HR Management & Compliance

How Soon Can a Worker Return to Work After Heat Illness?

Yesterday’s Advisor busted some myths about heat-related illness; today, we’ll look at some factors affecting an employee’s ability to return to work following a heat illness incident. Protecting workers from sweltering temperatures is a critical issue during the months of July and August—and OSHA is ramping up its awareness and enforcement efforts this year. Be sure that those who manage or supervise employees outdoors know the facts.

Most guidelines suggest that workers can easily relapse within a week after suffering heat illness, but new research indicates that their susceptibility may last far longer. “Heat intolerance” has been most closely studied in the armed forces because soldiers who collapse under conditions of high heat and heavy exertion and return to duty too soon tend to suffer secondary collapses.

Understanding and Assessing Heat Intolerance

Many factors can affect a worker’s underlying heat tolerance, including:

  • Air temperature and humidity,
  • Direct sun exposure,
  • Indoor radiant heat sources (ovens, hot manufacturing processes, etc.),
  • Limited air movement,
  • Physical exertion,
  • Not drinking enough fluids (dehydration),
  • Personal protective equipment (PPE) or clothing,
  • Certain medications,
  • Physical condition,
  • Lack of recent exposure (not acclimated), and
  • Advanced age (65+).


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But once a worker has suffered a heat illness, the illness itself becomes a predisposing condition that can indicate an increased susceptibility to heat illness. Workers who have suffered heat illness should not return to work right away.
Whether and when a worker can return to working in the heat after suffering heat-related illness is affected by:

  • Preexisting susceptibility. Some people are naturally heat-intolerant, but this is not usually discovered until after they suffer a heat-related illness. These workers will remain intolerant to heat; acclimatization will not help them.
  • Recovery time. Some workers develop temporary intolerance to heat after suffering a heat-related illness. They may remain heat intolerant for several months. A heat-tolerance test, in which the individual walks on a treadmill in a climatic chamber while being closely monitored, can determine whether he or she can safely return to work.


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One factor that doesn’t seem to affect a person’s heat tolerance following heat illness is the individual’s overall physical fitness. Overall fitness has no effect on whether a person becomes heat intolerant after suffering a heat-related illness, so don’t use that as a return-to-duty criterion.