Heat Stress Hazard Assessment: The OSHA Approach

Date First Published on SafetySmart Compliance: February 2nd, 2012
Topics: Heat & Cold Stress |

Like any other hazard, heat stress has to be managed by a process that begins with hazard assessment and detection. OSHA doesn’t have a heat stress standard. However, an OSHA Directive (TED 01-00-015) includes instructions to help OSHA inspectors determine whether heat stress dangers are present at a worksite. Knowing how OSHA assesses heat stress hazards is—or should be—of obvious relevance to your own heat stress control efforts.

Click here for a Heat Stress Hazard Assessment Checklist Work-Load Assessment

The first phase in assessment is consideration of the work-load for particular jobs. Under conditions of high temperature and heavy workload, the inspector should determine the work-load category of each job by averaging metabolic rates for the tasks and then ranking them:

  1. Light work: up to 200 kcal/hour
  2. Medium work: 200-350 kcal/hour
  3. Heavy work: 350-500 kcal/hour

Cool Rest Area: Where heat conditions in the rest area are different from the work area, the metabolic rate (M) should be calculated using a time-weighted average, as follows:

Equation III: 4-1. Average Metabolic Rate

AverageM = (M1)(t1)+ (M2)(t2)+…+(Mn)(tn) 

where: M = metabolic rate
t = time in minutes

A videotape may be helpful in evaluating work practices and metabolic load, according to the Directive.  The Directive includes a chart listing examples of activities and a calculation:

  • Light hand work: writing, hand knitting
  • Heavy hand work: typewriting
  • Heavy work with one arm: hammering in nails (shoemaker, upholsterer)
  • Light work with two arms: filing metal, planing wood, raking the garden
  • Moderate work with the body: cleaning a floor, beating a carpet
  • Heavy work with the body: railroad track laying, digging, barking trees
Sample Calculation: Assembly line work using a heavy hand tool
Walking along 2.0 kcal/min
Intermediate value between heavy work with two arms and light work with the body 3.0 kcal/min
Add for basal metabolism 1.0 kcal/min
Total:   6.0 kcal/min

The following table shows an assessment of work:

Body position and movement kcal/min*
Sitting 0.3
Standing 0.6
Walking 2.0-3.0
Walking uphill add 0.8 for every meter (yard) rise

Type of work Average kcal/min Range kcal/min
Hand work
Light 0.4 0.2-1.2
Heavy 0.9
Work: One arm
Light 1.0 0.7-2.5
Heavy 1.7
Work: Both arms
Light 1.5 1.0-3.5
Heavy 2.5
Work: Whole body
Light 3.5 2.5-15.0
Moderate 5.0
Heavy 7.0
Very heavy 9.0
* For a “standard” worker of 70 kg body weight (154 lbs) and 1.8m2 body surface (19.4 ft2).

Sampling Methods

The Directive next addresses 5 methods that can be used to measure heat stress conditions.

Body Temperature Measurements: Instruments used to estimate deep body temperature by measuring the temperature in the ear canal or on the skin aren’t “sufficiently reliable” to use in compliance evaluations, says the Directive.

Environmental Heat Measurements: According to the Directive, environmental heat measurements should be made at, or as close as possible to, the specific work area where the worker is exposed. When a worker isn’t continuously exposed in a single hot area but moves between 2 or more areas having different levels of environmental heat, or when the environmental heat varies substantially at a single hot area, environmental heat exposures should be measured for each area and for each level of environmental heat to which employees are exposed.

Wet Bulb Globe Temperature Index: Inspectors should calculate Wet Bulb Globe Temperature (WBGT) using the appropriate formula in Appendix III:4-2 of the Directive. The WBGT for continuous all-day or several hour exposures should be averaged over a 60-minute period. Intermittent exposures should be averaged over a 120-minute period. These averages should be calculated using the following formula:

Equation III:4-2. Average Web Bulb Globe Temperature (WBGT)


AverageWBGT = (WBGT1)(t1)+ (WBGT2)(t2)+…+(WBGTn)(tn) 



For indoor and outdoor conditions with no solar load, WBGT is calculated as:

WBGT = 0.7NWB + 0.3GT

For outdoors with a solar load, WBGT is calculated as

WBGT = 0.7NWB + 0.2GT + 0.1DB 

where: WBGT = Wet Bulb Globe Temperature Index
NWB = Nature Wet-Bulb Temperature
DB = Dry-Bulb Temperature
GT = Globe Temperature


Measurement of Heat Conditions: The Directive notes that portable heat stress meters or monitors can be used to measure heat conditions and that such instruments can calculate both the indoor and outdoor WBGT index according to established ACGIH Threshold Limit Value equations. With this information and information on the type of work being performed, heat stress meters can determine how long a person can safely work or remain in a particular hot environment. Appendix III:4-2 lists an alternate method of calculation.

The Directive includes the following table illustrating permissible heat exposure TLVs:



————- Work Load* ————
Work/rest regimen Light Moderate Heavy
Continuous work 30.0°C (86°F) 26.7°C (80°F) 25.0°C (77°F)
75% Work, 25% rest, each hour 30.6°C (87°F) 28.0°C (82°F) 25.9°C (78°F)
50% Work, 50% rest, each hour 31.4°C (89°F) 29.4°C (85°F) 27.9°C (82°F)
25% Work, 75% rest, each hour 32.2°C (90°F) 31.1°C (88°F) 30.0°C (86°F)
*Values are in °C and °F, WBGT. 

These TLV’s are based on the assumption that nearly all acclimatized, fully clothed workers with adequate water and salt intake should be able to function effectively under the given working conditions without exceeding a deep body temperature of 38°C (100.4° F). They are also based on the assumption that the WBGT of the resting place is the same or very close to that of the workplace. Where the WBGT of the work area is different from that of the rest area, a time-weighted average should be used (consult the ACGIH 1992-1993 Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices (1992).

These TLV’s apply to physically fit and acclimatized individuals wearing light summer clothing. If heavier clothing that impedes sweat or has a higher insulation value is required, the permissible heat exposure TLV’s in Table III:4-2 must be reduced by the corrections shown in Table III:4-3.

Other Thermal Stress Indices: The Directive refers to 2 more indices that can be used to calculate thermal stress:

  • The Effective Temperature index (ET) combines the temperature, the humidity of the air, and air velocity. This index has been used extensively in the field of comfort ventilation and air-conditioning. ET remains a useful measurement technique in mines and other places where humidity is high and radiant heat is low; and
  • The Heat-Stress Index (HSI) considers all environmental factors and work rate, but isn’t completely satisfactory for determining an individual worker’s heat stress and is also difficult to use.

Click here for a Heat Stress Hazard Assessment Checklist

Click here for a Comprehensive Model Heat Stress Plan

Click here for a short version of a Model Heat Stress Plan