As the saliva evaporates from their fur it cools them in a fashion similar to sweat evaporating off of our skin. But once the air temperature is close to, or exceeds, body temperature then the cooling by licking method begins to fail and cats are left with only their tongue 's rapid breathing as a cooling mechanism. As you can imagine this is not sufficient and the possibility of cat heat stroke rises with increasing temperatures. Persians and other short nosed breeds are especially susceptible to heat stroke. Cats should never be left in a hot car or crate.
Heat stroke requires immediate attention or it can be fatal.
Wrap in cold wet towels, immerse in water or find some way to get the cat wet. Stop the cooling treatment when the rectal temperature drops down to 103 F and let their body finish the cool down process. If you continue with the cool down past 103 F you could produce hypothermia. Of course even in this weakened state some cats will never allow you to get in a rectal thermometer.