You cannot judge heat danger by the air temperature alone. Humidity, sun, wind and the work you do change how hot your body gets. This lesson shows how to read the conditions.
The heat index
The heat index combines air temperature and humidity to show how hot it feels. The US National Weather Service (NWS) publishes it. For example, air at 96 degrees Fahrenheit with 65 percent humidity feels like about 121 degrees.
NWS heat index categories:
- Caution: 80 to 90 degrees F,
- Extreme caution: 90 to 103 degrees F,
- Danger: 103 to 124 degrees F,
- Extreme danger: 125 degrees F and above.
The heat index is calculated for shade
The NWS values assume shade and light wind. Full sun can add up to 15 degrees F to the heat index. Very hot, dry wind can also be harmful. So a heat index of 95 degrees in shade can feel like 110 in direct sun.
Why 80 degrees keeps coming up
Many rules and guides use a heat index of about 80 degrees F as the point where heat precautions begin. California's outdoor standard requires shade when the temperature is above 80 degrees F. Oregon's rule applies when the heat index is 80 degrees or more. Maryland's standard also starts at a heat index of 80 degrees. A higher level, such as 90 or 95 degrees, often triggers extra steps, such as closer watching of workers.
Those are legal triggers in those states. They are not safe limits. Heat illness can happen below them, especially for a worker who is not used to the heat.
Other ways to measure heat
- WBGT (wet bulb globe temperature) takes into account temperature, humidity, wind and sun or radiant heat. Professionals use it with a special meter, especially for hard work and indoor heat sources.
- The NIOSH Heat Safety Tool is a free phone app that shows the heat index and the risk level for your location.
- NWS heat alerts, such as a heat advisory or an extreme heat warning (called an excessive heat warning until 2025), tell you a heat wave is coming.
Things that make it feel hotter
- Radiant heat from furnaces, ovens, hot metal, a roof or the sun on a wall,
- poor air movement in a closed space,
- protective clothing, such as coveralls, a respirator or chemical suits,
- hard physical work,
- the first hot days of the season, and heat waves,
- a hot vehicle, trailer or container.
Plan for the day
Before the shift, check the forecast and the heat index for the hottest part of the day. Then:
- do the hardest work in the cooler hours when you can,
- plan for more water and rest when the index is high,
- expect new or returning workers to need extra care,
- check that water, shade or a cool place is ready before you need it.
What to remember
- The heat index combines temperature and humidity. Sun can add up to 15 degrees F.
- NWS categories rise from caution at 80 to extreme danger at 125 degrees and above.
- A legal trigger such as 80 degrees is not a safe limit.
- Radiant heat and clothing make the real conditions worse than the number.