What OSHA recommends at this level
- Remind workers to drink water roughly every 15 minutes, before they feel thirsty.
- Provide shade and schedule rest breaks somewhere genuinely cool.
- Schedule the heaviest work for the coolest part of the day.
- Set up a buddy system so nobody works unobserved.
- Keep a trained supervisor or attendant on site.
Heat index from the National Weather Service Rothfusz regression. Risk bands and protective measures from OSHA-NIOSH heat guidance. Shade and still air are assumed; see below for why full sun is different.
How the heat index is calculated
The heat index is what a given temperature feels like once humidity is taken into account. Sweat is how the body sheds heat, and humid air slows evaporation, so the same 95°F is far more dangerous at 70% humidity than at 20%.
This calculator uses the Rothfusz regression published by the National Weather Service, including both of its correction terms for very dry and very humid air. It is the same equation behind the figure in a weather forecast, so the number here should match what you see elsewhere rather than approximate it.
Heat index is not wet bulb globe temperature
These get used interchangeably and they are not the same measurement. The heat index comes from temperature and humidity and assumes shade with light wind. Wet bulb globe temperature also accounts for direct sunlight and air movement, which is why it needs an instrument on site rather than a formula. Working in full sun can add roughly 15°F to the effective heat stress, so treat the figure above as a floor for outdoor work, not a ceiling.
What the law actually requires
There is no federal OSHA heat standard. Heat cases are cited under the General Duty Clause, Section 5(a)(1) of the OSH Act, which requires a workplace free from recognised hazards likely to cause death or serious harm. That means there is no temperature at which federal law says work must stop, and it also means an employer cannot point at a missing rule as a defence.
Several states go further. California requires heat illness prevention training, water, shade and cool-down rests under Title 8 CCR 3395, with additional duties above set temperature thresholds. If you operate in a state-plan state, check the state rule rather than assuming the federal position applies.
Common questions
What heat index is dangerous for workers?
OSHA treats a heat index below 91°F as lower risk, 91°F to 103°F as moderate, 103°F to 115°F as high, and above 115°F as very high to extreme. The bands are guidance for planning rather than a legal threshold: there is no federal heat standard setting a temperature at which work must stop.
Is the heat index the same as wet bulb globe temperature?
No. The heat index is calculated from air temperature and relative humidity, and it assumes shade and light wind. Wet bulb globe temperature also accounts for direct sun and air movement, which means it has to be measured with an instrument rather than calculated from a forecast. In full sun the effective heat stress can be roughly 15°F higher than the heat index suggests, so a shade-based figure will understate risk for outdoor work.
Does OSHA require heat illness training?
There is no federal OSHA heat standard, so heat cases are cited under the General Duty Clause, Section 5(a)(1) of the OSH Act. Several states do have their own enforceable rules. California requires heat illness prevention training under Title 8 CCR 3395, and other state plans have adopted or proposed similar requirements.
How often should workers drink water in the heat?
OSHA guidance is roughly one cup of water every 15 to 20 minutes during work in the heat, and to drink before feeling thirsty. Thirst lags behind actual fluid loss, so waiting for it means starting the shift already behind.
Training that covers this
A calculator tells you today's risk. Training is what the rule actually asks for, and in California it is explicitly required.