Wind blockage - Some worksites may be hotter than surrounding areas because of structures that block air movement.Heat sources - Weather reports cannot account for the heat generated by fires, hot tar or other materials, ovens, or other hot equipment, or heat-absorbing surfaces such as roads and roof surfaces.Direct sunlight can increase Heat Index by up to 13.5☏ (7.5☌). Work in the sun may be considerably hotter. Direct sunlight - Weather services measure temperature and Heat Index in the shade.Indoor work - A weather report cannot gauge conditions inside a building.In addition to possible distance-based errors, weather reports can be inaccurate if the worksite has features that affect heat conditions. The potential error increases with distance from the weather station. Heat conditions at the worksite may be different for multiple reasons, from cloud cover and humidity to local heat sinks. While local weather reports based on meteorological data from observation stations can be useful, the readings from these stations may not reflect the conditions at the specific worksite. Use of heat index is a less desirable substitute. Workplace environmental heat should be measured on-site using WBGT meters. All these factors can influence the total heat stress experienced by workers. Air (dry bulb) temperature also ignore relative humidity. The heat index does not account for the effects of wind, sunlight, radiant heat sources, or workload. It is measured in the shade and combines air temperature and relative humidity to represent how hot the conditions feel at rest. Heat Index is another common way to measure heat stress. In contrast, standard thermometers only assess one factor (air temperature). One major advantage is that WBGT accounts for all four major environmental heat factors - temperature, humidity, radiant heat, and wind. WBGT has important advantages over other environmental heat measurements. Employers should always follow the WBGT manufacturer's instructions about setup, calibration, and use. For example, if the work is in direct sunlight, then the WBGT instrument should be in the sun. The WBGT instrument should be placed close to the work location. A black globe thermometer to measure radiant heat. ![]()
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