A book has mold if you see fuzzy, powdery, or web-like growth — white, green, gray, or black — usually raised from the surface and accompanied by a strong musty smell, often starting at the edges, gutter, endpapers, and cover. Mold has texture, can smear, and spreads, which sets it apart from flat reddish foxing spots and from ordinary dust. Because mold releases spores that can irritate the lungs, isolate a moldy book in a sealed bag away from your other books and handle it in a ventilated area with a mask and gloves.
Published June 2026 · By Josh Eldred, New Mexico Literacy Project
What mold looks and smells like
Mold (and mildew, a common surface mold) shows up as fuzzy, powdery, cobwebby, or slightly slimy growth that sits on or in the paper and binding. It can be white, green, gray, black, or even pinkish, and it usually brings a strong musty odor. Check in good light along the most vulnerable spots: the page edges, the gutter (where pages meet the spine), the endpapers, and the inside of the covers. If a patch looks raised or furry and the book smells damp and sour, assume it's mold.
Mold vs. foxing vs. dust
Mold / mildew
Living growth — fuzzy, powdery, or slimy, with texture you can see and feel. Many colors, musty smell, and it spreads in humidity. Needs action.
Foxing
Flat, rusty-brown age spots with no texture; it doesn't grow or spread like an organism. A cosmetic age condition, not a contamination — see what is foxing.
Dust & dirt
Loose surface grime that brushes off cleanly, with no growth pattern and no musty smell. Handled with simple dry cleaning.
The quick test: texture and spread. Fuzzy/powdery and getting worse = mold. Flat and stable = foxing. Wipes away = dust.
Why it matters: health and spread
Two reasons to take mold seriously. First, health: mold releases spores that can trigger allergic reactions and respiratory irritation, and anyone with asthma, allergies, or a weakened immune system should not handle moldy books at all. Second, cross-contamination: spores travel, so one moldy book shelved against clean ones can spread the problem through a collection. Treat a moldy book as something to isolate, not to slot back on the shelf.
How to handle a moldy book
1. Isolate it
Put the book in a sealed bag, kept apart from your other books, until you can deal with it. This stops spores from spreading.
2. Dry it out
Mold needs moisture; lowering humidity halts active growth. New Mexico's dry air helps, but get the book out of any damp space first. (If it's actually wet, see water-damaged books.)
3. Remove dead spores outdoors
Once dry, gently brush off the dead, powdery growth outdoors wearing a mask and gloves — never indoors, where you'd just scatter spores into the room. Brush away from yourself.
4. Call a conservator for valuable or heavy cases
Significant mold, or a book that may be valuable, is a job for a professional conservator rather than home treatment.
Frequently asked questions
How can you tell if a book has mold?
Look for fuzzy, powdery, or web-like growth (white/green/gray/black) with texture and a strong musty smell, usually at edges, gutter, and endpapers. Texture and spreading distinguish it from flat foxing or loose dust.
Is mold on books dangerous?
The spores can cause allergic and respiratory reactions; at-risk people should avoid handling moldy books. Work in a ventilated area or outdoors with a mask and gloves, and keep moldy books away from clean ones.
Mold or foxing?
Foxing is flat rust-colored spotting that doesn't spread; mold is fuzzy/powdery, musty, and grows in humidity.
Cite This Guide
Eldred, J. (June 2026). How to Tell If a Book Has Mold. New Mexico Literacy Project.
https://newmexicoliteracyproject.org/how-to-tell-if-a-book-has-mold
Licensed under CC BY 4.0.