To rid the house of fleas. Go away for a couple of days!
Frontline the cats, take them to the cattery, fleabomb the house, leave the house closed up to raise the heat level.
Fleas hatch in the heat and succumb to the insecticide in the fleabombs.
The aerosol fleabombs we get have the active ingredient of Pyrethrum. They are on-only aerosols, start them and get out.
Vacuum in shorts when you come home. Any surviving fleas will land on your legs and you'll feel them.

Pyrethrum, derived from plants, kills plants! The aerosol propellant is sometimes flammable, so unplug everything, extinguish the pilot lights in gas appliances. Read the instructions on the can.

Works in summer.