Introduction to Heat Stress

Workers indoors or outdoors are susceptible to stress in high temperature environments, requiring an instrument which can monitor temperatures, calculate possible heat stroke conditions, and alert the workers so they can stop, cool down, drink water, etc. The Heat Stress instrument measures WBGT (Wet Bulb Globe Temperature) for both indoor and outdoor environments.

How Does a Heat Stress Monitor Work?

The WBGT instruments can measure temperature of wet and dry bulbs and a black globe to estimate stress in a high temperature working environment, indoors or outdoors. The WBGT index is a single number derived mathematically from three distinct temperature measurements: wet bulb temperatures, dry bulb temperatures, and globe temperatures. Heat injuries can be expected at WBGT readings of 75°F and above unless preventative measures are taken.

The heat stress formula referenced by the U.S. Government Occupational and Environmental Health Bulletin is:

WBGT (outdoor) = 0.7 x WB + 0.2 x BG + 0.1 x DB
WBGT (indoor) = 0.7 x WB + 0.3 x BG


  • WB is the wet-bulb temperature
  • BG is the 6-inch Vernon Black Globe temperature
  • DB is the dry-bulb (ambient) temperature
  • WBGT is the wet-bulb globe temperature index

WBGT Index Guidelines

The U.S. Government Occupational and Environmental Health Bulletin lists several of the WBGT index general guidelines, which include the following:

WBGT Index = 78.0 - 81.9°F. Extremely intense physical exertion may precipitate heat exhaustion or heat stroke; therefore caution should be taken.

WBGT Index = 82.0 - 84.9°F. Discretion should be used in planning heavy exercise for unseasoned personnel. This is sometimes used as the marginal limit of environmental heat stress.

WBGT Index = 85.0 - 87.9°F. Strenuous exercise such as marching at standard cadence should be suspended in unseasoned personnel during their first three weeks of training. Training activities may be continued on a reduced scale after the second week of training. Outdoor classes in the sun should be avoided above this temperature.

WBGT Index = 85.0°F or greater. Outdoor classes in the sun should be avoided.

WBGT Index = 88.0°F. Strenuous exercise should be curtailed for all recruits and other trainees with less than 12 weeks training in hot weather.

WBGT Index = 90.0°F or greater. Physical training and strenuous exercise should be suspended for all personnel (excluding essential operational commitments not for training purposes, where risk of heat casualties may be warranted).

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