Check heat index against danger thresholds.
The heat index is the shade value: national weather services calculate it for a shaded, lightly breezy environment, and full sun adds up to about 8 C to what a body actually experiences. The four warning bands — caution, extreme caution, danger and extreme danger — are the thresholds that trigger public advisories, and the exposure limits shown follow from them for unacclimatised people doing light activity.
Heat index with sun correction
Heat index from the Rothfusz regression in Fahrenheit, plus 15 F when in direct sun, converted back to Celsius
Heat illness can be fatal and individual susceptibility varies widely. These bands are general public guidance, not medical or occupational safety advice — follow official heat warnings and workplace heat stress rules.
Above about 32 C heat exhaustion becomes possible with prolonged exertion; above 41 C it becomes likely; above 54 C heat stroke is imminent.
Yes. Official values are shade values, which is why full sun is treated as adding several degrees to the effective exposure.
Older adults, infants, people with cardiovascular or renal conditions, those on certain medications, and anyone unacclimatised or working outdoors.