Bats leave dropppings. The spores of mold that grows on the droppings can be harmful. Enough droppings and urin and you start getting rot in wood members. enough dead carcases, droppings etc., you get carpet beetles that eat stuff....the beetles end up inside the building eating stuff...this is a problem in museums where we like to keep the stuff. Purdue has a web site with a lot of really good pest management literature. They call it their E-series. I I think the E stands for entomology. I'll bet they've got some literature on Bats. The complication is that bats are not mice. Bats are more beneficial than harmful and its only that we don't want them in the building. Want more? -jc