Heat index or temperature: which one your rule reads
Some states trigger on air temperature and some on heat index. Recording the wrong one leaves the rule unchecked.
California outdoor and Washington outdoor heat rules trigger on plain air temperature. Oregon, Maryland, California indoor control measures, and the federal heat NEP trigger on heat index. The two numbers can differ by more than fifteen degrees on the same day, in either direction, so recording only one can leave a rule unevaluated or make a safe day look dangerous.
Takeaways
- Say which metric each of your states reads.
- Explain why the two numbers diverge, and in which direction.
- Record both so no rule goes unevaluated.
- Know where a heat index number should come from.
Which rule reads which number
How far apart they get
Where a heat index number should come from
What to record in practice
Frequently asked questions
- Is heat index the same as the feels like temperature on my phone? Usually close but not always identical. Consumer apps sometimes use a wind chill and heat index composite, or a proprietary apparent temperature. For a record, use a heat index from a known source and say which source it was.
- Does heat index account for sun exposure? No. Standard heat index assumes shade. Full sun can add roughly 15 F to the apparent temperature, which is why NOAA notes that the index understates exposure for outdoor work in direct sun.
- Can I use heat index for a California outdoor site? You can record it, and doing so helps with the federal NEP. But the 3395 triggers are written in air temperature, so temperature is the number that decides shade and high heat.
- What about wind? Heat index does not incorporate wind. WBGT does, which is part of why Minnesota and heat stress professionals prefer it for indoor and high-radiant environments.