average (range) 107 (84–127) BPM 118 (105–134) BPM <0.01

CBT > Criterion,* number (%) 10 (20%) 7 (88%) <0.01

 Minutes above criterion, median (range) 28 (6–112) 46 (11–597)

 Period monitored above criterion,

 average (%) 9% 22%

Sustained† HR > (180 – age), number (%) 19 (40%) 5 (63%) 0.27

 Minutes above criterion, average 53 53

 Period monitored above criterion,

 average (%) 13% 12%

Average HR > 115, number (%) 8 (17%) 3 (38%) 0.18

*ACGIH Heat Strain Criteria: CBT greater than 101.3oF for acclimatized employees or greater than 100.4oF for 

unacclimatized employees.

†Values sustained for greater than or equal to 5 continuous minutes.
Page 20 Health Hazard Evaluation Report 2006-0307-3139

Appendix A: Tabl   (contin

Table A11. Comparison of CBT and HR values based on symptom presentation

Symptom presentation N* Mean 

CBT (oF)

P 

value

Mean Peak 

CBT (oF)

P 

value

Mean HR 

(BPM)

P 

value

Racing heart beat or 

palpitations

Yes 16–17 99.7

0.39

100.9

0.45

111.9

0.11

No 40–41 99.6 100.7 107.1

Headache

Yes 10–12 99.6

0.75

101.0

0.25

109.6

0.69

No 46 99.6 100.7 108.2

Muscle cramps

Yes 7–8 99.6

0.85

100.9

0.73

109.0

0.88

No 49–50 99.6 100.8 108.4

Lightheadedness or 

dizziness

Yes 8 99.6

0.69

101.0

0.39

109.3

0.82

No 48–50 99.6 100.8 108.4

Confusion or 

disorientation

Yes 5 99.8

0.44

100.9

0.70

108.4

0.99

No 51–53 99.6 100.8 108.5

Unsteady walk

Yes 3 99.6

0.92

101.0

0.57

104.6

0.50

No 53–55 99.6 100.8 108.7

Nausea or vomiting

Yes 1 99.8

†

102.8

†

109.3

†

No 55–57 99.6 100.8 108.5

*Number of participants with or without a specified symptom.

†Statistical testing was not performed because N = 1 in one of the groups.
Health Hazard Evaluation Report 2006-0307-3139 Page 21

Biological Monitoring 

We used the i-STAT® 1 handheld analyzer and i-STAT® 8+ cartridges (Abbott Point of Care, Inc., 

East Windsor, New Jersey) to measure serum electrolytes. Approximately 2–3 drops of whole blood was 

collected via finger prick and placed in the well of the cartridge prior to inserting the cartridge into 

the analyzer. Blood was collected following the standard precautions for working with blood and blood 

products specified by CDC and OSHA [CDC 1998; 29 CFR 1910.1000].

The Atago Uricon-Ne Refractometer® (Atago USA, Inc., Bellevue, Washington) was used to measure 

urine specific gravity. The refractometer measures specific gravity from 1.000 to 1.050 and is accurate to 

within 0.001.

Physiologic Monitoring 

We used the VitalSenseTM (Mini-Mitter Company, Inc., Bend, Oregon) physiological monitoring system 

to continuously measure CBT to within ± 0.2°F. As the temperature sensor pill travels through the 

gastrointestinal tract it transmits CBT data to a receiver where it is stored to memory. Once swallowed the 

normal passage time of the temperature sensor pill is 1–5 days. The Mini-Mitter ActiHeart® HR monitor 

recorded data at 15-second intervals.

Wet Bulb Globe Temperature 

We measured the WBGT (calculated using wet, dry, and globe bulb temperatures) and wind speed using 

QUESTemp °36 and QUESTemp °34 instruments (Quest Technologies, Inc., Oconomowoc, Wisconsin). 

These instruments measure temperatures of 23°F–212°F and are accurate to within ± 0.9°F. These 

instruments were allowed to equilibrate for a minimum of 15 minutes before data were collected unless 

otherwise noted in the report.

Weather Conditions 

We measured weather conditions including temperature, relative humidity, and wind speed at the plant 

using a HOBO® weather station (Onset Computer Corporation, Bourne, Massachusetts). The weather 

station was set up near the entrance to the plant and allowed to continually monitor throughout the 

survey.

Appendix B: Metho
Page 22 Health Hazard Evaluation Report 2006-0307-3139

Appendix B: Method  (continu

Statistical Analysis 

Statistical analysis was carried out with SAS version 9.1.3 software (SAS Institute, Cary, North Carolina). 

A P value ≤ 0.05 was considered statistically significant. Paired t-tests were used to compare preshift and 

postshift blood and urine laboratory means. When the data were normally distributed, the t-test was 

used to compare body mass index and changes in preshift and postshift lab values between employees 

who did and did not meet ACGIH criteria for heat strain. In cases where the data were not normally 

distributed, the nonparametric Wilcoxon test was used to compare differences in preshift and postshift 

lab values between employees who did and did not meet ACGIH criteria for heat strain. The t-test was 

also used to compare average CBT, peak CBT, and average HR between employees who were and were not 

acclimatized. Fisher’s exact test was used to compare differences in the percentages of employees meeting 

each ACGIH criterion in employees who were and were not acclimatized. T-tests or Wilcoxon tests were 

used to compare average CBT, peak CBT, and average HR between employees with and without workrelated symptoms.

References

CDC [1998]. Guideline for infection control in health care personnel. Am J Infec Control 26:289–354.

CFR. Code of Federal Regulations. Washington, DC: U.S. Government Printing Office, Office of the 

Federal Register.
Health Hazard Evaluation Report 2006-0307-3139 Page 23

In evaluating the hazards posed by workplace exposures, NIOSH investigators use both mandatory (legally 

enforceable) and recommended OELs for chemical, physical, and biological agents as a guide for making 

recommendations. OELs have been developed by federal agencies and safety and health organizations to 

prevent the occurrence of adverse health effects from workplace exposures. Generally, OELs suggest levels 

of exposure that most employees may be exposed to for up to 10 hours per day, 40 hours per week, for a 

working lifetime, without experiencing adverse health effects. However, not all employees will be protected 

from adverse health effects even if their exposures are maintained below these levels. A small percentage 

may experience adverse health effects because of individual susceptibility, a preexisting medical condition, 

and/or a hypersensitivity (allergy). In addition, some hazardous substances may act in combination with 

other workplace exposures, the general environment, or with medications or personal habits of the 

employee to produce adverse health effects even if the occupational exposures are controlled at the level set 

by the exposure limit. Also, some substances can be absorbed by direct contact with the skin and mucous 

membranes in addition to being inhaled, which contributes to the individual’s overall exposure.

Most OELs are expressed as a TWA exposure. A TWA refers to the average exposure during a normal 8- to 

10-hour workday. Some chemical substances and physical agents have recommended short term exposure 

limit or ceiling values where adverse health effects are caused by exposures over a short period. Unless 

otherwise noted, the short term exposure limit is a 15-minute TWA exposure that should not be exceeded 

at any time during a workday, and the ceiling limit is an exposure that should not be exceeded at any time.

In the United States, OELs have been established by federal agencies, professional organizations, state 

and local governments, and other entities. Some OELs are legally enforceable limits, while others are 

recommendations. The U.S. Department of Labor OSHA PELs (29 CFR 1910 [general industry]; 29 

CFR 1926 [construction industry]; and 29 CFR 1917 [maritime industry]) are legal limits enforceable 

in workplaces covered under the Occupational Safety and Health Act of 1970. NIOSH RELs are 

recommendations based on a critical review of the scientific and technical information available on a 

given hazard and the adequacy of methods to identify and control the hazard. NIOSH RELs can be 

found in the NIOSH Pocket Guide to Chemical Hazards [NIOSH 2010]. NIOSH also recommends different 

types of risk management practices (e.g., engineering controls, safe work practices, employee education/

training, personal protective equipment, and exposure and medical monitoring) to minimize the risk of 

exposure and adverse health effects from these hazards. Other OELs that are commonly used and cited 

in the United States include the TLVs recommended by ACGIH, a professional organization, and the 

WEELs recommended by the American Industrial Hygiene Association, another professional organization. 

The TLVs and WEELs are developed by committee members of these associations from a review of the 

published, peer-reviewed literature. They are not consensus standards. ACGIH TLVs are considered 

voluntary exposure guidelines for use by industrial hygienists and others trained in this discipline “to 

assist in the control of health hazards” [ACGIH 2011a]. WEELs have been established for some chemicals 

“when no other legal or authoritative limits exist” [AIHA 2010].

Outside the United States, OELs have been established by various agencies and organizations and 

include both legal and recommended limits. The Institut für Arbeitsschutz der Deutschen Gesetzlichen 

Unfallversicherung (IFA, Institute for Occupational Safety and Health of the German Social Accident 

Insurance) maintains a database of international OELs from European Union member states, Canada 

(Québec), Japan, Switzerland, and the United States. The database, available at http://www.dguv.de/ifa/

en/gestis/limit_values/index.jsp, contains international limits for over 1,500 hazardous substances and is 

updated periodically.

Appendix C: Occupational Exposure Limits and H
Page 24 Health Hazard Evaluation Report 2006-0307-3139

Appendix C: Occupational Exposure Limits and Helth Effect

Employers should understand that not all hazardous chemicals have specific OSHA PELs, and for some 

agents the legally enforceable and recommended limits may not reflect current health-based information. 

However, an employer is still required by OSHA to protect its employees from hazards even in the absence 

of a specific OSHA PEL. OSHA requires an employer to furnish employees a place of employment free 

from recognized hazards that cause or are likely to cause death or serious physical harm [Occupational 

Safety and Health Act of 1970 (Public Law 91–596, sec. 5(a)(1))]. Thus, NIOSH investigators encourage 

employers to make use of other OELs when making risk assessments and risk management decisions to 

best protect the health of their employees. NIOSH investigators also encourage the use of the traditional 

hierarchy of controls approach to eliminate or minimize identified workplace hazards. This includes, in 

order of preference, the use of (1) substitution or elimination of the hazardous agent, (2) engineering 

controls (e.g., local exhaust ventilation, process enclosure, dilution ventilation), (3) administrative controls 

(e.g., limiting time of exposure, employee training, work practice changes, medical surveillance), and (4) 

personal protective equipment (e.g., respiratory protection, gloves, eye protection, hearing protection). 

Control banding, a qualitative risk assessment and risk management tool, is a complementary approach 

to protecting employee health that focuses resources on exposure controls by describing how a risk 

needs to be managed. Information on control banding is available at http://www.cdc.gov/niosh/topics/

ctrlbanding/. This approach can be applied in situations where OELs have not been established or can be 

used to supplement the OELs, when available.

Heat Stress 

NIOSH defines heat stress exposure as the sum of the heat generated in the body (metabolic heat) plus 

the heat gained from the environment (environmental heat) minus the heat lost from the body to the 

environment, primarily through evaporation. Many bodily responses to heat stress are desirable and 

beneficial because they help regulate internal temperature and, in situations of appropriate repeated 

exposure, help the body adapt (acclimatize) to the work environment. However, at some stage of heat 

stress, the body’s compensatory measures cannot maintain internal body temperature at the level required 

for normal functioning. As a result, the risk of heat-induced illnesses, disorders, and accidents substantially 

increases. Increases in unsafe behavior, which may lead to accidents, are also seen as the level of physical 

work of the job increases [NIOSH 1986].

Many heat stress guidelines have been developed to protect people against heat-related illnesses. The 

objective of any heat stress index is to prevent a person’s CBT from rising excessively. The World Health 

Organization concluded that, “it is inadvisable for CBT to exceed 100.4°F or for oral temperature to 

exceed 99.5°F in prolonged daily exposure to heavy work and/or heat” [WHO 1969]. According to 

NIOSH, a CBT of 102.2°F should be considered reason to terminate exposure even when CBT is being 

monitored. This does not mean that an employee with a CBT exceeding those levels will necessarily 

experience adverse health effects; however, the number of unsafe acts increases as does the risk of 

developing heat stress illnesses [NIOSH 1986].
Health Hazard Evaluation Report 2006-0307-3139 Page 25

Appendix C: Occupational Exposure Limits and Helth Effect

NIOSH recommends controlling total heat exposure so that unprotected healthy employees are not 

exposed to metabolic and environmental heat combinations that exceed the applicable NIOSH criteria. 

These criteria state that most healthy employees who work in hot environments and are exposed to 

combinations of environmental and metabolic heat less than the NIOSH recommended action limit for 

non-acclimatized employees or the NIOSH REL for acclimatized employees should be able to tolerate total 

heat stress without substantially increasing their risk of incurring acute adverse health effects. Also, no 

employee should be exposed to metabolic and environmental heat combinations that exceed applicable 

ceiling limits without being provided with and properly using appropriate and adequate heat-protective 

clothing and equipment [NIOSH 1986].

ACGIH guidelines require the use of a decision-making process that provides step-by-step situationdependent instructions that factor in clothing insulation values and physiological evaluation of heat strain 

[ACGIH 2011b]. ACGIH WBGT screening criteria factor in the ability of the body to cool itself (clothing 

insulation value, humidity, and wind) and, like the NIOSH criteria, can be used to develop work/

rest regimens for acclimatized and unacclimatized employees. The ACGIH WBGT-based heat exposure 

assessment was developed for a traditional work uniform of long-sleeved shirt and pants, and represents 

conditions under which it is believed that nearly all adequately hydrated, unmedicated, healthy employees 

may be repeatedly exposed without adverse health effects. Clothing insulation values and the appropriate 

WBGT adjustments, as well as descriptors of the other decision-making process components can be found 

in ACGIH’s “Documentation of the Threshold Limit Values for Chemical Substances and Physical Agents 

and Biological Exposure Indices” [ACGIH 2011b]. The ACGIH TLV for heat stress provides a framework 

for the control of heat-related illnesses only. Although accidents and injuries can increase with increasing 

levels of heat stress, it is important to note that the TLVs are not directed toward controlling these 

[ACGIH 2011b].

NIOSH and ACGIH criteria can only be used when WBGT data for the immediate work area are available 

and must not be used when employees wear encapsulating suits or garments that are impermeable or 

highly resistant to water vapor or air movement. Further assumptions regarding work demands include 

an 8-hour work day, 5-day work week, two 15-minute breaks, and a 30-minute lunch break, with rest 

area temperatures the same as, or less than, those in work areas, and at least some air movement. While 

NIOSH and ACGIH guidelines distinguish between safe and dangerous levels, professional judgment 

must be used in administering a heat stress management program to ensure adequate protection. The 

OSHA technical manual’s section on heat stress refers to the ACGIH document for guidelines to evaluate 

employee heat stress and how to investigate the workplace [OSHA 1999].

Heat Strain 

The body’s response to heat stress is called heat strain. Operations involving high air temperatures, radiant 

heat sources, high humidity, direct physical contact with hot objects, and strenuous physical activities 

have a high potential for inducing heat strain in employees. Heat strain is highly individual and cannot be 

predicted on the basis of environmental heat stress measurements. Physiological monitoring for heat strain 

becomes necessary when impermeable clothing is worn, when heat stress screening criteria are exceeded, 

or when data from a detailed analysis (such as the International Standards Organization required sweat 

rate index) shows excess heat stress.
Page 26 Health Hazard Evaluation Report 2006-0307-3139

Appendix C: Occupational Exposure Limits and Helth Effect

ACGIH considers one indicator of physiological strain, sustained peak HR, to be the best sign of 

acute, high-level exposure to heat stress. Sustained peak HR, defined by ACGIH as 180 BPM minus an 

individual’s age, is a leading indicator that thermal regulatory control may not be adequate and that 

increases in CBTs have or will soon occur [ACGIH 2011b].

According to ACGIH, an individual’s heat stress exposure should be discontinued when any of the 

following heat strain indicators occur:

● Sustained (over several minutes) HR exceeds 180 BPM minus the individual’s age in years, (180 BPM 

– age) for those with normal cardiac performance

● CBT is greater than 100.4°F for unselected, unacclimatized personnel and greater than 101.3°F for 

medically fit, heat-acclimatized personnel

● Recovery HR at 1 minute after a peak work effort exceeds 110 BPM

● Presence of symptoms of sudden and severe fatigue, nausea, dizziness, or lightheadedness

An individual may be at greater risk of heat strain if:

● Profuse sweating is sustained over several hours

● Weight loss over a shift is greater than 1.5% of body weight

● 24-hour urinary sodium excretion is less than 55 millimoles

Acclimatization

When employees are first exposed to a hot environment, they show signs of distress and discomfort, 

experience increased CBTs and HRs, and may have headaches and/or nausea. On repeated exposure 

marked adaptation to the hot environment known as acclimatization occurs.

The loss of acclimatization begins when the activity under those heat stress conditions is discontinued, 

and a noticeable loss occurs after 4 days. This loss is usually rapidly made up so that by Tuesday, employees 

who were off on the weekend are as well acclimatized as they were on the preceding Friday. Chronic 

illness, an acute episode of mild illness (e.g., gastroenteritis), the use or misuse of pharmacologic agents, 

a sleep deficit, a suboptimal nutritional state, or a disturbed water and electrolyte balance may reduce an 

employee’s capacity to acclimatize [ACGIH 2011b].

Volume Depletion 

When working in hot environments, it is often difficult to completely replace lost fluids as the day’s work 

proceeds. Sweat contains water and salt, and excessive sweating can cause volume depletion and electrolyte 

imbalances. Volume depletion is different from pure dehydration, and occurs when loss of both water and 

salt/sodium results in a reduced circulatory blood volume [Mange 1997]. Some studies have shown that 

even small deficits adversely affect performance. Volume depletion also negates the advantage granted by 

high levels of aerobic fitness and heat acclimatization.
Health Hazard Evaluation Report 2006-0307-3139 Page 27

Appendix C: Occupational Exposure Limits and Helth Effect

Several studies have shown that volume depletion or dehydration increases CBT during exercise in 

temperate and hot environments. Because water is the most abundant constituent in the body, comprising 

approximately 60% of the body weight in men and 50% in women, maintaining enough water improves 

the body’s overall function. Disorders of water regulation result in hyponatremia or hypernatremia. Most 

individuals with acute exercise-induced heat disorder are volume depleted with normal to mildly increased 

serum sodium and serum osmolality (hypernatremia). Increased water intake before and during activities 

in hot environments is highly emphasized to prevent volume depletion and heat illness. However, drinking 

too much water can lead to decreased serum sodium concentrations (water toxicity or hyponatremia). 

Many people with hyponatremia due to water overload have increased their total body water by about 1 

gallon to achieve such low serum sodium values.

The most significant clinical signs of hyponatremia involve the central nervous system, and symptoms vary 

from subtle changes in one’s ability to think, to decreases in energy levels, to severe alterations such as 

coma or seizure. Symptoms generally parallel the rate of development and degree of hyponatremia.

Fluid Replacement 

Palatability of any fluid replacement solution is important to ensure adequate rehydration. Evidence shows 

that adding sweeteners to drinks leads to increased consumption. Glucose-electrolyte solutions have been 

shown to facilitate sodium and water absorption. Also, the glucose in these solutions provides energy for 

muscular activity in endurance events that require vigorous exercise [Rolls 1990]. However, employees 

should be cautioned to avoid drinking large amounts of sugar-laden beverages in hot climates as this causes 

an osmotic diuresis (increased urine production) that increases fluid loss through urination. Caffeinated 

beverages and alcohol intake also increase urinary fluid loss and should be avoided. The temperature 

of the drink also influences consumption of fluids. Ideally, fluids should be ingested at temperatures of 

50°F–60°F, in small quantities (5–7 ounces), and at frequent intervals (every 15–20 minutes).

Average Americans consume adequate, if not excessive, amounts of sodium in their usual diet such that 

for mild volume depletion, only water replacement is needed. However, in moderate volume depletion 

or when involved in events resulting in prolonged sweating, electrolyte (i.e., sodium) replacement is 

indicated. Many oral electrolyte replacement formulas such as Gatorade® are available. Salt tablets are not 

recommended as they can irritate the stomach, leading to vomiting, which can exacerbate fluid losses and 

do not address water replacement needs. Those with nausea and vomiting from heat stress may require 

intravenous saline administration to replace their water and sodium.

References 

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Industrial Hygienists.

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Cincinnati, OH: American Conference of Governmental Industrial Hygienists; 2002–2011 Suppl.
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AIHA [2010]. AIHA 2010 Emergency response planning guidelines (ERPG) & workplace environmental 

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accessed: August 2011.

OSHA [1999]. Heat stress. In: OSHA technical manual. Sec 3 Chap 4. Washington, D.C. U.S. 

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Health Hazard Evaluation Report 2006-0307-3139 Page 29

Acknowledgments a

Availability of Re

The Hazard Evaluations and Technical Assistance Branch 

(HETAB) of the National Institute for Occupational Safety and 

Health (NIOSH) conducts field investigations of possible health 

hazards in the workplace. These investigations are conducted 

under the authority of Section 20(a)(6) of the Occupational Safety 

and Health Act of 1970, 29 U.S.C. 669(a)(6) which authorizes 

the Secretary of Health and Human Services, following a written 

request from any employer or authorized representative of 

employees, to determine whether any substance normally found 

in the place of employment has potentially toxic effects in such 

concentrations as used or found. HETAB also provides, upon 

request, technical and consultative assistance to federal, state, and 

local agencies; labor; industry; and other groups or individuals to 

control occupational health hazards and to prevent related trauma 

and disease.

Mention of any company or product does not constitute 

endorsement by NIOSH. In addition, citations to websites 

external to NIOSH do not constitute NIOSH endorsement of 

the sponsoring organizations or their programs or products. 

Furthermore, NIOSH is not responsible for the content of these 

websites. All Web addresses referenced in this document were 

accessible as of the publication date.

This report was prepared by Bich Dang, Chad H. Dowell, and 

Charles Mueller of HETAB, Division of Surveillance, Hazard 

Evaluations and Field Studies. Industrial hygiene field assistance 

was provided by Chandran Achutan, Salena Barnes, Donnie 

Booher, Karl Feldmann, and Alberto Garcia. Medical field 

assistance was provided by Carlos Aristeguieta, John Clark, Sara 

Luckhaupt, and Natalie Madero. Health communication assistance 

was provided by Stefanie Evans. Editorial assistance was provided 

by Ellen Galloway. Desktop publishing was performed by Robin 

Smith.

Copies of this report have been sent to employee and 

management representatives at the aluminum smelter, the state 

health department, and the Occupational Safety and Health 

Administration Regional Office. This report is not copyrighted and 

may be freely reproduced. The report may be viewed and printed at 

http://www.cdc.gov/niosh/hhe/. Copies may be purchased from 

the National Technical Information Service at 5825 Port Royal 

Road, Springfield, Virginia 22161.
Below is a recommended citation for this report:

NIOSH [2011]. Health hazard evaluation report: heat stress and strain evaluation 

among aluminum potroom employees – Texas. By Dang B, Dowell CH, Muller C. 

Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease 

Control and Prevention, National Institute for Occupational Safety and Health, NIOSH 

HETA No. 2006-0307-3139.

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