air drafts can *sometimes* be harmful to them. i’d definitely recommend not positioning the fan directly on them, & ensure there is enough room for them to move away from the air if it causes them unpleasantness. it doesn’t cause them illness, although it can exacerbate existing illness (as it does to humans). using a cold rag (rung out, not dripping) & hanging it up on their cage as a shaded area is a good way to keep them cool. another way of keeping them cool is feeding them chilled vegetables such as cold cucumbers & bell peppers. some people also take a cool rag & place it over their coat to relieve them from the heat.