Heat does kill fleas. They’ll die in temperatures above 95°F (35°C). Outdoors, fleas die when temperatures rise above 95°F for more than 40 hours a month. In dryers, it’s recommended to use the highest heat setting to kill fleas.
Hot Temperatures Kill Fleas
Adult cat fleas can’t survive in temperatures above 95°F (35°C). At this temperature, they’ll die within two days unless relative humidity exceeds 75% Fig 1.
Fig 1 Days it takes (y-axis) for 90% of unfed adult fleas to die at various relative humidity percentages (x-axis) while temperature is kept constant at 95°F.
Flea Eggs & Larvae
Flea eggs die in temperatures above 100.4°F (38°C). In slightly cooler temperatures, 95°F (35°C), 40% of eggs will survive to hatch. However, the eggs will desiccate if relative humidity is less than 75%. And the eggs will die in fully saturated air because heat accumulates within the shells.
Though some eggs may hatch at 95°F, it’s still too hot for the larvae to mature into adults. Around 30% of larvae will live long enough to spin cocoons and complete their pupal-imaginal molt, but 100% of the fleas will die within the cocoon, never to emerge.
Outdoors in Summer
In the summertime, fleas will die outdoors when temperatures exceed 95°F for more than 40 hours a month. In hot climates, fleas will die in structures that trap heat, such as dog houses, cars, or RVs.
In a Dryer
Fleas can’t survive in a dryer. To kill fleas on garments and bedding, it’s recommended to launder the items for ten minutes at 140°F, and then dry at the highest heat setting. Multiple factors will prevent survival, even if the heat somehow doesn’t kill the fleas. The dry conditions will cause desiccation, and the tumbling action will cause physical damage. Plus, the washer’s water and detergent will likely wash away and drown the fleas.