It will provide a quick knockdown of flies in any environment. It is most commonly used in a fogging machine as a space spray.
Pyrethrum is a natural pesticide which provides a quick kill but no residual.
Pyrethrum will work well on any flying insect including flies. Inside and outside. This varies on the target pest. You may use as little as one ounce per gallon or as much as 4 ounces per gallon. For fly control, you will generally use 1-2 ounces per gallon.