The Question Before You Test, and the Cheap Tape Check

Before you take a single sample, write down what you want to find out. "Did the toilet overflow years ago leave mold inside the wall?" is a good question. It tells you exactly where to sample: under the base molding beside the toilet, in a few spots. Random samples of dust from walls, fans or rugs usually waste effort, except in a very humid house where mold may be growing on furniture.

Think the way mold does. It wants food, mostly wood, paper and natural fibers, and it wants water. It is less likely, though not impossible, on synthetics, on most paints and on bare concrete. If you can think of no reason for mold to grow at a spot in a living room, it may not be worth testing there.

Also sample the places that look clean. If one basement joist is discolored, take one sample from the stain and another from clean-looking joists nearby. The clean sample works as a control: it shows whether you have a single spot to wipe or a whole ceiling to deal with.

Here is a free first look at any suspicious stain. Press a strip of clear, glossy tape against it, peel it off and hold it up to a window or lamp.

• Nothing on the tape: probably just a stain.
• Gritty or crumbly bits: probably dirt.
• A thin film or filmy discoloration: possibly mold, worth a closer look.

Your eye cannot confirm mold; only a microscope can. But this simple check has kept homeowners from paying for large cleanups of stains that turned out to be nothing.

A second trick: hold a flashlight against the surface so its beam skims along it. Fuzzy growth stands up and casts small shadows that are easy to miss in normal light.

Evidence: Inspector's practice (precautionary). Current guidance: EPA and IICRC S520 treat surface sampling as a supporting tool rather than a required step, and anything headed for court or an insurance claim should go through an accredited lab.

• Use clear, glossy tape about three-quarters of an inch wide. Tear off about three inches, roughly the length of a microscope slide.
• Loop it over your index finger, sticky side out, and press the center onto the surface. You want only a little material in the middle; an overloaded tape shows nothing but a dark smear.
• If nothing is visible, press that one strip against several similar spots close by. That improves the odds of catching growth.
• Choose rough, unfinished surfaces and the undersides of shelves rather than dusty tops. You are hunting for mold, not dust.
• To reach inside a wall from outside it, stick a strip of tape over the tip of a putty knife or an old credit card, then ease it beneath the base molding at several points.
• For furniture, touch one tape to many spots under and behind each piece in a room. If it comes back positive, test each piece separately to find the source.
• Fold a small tab at one end, write a number on the tape itself, and press it onto the outside of a sturdy plastic bag. Keep a numbered list of where each tape came from.

If mold is visible, touch only the very edge of it. A tiny dot is enough to identify it, and pressing tape across a whole patch can release a cloud of particles. Wear a well-fitted N95 or P100 respirator whenever you might disturb mold. If you react strongly to mold, ask someone less sensitive to take the samples.

Evidence: Well supported (EPA advises respiratory protection when disturbing mold).

A clean tape does not prove a wall is clean; it is only a spot check. But it is cheap, so take plenty.

This page is for education only. It is not medical, legal or engineering advice. The inspector whose work this builds on was an environmental consultant, not a physician. Talk with your doctor about health concerns, and use licensed professionals for electrical work and large mold jobs. Products, prices and labs mentioned reflect the years they were recorded and may have changed.

Source: the field work of environmental inspector May Dooley, MS, MA, CMC

Mold Testing & Remediation Guide →