What is bat guano? We’ve had bats living in our shed for the last year. A neighbor said that guano left by bats could be a health hazard. Is that true? Should we be worried?

L. B., Holden, MA

You should maybe be careful, but probably no need to worry. Guano is bat “poop” and is, in fact, prized by gardeners as a fertilizer.  Nevertheless, in certain situations, there is a slight risk of the respiratory disease histoplasmosis when disturbing or handling bat guano.

Inhaling Fungal Spores Might Be a Concern

Many experts say we overinflate the risk of histoplasmosis. It is only a concern in long-term bat roosts where the guano has accumulated for some time. It’s in this accumulated guano that the Histoplasma fungus can grow. All agree that if you are working around, or cleaning up, large amounts of guano, you should wear gloves and a HEPA filter respirator to avoid inhaling any fungal spores that may be present. You should also wet down the material with a bleach solution to keep it from becoming airborne during the cleanup process.

Roosting Bats Can Make a Smelly Mess

Bat guano can be a nuisance problem simply because it falls from above onto anything stored below and can make quite a mess. Some folks that appreciate the bats (and the guano) rig a tarp below the ceiling to catch the stuff. They periodically lower the tarp, collect the guano for the garden, then hose the whole thing off before reinstalling.

Bat guano and bat urine can stain walls or floors, it can smell, and may also attract insects. Another risk from accumulated guano is the weight on a ceiling below, probably not a concern in your shed.

The Bottom Line

Any health risks from the guano depend on how much is present, how long it has accumulated, how often you use this shed, and whether guano is disturbed when you do so. It goes without saying that if the bats, and the guano, were in the attic of your home instead, there would be a higher level of concern.

If it was my shed and I accessed it often, I would have the guano cleaned up and the shed bat-proofed, if possible, to keep bats out. That means sealing up any openings that bats could use to enter. Since bats don’t need much of an opening, a shed could be a challenge. Bat-proofing (also called bat exclusion) should be done during a time of year when bats are not rearing young in the shed. Fall or winter is a good time unless you have a winter bat roost in that shed (see What Do Bats Do in the Winter?).

If your bats have only been present for a year, there are probably not large amounts of guano present and cleanup should be relatively simple. You might want to contact your local health department if you have concerns. Give us a call about bat exclusion. For more on living with bats and bat exclusion, see these Colonial blogs:

• What’s So Bad About Having Bats in My Attic?
• Bat Removal is Not a Do-it-Yourself Job!
• Sealing Out Bats – Advice From the Pros