Heat Stress Monitors

Heat Stress Monitors – Important, but Often Overlooked Safety Tools

Heat stress is the reaction of the body to working in warm/hot environments – the combined effect of temperature, humidity and radiant heat. Individuals who routinely work in high temperature environments become heat-acclimated and though still subject to heat induced illnesses, are more tolerant of high temperatures.

Heat stress ideally is monitored by measuring core body temperature. This is impractical and based on extensive experimental data, commercial heat stress monitors measure air temperature, humidity and radiant temperature to calculate a Heat Index for indoor and outdoor work environments which is correlated to risk levels. The table below summarizes OSHA’s Heat Index Guidelines. Additional guidance from OSHA’s website can be accessed here.

Summary of Heat-Related Risk Levels and Associated Protective Measures

The most critical actions employers should take to help prevent heat-related illness at each risk level:

Heat Index Risk Level Protective Measures
<91°F

Lower (Caution)
  • Provide drinking water
  • Ensure that adequate medical services are available
  • Plan ahead for times when heat index is higher, including worker heat safety training
  • Encourage workers to wear sunscreen
  • Acclimatize workers

If workers must wear heavy protective clothing, perform strenuous activity or work in the direct sun, additional precautions are recommended to protect workers from heat-related illness.*

91°F to 103°F
Moderate In addition to the steps listed above:

  • Remind workers to drink water often (about 4 cups/hour)**
  • Review heat-related illness topics with workers: how to recognize heat-related illness, how to prevent it, and what to do if someone gets sick
  • Schedule frequent breaks in a cool, shaded area
  • Acclimatize workers
  • Set up buddy system/instruct supervisors to watch workers for signs of heat-related illness

If workers must wear heavy protective clothing, perform strenuous activity or work in the direct sun, additional precautions are recommended to protect workers from heat-related illness.*

  • Schedule activities at a time when the heat index is lower
  • Develop work/rest schedules
  • Monitor workers closely
103°F to 115°F High In addition to the steps listed above:

  • Alert workers of high risk conditions
  • Actively encourage workers to drink plenty of water (about 4 cups/hour)**
  • Limit physical exertion (e.g. use mechanical lifts)
  • Have a knowledgeable person at the worksite who is well-informed about heat-related illness and able to determine appropriate work/rest schedules
  • Establish and enforce work/rest schedules
  • Adjust work activities (e.g., reschedule work, pace/rotate jobs)
  • Use cooling techniques
  • Watch/communicate with workers at all times

When possible, reschedule activities to a time when heat index is lower

>115°F Very High to Extreme Reschedule non-essential activity for days with a reduced heat index or to a time when the heat index is lower

Move essential work tasks to the coolest part of the work shift; consider earlier start times, split shifts, or evening and night shifts.

Strenuous work tasks and those requiring the use of heavy or non-breathable clothing or impermeable chemical protective clothing should not be conducted when the heat index is at or above 115°F.

If essential work must be done, in addition to the steps listed above:

  • Alert workers of extreme heat hazards
  • Establish water drinking schedule (about 4 cups/hour)**
  • Develop and enforce protective work/rest schedules
  • Conduct physiological monitoring (e.g., pulse, temperature, etc)
  • Stop work if essential control methods are inadequate or unavailable.
*The heat index is a simple tool and a useful guide for employers making decisions about protecting workers in hot weather. It does not account for certain conditions that contribute additional risk, such as physical exertion. Consider taking the steps at the next highest risk level to protect workers from the added risks posed by:

  • Working in the direct sun (can add up to 15°F to the heat index value)
  • Wearing heavy clothing or protective gear

**Under most circumstances, fluid intake should not exceed 6 cups per hour or 12 quarts per day. This makes it particularly important to reduce work rates, reschedule work, or enforce work/rest schedules.

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