What OSHA Heat Index Risk Means
OSHA uses heat index guidance to help employers recognize when outdoor heat can become hazardous. The risk is not based on weather alone. Heavy work, personal protective equipment, lack of acclimatization, direct sunlight, and limited air movement can make the real risk higher than the posted heat index.
The National Weather Service heat index is calculated for shady, light-wind conditions. In direct sun, the apparent heat burden can be higher. For detailed industrial hygiene assessment, wet bulb globe temperature is often a better worksite measure because it accounts for radiant heat and wind.
- Employers use heat index to trigger preventive actions.
- Supervisors use it to plan water, rest, and shade.
- Workers use it to recognize when symptoms require action.
How to Calculate Heat Index
A common U.S. heat index equation is the Rothfusz regression: HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T^2 - 0.05481717R^2 + 0.00122874T^2R + 0.00085282TR^2 - 0.00000199T^2R^2, where T is air temperature in degrees F and R is relative humidity percent.
For example, at 90 F and 60 percent relative humidity, the formula gives a heat index of about 100 F before special adjustment rules. That number should be treated as a warning level, not a complete job hazard analysis.
Turning the Number Into Controls
A heat plan should connect rising heat index values to practical controls. Those controls can include cool drinking water, scheduled rest breaks, shaded or cooled recovery areas, acclimatization for new or returning workers, emergency response procedures, and supervisor checks.
OSHA does not treat a simple calculator result as a complete compliance program. Employers still need to evaluate the job, train workers, monitor symptoms, and adjust work practices when conditions change during the day.
- Increase observation during heat waves and after days off.
- Watch new workers closely during the first one to two weeks.
- Do not rely on thirst alone as a hydration control.
Common Heat Risk Mistakes
One mistake is checking the forecast once and ignoring the work area. Roofs, asphalt, attics, kitchens, foundries, and enclosed spaces can be hotter than nearby weather stations. Another mistake is treating acclimatized and unacclimatized workers the same.
Heat illness can escalate quickly. Muscle cramps, dizziness, headache, confusion, fainting, and cessation of sweating in severe cases should be taken seriously. Emergency procedures should be understood before the first hot day, not improvised during an incident.
- Do not ignore humidity on days that are not extremely hot.
- Do not overlook medications, illness, or PPE that increase heat strain.
- Do not use heat index alone for indoor radiant heat sources.
Frequently asked questions
Is heat index the same as wet bulb globe temperature?
No. Heat index uses temperature and humidity. Wet bulb globe temperature also reflects radiant heat and air movement, so it is often better for detailed occupational heat assessment.
Does OSHA require a specific work-rest schedule at each heat index?
OSHA guidance supports preventive controls, but requirements can depend on the standard, state plan, industry, and facts of the worksite. Employers should follow applicable federal, state, and local rules.
Why is direct sunlight important?
The standard heat index assumes shade. Direct sun increases radiant heat load, so workers in sun may face higher heat stress than the basic heat index suggests.
Who is at highest risk during hot work?
New workers, workers returning after absence, people doing heavy labor, workers wearing PPE, and people with certain medical conditions or medications can be at higher risk.