Why do bat droppings smell so bad?

Bat guano is rich in uric acid, nitrogen compounds, and undigested insect fragments. Left in place, bacteria and fungi break those compounds down, releasing ammonia and a mix of other volatile organic compounds — the sharp, acrid smell people describe. Importantly, most of what you're smelling in a long-occupied roost isn't the droppings themselves; it's urine that has soaked into insulation, wood, and drywall over repeated seasons, which holds odor far longer than dry guano pellets do.

This is why the smell often seems to come from everywhere in an attic rather than from one obvious pile — the source is saturated material, not a single spot you can simply scoop up.

Ammonia release accelerates with heat and humidity, and New Hampshire attics run hot and poorly ventilated through the summer — exactly the maternity period when colonies are most active and producing waste continuously. Warm, humid air also slows drying, which keeps the microbial breakdown process going longer than it would in a cool, dry space. The result is a smell that peaks in summer and fades somewhat as the attic cools and the colony's activity drops off going into fall.

It's worth separating two things that get conflated constantly: the odor, and histoplasmosis. The odor is ammonia and other VOCs off-gassing from decomposing waste — unpleasant, but not itself a toxic exposure. Histoplasmosis is a fungal respiratory infection from inhaling Histoplasma capsulatum spores, and CDC is direct about this: most people who breathe in the spores do not get sick at all. The real exposure risk comes from physically disturbing dried, accumulated guano — sweeping it, vacuuming it, handling contaminated insulation — which aerosolizes spores. Simply smelling a strong odor from below is not the same event as that kind of disturbance. That distinction matters because it changes what you should actually do: leave the material alone and get the colony excluded and the space professionally cleaned, rather than trying to "clean out the smell" yourself.

Because bats are a protected species in New Hampshire, there's no lawful extermination option — and no service, from Anchor or anyone else, that can legally kill a colony to solve an odor problem. The fix is exclusion: one-way devices installed by a wildlife specialist that let bats leave to feed and prevent them from re-entering, timed to avoid the maternity period when pups can't yet fly. Once the colony is out, the odor stops accumulating, and remediation can address the material that's already saturated. Trying to seal an active roost yourself — especially during summer — risks trapping pups inside, which makes both the smell and the underlying problem considerably worse.