A.Jeffrey May, principal scientist at May Indoor Air Investigations in Cambridge, Mass., responds: Because the exposed fibers in fiberglass batts are largely glued together, they are only aerosolized (broken up and suspended in the air) when there is a direct physical disturbance, such as during installation or while mechanical work is being performed in an insulated area. Even then, these fibers don't stay in the air long, and eventually they accumulate in the floor dust. Otherwise, the number of fiberglass fibers in the air is probably rather low, with most of them too large to be inhaled into the lungs.

Occasionally, there are a few respirable fibers (ones that are short and thin enough to be inhaled), but the theory is that — unlike highly stable asbestos fibers — they ultimately dissolve in the lung fluids. Fiberglass fibers are still considered to be "possible" carcinogens, but they have remained in this category for some time and...

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