Human and environmental heat
Heat Index & Heat Stress Explorer
Calculate the National Weather Service heat index from air temperature and relative humidity, then see why that number is only one part of understanding human heat stress.
Calculate heat index
National Weather Service heat index estimate
Heat index is a screening measure, not a safety verdict
NIOSH explains that occupational heat stress combines environmental heat, metabolic heat and the effects of clothing or personal protective equipment. A single temperature-humidity index cannot represent all of those factors. NIOSH recommends wet bulb globe temperature (WBGT)-based methods when suitable measurements and resources are available for hot work environments.
The NIOSH and OSHA Heat Safety Tool also notes that heat index values were developed for shade and light wind. Full sunshine can make conditions more severe, and additional radiant sources such as hot machinery create conditions that ordinary heat index does not represent well.
Why this tool does not prescribe work/rest schedules
Work/rest guidance depends on more than the heat index. Work intensity, acclimatisation, clothing, radiant heat and applicable workplace rules matter. This site therefore calculates the environmental heat-index quantity and links to authoritative occupational guidance rather than issuing a personalised declaration that work is safe or unsafe.
Method
The implementation follows the National Weather Service procedure: it first evaluates a simpler apparent-temperature expression and applies the Rothfusz regression when conditions warrant it, including the published low-humidity and high-humidity adjustments. The full regression is not intended for extreme conditions outside the data range on which it was based.