Cooking encyclopedia Food safety
The temperature range in which bacteria multiply fastest, and how long food may spend there.
4–60 °C (39–140 °F)
USDA FSIS gives the danger zone as 4 °C to 60 °C (40 °F to 140 °F) and the safe cumulative limit in it as two hours — one hour if the surroundings are above 32 °C (90 °F). The clock is cumulative across the whole life of the food, not reset by refrigeration.
Bacterial growth is slow when cold and stops when hot; in between it is exponential, doubling in as little as twenty minutes at the warm end. Cooking kills bacteria but does not always destroy toxins they have already produced, which is why time in the zone matters even for food you intend to cook thoroughly.
Thawing, marinating, cooling, transporting, buffets, packed lunches, slow-cooked dishes started from cold.
Cooling a large pot on the counter overnight; marinating at room temperature; leaving a buffet out through an afternoon; adding the times up wrong because the food went back in the fridge in between.