How to Test for Black Mold in Your NYC Apartment: Expert Tips

Black mold — usually Stachybotrys chartarum — is the type New Yorkers worry about most. It grows on materials that stay wet for days, like drywall behind a leaking riser or the ceiling under a neighbor's bathroom. The good news: you can do a lot to check for it yourself before calling a professional.

Start with your nose and your eyes

A persistent musty, earthy smell is the number-one sign of hidden mold. Check closets on exterior walls, the cabinet under the kitchen sink, behind the toilet, around radiators and window AC units, and any ceiling below another apartment's bathroom.

Look for dark spots, bubbling paint, water stains, or soft drywall. In pre-war buildings, look closely at plaster around windows and along exterior walls where moisture comes through the masonry.

Track humidity

A $15 hygrometer tells you a lot. Steam-heated NYC apartments often swing from very dry to very humid. If readings stay above 60% for long stretches, mold has what it needs to grow.

What DIY test kits can't tell you

Petri-dish kits from the hardware store will always grow something, because mold spores are everywhere. They don't compare your indoor air to the outdoor air, can't measure how much mold is present, and won't find the moisture source.

Professional air sampling uses calibrated pumps and an accredited lab to compare indoor and outdoor spore counts — the comparison that actually tells you whether you have a problem.

Don't disturb it

If you find a patch larger than about 10 square feet, don't scrub, sand, or spray it. Disturbing mold releases spores into the air and through the building. In New York, mold over 10 square feet in a multi-unit building falls under NYC Local Law 55 and NYS mold licensing rules.

When to call a licensed mold assessor

Call a professional if you smell mold but can't find it, if you've had a leak from above, if someone in the home has new allergy or asthma symptoms, or if you need documentation for a landlord, co-op board, or insurance claim.

**Concerned about black mold?** HBH's NYS-licensed assessors serve all five boroughs. Call or text (631) 774-6502.