Heat Stress Basics

In my experience, quantifying heat stress is a task many safety professionals avoid. This may derive from the fact that there is no specific OSHA regulation governing allowable exposures to heat stress. Also, several components of the typical heat stress evaluation are subjective. Attributing numbers to the contribution of clothing and activity level is not as straightforward as reading an instrument or interpreting a laboratory report.

This article presents, in a simplified fashion, the decision-making process described by the American Conference of Governmental Industrial Hygienists (ACGIH) to screen jobs for risk due to heat.

Recognizing Risk – Heat stress must be taken seriously because its potential effects are severe and acute. Heat can strain the body and result in death in a short time. Pre-existing health conditions can make some workers more susceptible to the adverse effects of heat stress.



The ACGIH evaluation protocol consists of these components:

  • Temperature index (also referred to as wet-bulb globe temperature, or WBGT) – This index can be obtained with a simple instrument that calculates the WBGT based on input from three thermometers. An ordinary dry-bulb thermometer measures simple air temperature; a thermometer with its bulb enveloped in a water-soaked cloth accounts for the effect of humidity; and a thermometer inside a black globe measures radiant heat.
  • Work schedule – Continuous work in a hot environment strains the body significantly more than a work regimen that involves resting for only a portion of each hour.
  • Physical exertion – The subject’s WBGT readings are compared with criteria that vary based on the level of physical exertion. This is where absolute numbers are not easily assigned without a detailed physiological study involving factors such as oxygen consumption and calories burned. For screening purposes, however, work can be subjectively categorized as light, medium, or heavy. Standing with minor hand/arm movement is considered light work, using a table saw is an example of moderate work, and shoveling wet sand would be heavy work.
  • Clothing – As with work schedule and physical exertion levels, evaluating a worker’s clothing is an important factor. The evaluation criteria are adjusted for different levels of clothing. A one-layer uniform with a long-sleeved shirt and long pants is more restrictive than shorts and a T-shirt. Adjustment factors are applied to screening criteria when restrictive clothing is worn.
  • Acclimatization – It is a well-established physiological principle that body chemistry adjusts to working in hot environments over time. After gradual introduction to longer durations of work in hot environments, the body’s tolerance increases significantly.



The screening criteria discussed above are used to decide if one needs either a more detailed evaluation or implementation of a heat stress management program. Since the detailed evaluation can be complicated, many workplaces find it easier to implement a management program. This approach errs on the side of caution rather than collecting more data. Following are the typical elements of a heat stress management plan:

  • Monitor employees for the signs of heat strain. This often involves training of observers or coworkers to identify the symptoms.
  • Perform medical evaluations to identify employees susceptible to heat injury due to physical condition or the use of medication.
  • Supply clothing that is less restrictive.
  • Allow personnel to self-limit exposures and take frequent water and rest breaks.
  • Institute engineering controls such as the use of cooling fans and the installation of materials that shield radiant heat.
  • Institute administrative controls that allow rotation of workers into and out of hot areas.
  • Coordinate heat strain management with your ergonomics program for a synergistic effect. Elimination of heavy lifting and awkward repetitive body movements will also reduce heat produced by the body and reduce the risk of heat strain.
  • Allow workers to acclimatize over time to hot jobs.



Threshold Limit Values for Chemical Substances and Physical Agents, American Conference of Governmental Industrial Hygienists, 2015.

OSHA Technical Manual, Section 2, Chapter 4, Heat Stress.


Christopher J. Wesley, CIH

Vice President, EHS Services