Is the heat at your worksite over the NIOSH limits?

What WBGT measures and why it is used

A thermometer in the shade tells you little about a roofer, a foundry worker or a soldier on a march. Heat reaches the body from the air, from the sun and hot surfaces by radiation, and it leaves mainly as sweat that evaporates, which depends on humidity and wind. WBGT puts these together on the Celsius or Fahrenheit scale. The globe thermometer, a thermometer in a black sphere, picks up radiant heat; the natural wet bulb, a thermometer with a wetted wick left in the open air, picks up humidity and wind. NIOSH's heat criteria document (2016-106) takes WBGT as the standard measure for workplace limits. The heat index calculator is the right tool for a weather report; this one is for a work site.

The method, step by step

First compute WBGT: 0.7 × natural wet bulb + 0.2 × globe + 0.1 × air outdoors in sunshine, or 0.7 × natural wet bulb + 0.3 × globe indoors. Then find the metabolic rate M for the work in watts, averaged over the hour, and read the two limits from NIOSH's equations: RAL = 59.9 − 14.1 × log₁₀ M (workers not acclimatized) and REL = 56.7 − 11.5 × log₁₀ M (acclimatized), in °C-WBGT. If the WBGT is above the limit that applies, the heat is beyond what NIOSH recommends without added controls, and the metabolic rate should be measured rather than guessed. The limits are for an unprotected, healthy 70 kg worker in light clothing doing a continuous hour of work.

Worked example

A crew lays pavers in the sun. A WBGT meter shows a natural wet bulb of 78 °F, a globe of 100 °F and air at 90 °F, so WBGT = 0.7 × 78 + 0.2 × 100 + 0.1 × 90 = 83.6 °F (28.7 °C). The work is moderate, 300 kcal per hour, which is 348.9 W. NIOSH's limits at that rate are 75.3 °F (alert) and 81.4 °F (exposure). The WBGT is above both, by 2.2 °F over the exposure limit, so even an acclimatized crew needs the heat controlled. At a lighter 180 kcal per hour the exposure limit rises to 86.0 °F; at a heavier 415 kcal per hour it falls to 78.5 °F. The calculator opens with this crew's readings.

Limits of this method

• Clothing. The limits assume light summer clothing. NIOSH says WBGT is not the right measure for vapor-barrier or encapsulating clothing and other protective equipment, because they block evaporation; such work needs other methods and, often, physiological monitoring. ACGIH publishes adjustment values for clothing, but its Threshold Limit Values are copyrighted, so this page does not reproduce them; look them up in the ACGIH TLV book if you work to them.
• Averages hide the peak. The limits are for a one-hour average. NIOSH asks that heat be measured at least hourly in the hottest part of the shift, in the hottest months and during heat waves, and that the average cover every area a worker passes through.
• Chart versus equation. The printed equations and the charts differ by a degree or so in places (see the question above). Work near a limit is a reason to measure, not to argue about decimals.
• The person matters. Obesity, medications, a recent illness, dehydration and not being acclimatized all lower a worker's tolerance; NIOSH recommends medical monitoring for workers exposed above the alert limit.

Common questions

What is WBGT?

Wet bulb globe temperature is one number that combines the four things that make a hot workplace hot: air temperature, humidity, radiant heat and air movement. NIOSH's heat criteria document (2016-106, section 9.3.2) gives WBGT = 0.7 × natural wet bulb + 0.3 × globe temperature indoors or with no sun, and WBGT = 0.7 × natural wet bulb + 0.2 × globe + 0.1 × air temperature outdoors in sunshine. The wet bulb carries most of the weight because it reflects both humidity and wind. With a natural wet bulb of 78 °F, a globe of 100 °F and air at 90 °F, the sunny formula gives 83.6 °F and the indoor formula 84.6 °F.

What are the NIOSH RAL and REL?

The recommended alert limit (RAL) is the WBGT above which workers who are not acclimatized to heat should be protected; the recommended exposure limit (REL) is the higher limit for acclimatized workers. Both fall as the metabolic rate rises, according to NIOSH's equations RAL = 59.9 − 14.1 × log₁₀ M and REL = 56.7 − 11.5 × log₁₀ M, in °C-WBGT with M in watts. At 300 kcal per hour (348.9 W, NIOSH's example of moderate work) they are 24.0 °C (75.3 °F) and 27.5 °C (81.4 °F).

Why do the limits differ from NIOSH's worked example?

NIOSH's own example reads 27.8 °C for the REL and 25 °C for the RAL at 300 kcal per hour from its charts (figures 8-1 and 8-2). The printed equations give 27.5 °C and 24.0 °C. This calculator uses the equations, which are the more cautious values, and shows the number of degrees of room left. If you must justify a decision, check it against the chart in NIOSH 2016-106.

How do I estimate the metabolic rate?

Add up the energy of the work, averaged over a full hour including rests. NIOSH describes the screening estimate as a one-hour time-weighted average and points to the Compendium of Physical Activities (Ainsworth and others, 2011) for activity values; its worked example treats continuous moderate work as about 300 kcal per hour (348.9 W). Where the result lands near a limit, the document says the metabolic heat should be measured, for example by indirect calorimetry, instead of estimated. One kcal per hour is 1.163 W.

Is WBGT the same as the heat index?

No. The heat index (the weather service's "feels like" figure) is built for a person at rest in the shade and uses only air temperature and humidity. WBGT adds radiant heat and wind, which is what a worker in the sun or next to a furnace feels, and it is the index OSHA and NIOSH use for heat in workplaces. Use the heat index calculator for a weather forecast and this page for a work site.

Can I use a wet globe (Botsball) reading?

Roughly. NIOSH says the wet globe temperature is easier to take and, for moderate radiant heat and humidity (about 72 °F and 50% relative humidity), WBGT is approximately wet globe temperature plus 2 °C (3.6 °F). It calls that adequate for general monitoring, and says high readings should be followed by a proper WBGT measurement. Do not use the shortcut to prove a site is safe.

This is a screening tool, not a safety assessment. Heat illness can kill: employers should follow a written heat illness prevention program with acclimatization, water, rest, shade and emergency response, and have a qualified person measure the site.

Sources: NIOSH, Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments, DHHS (NIOSH) Publication 2016-106 (Jacklitsch and others, February 2016), section 1.1, chapter 8 (equations for the RAL and REL, figures 8-1 and 8-2) and section 9.3 (WBGT equations and wet globe temperature); the Compendium of Physical Activities (Ainsworth and others, 2011) as cited there. Related: heat index calculator, wind chill calculator for cold, noise dose calculator. How we check our calculators.