Heat Index & WBGT Monitoring

Heat illness is preventable, but only if you know how hot the work environment really is. Two measures are used: the heat index (what weather reports give you) and the wet bulb globe temperature (WBGT) (what many safety programs use). They don’t measure the same thing, and the difference matters on a jobsite in full sun.

• Outdoor Workers
• Industrial
• Safety Managers
• California
• Maryland
• Nevada
• Oregon
• Washington

The heat index

• The heat index combines air temperature and relative humidity to estimate how hot it feels to the body. Humid air slows sweat evaporation, so the body cools less well.
• The National Weather Service gives heat index categories and the cautions that go with them:

• Heat index values are calculated for shady conditions with light wind. Direct sunlight can add up to about 15°F to the heat index. Heavy clothing, PPE and strenuous work add more strain.
• The heat index is easy to find, in forecasts, apps and alerts, and it is useful for planning. But it doesn’t account for sun radiation, wind, work rate or clothing.
• OSHA uses the heat index to set risk levels in its Heat Illness Prevention campaign (for example, many programs start special precautions at a heat index of 80°F and higher risk above 91°F), and offers a free OSHA-NIOSH heat safety app that shows the current heat index and precautions. Check OSHA’s current guidance.

WBGT: wet bulb globe temperature

• WBGT is a measure of heat stress that combines air temperature, humidity, radiant heat (from the sun and hot surfaces) and wind, measured with a special instrument. It is the standard used by NIOSH, ACGIH, the military and many sports and industrial programs.
• Outdoors in sun, WBGT takes in the globe temperature (radiant heat), natural wet bulb and dry bulb temperature. Indoors and without sun, it uses just wet bulb and globe readings.
• WBGT is better than the heat index for workplaces because it includes radiant heat, especially around furnaces, boilers, hot roofs, pavement and equipment, which the heat index leaves out.
• It is measured with a WBGT monitor (a handheld or tripod-mounted device) placed where the work happens, or estimated from weather data by an app or a nearby weather station. Estimates are less accurate in sun and radiant heat.
• Readings are in degrees, and exposure limits depend on how hard the work is, the workers’ acclimatization and clothing. Employers use limits from NIOSH, ACGIH or their own program to decide work and rest periods and when to stop work. See Heat Acclimatization.

Heat index versus WBGT

Don’t compare heat index and WBGT numbers directly. They are on different scales.

How to use these measures at work

• Check the forecast daily and plan heavy work for cooler parts of the day. Use heat alerts from the weather service. See Weather Planning.
• Use the heat index or WBGT to set precautions: water, shade or cooling, rest breaks, a buddy system and pace of work. Increase them as the level rises.
• Monitor on site. Where there is radiant heat or high risk, use a WBGT monitor in the work area, with readings taken at set intervals and when conditions change. Follow the instrument maker’s calibration and use instructions.
• Adjust for the worker, not just the weather. Heavy clothing, PPE, work rate, age, fitness, medications and illness raise risk at the same temperature. See Cooling PPE and Heat Illness.
• Acclimatize new and returning workers. Many heat deaths happen in the first days on the job. Increase exposure gradually over 1 to 2 weeks, starting with about 20 percent of the normal load for new workers, and building up. See Heat Acclimatization.
• Provide water and breaks: about a cup of water every 15 to 20 minutes while working in the heat, and shade or air-conditioned rest areas. Don’t wait for thirst.
• Know the signs of heat illness and act at once: heavy sweating, cramps, dizziness, headache, nausea, confusion, hot dry skin or fainting. Heat stroke is an emergency: call 911 and cool the person right away.
• Have an emergency plan that covers heat. See Emergency Action Plans.

Rules and standards

• OSHA doesn’t yet have a final, nationwide heat standard for all workplaces. It has been working on a proposed rule, and has a national emphasis program on heat. Employers are expected to protect workers from recognized hazards, including heat, under the General Duty Clause. See OSHA Emphasis Programs and How OSHA Makes New Rules.
• Several states have heat rules, such as California, Washington, Oregon, Maryland and Nevada, that set trigger temperatures, water, shade and rest. See State Heat Rules and California Indoor Heat Rule. Rules and trigger levels differ by state and change over time.
• NIOSH and ACGIH publish recommended WBGT limits that employers often use as a guide. Check the current criteria and consult an industrial hygienist.

Indoor heat

Hot indoor work, such as in foundries, bakeries, laundries, kitchens, steel mills, warehouses without cooling and attics, can expose workers to high heat and radiant heat, even when outdoor weather is mild. Use WBGT monitors in these places, and control heat with ventilation, shielding, cooling, schedule changes and PPE. See Industrial Ovens & Furnaces.

Heat monitoring checklist

• I check the daily heat index and heat alerts, and plan heavy work for cooler times
• I know that direct sun can add about 15°F to the heat index
• Where radiant heat exists, WBGT is measured on site and used to set work and rest
• New and returning workers are acclimatized gradually
• Water, shade or cooling and breaks are provided and used
• Everyone knows the signs of heat illness and that heat stroke needs 911 and rapid cooling

Last updated: October 4, 2026