A.Ted Cushman responds: I called Dr. Harold Friedman, an asthma specialist at the Dartmouth Hitchcock Medical Center, an Ivy League teaching hospital in Hanover, N.H. Dr. Friedman explained that asthma is caused by an allergic reaction in the airways. A few common allergens are usually to blame: the droppings of cockroaches or microscopic dust mites, and animal dander. In general, Dr. Friedman tells asthmatic patients to avoid carpets and to keep their homes clean and well-ventilated. Pets, unfortunately, have to go. But Dr. Friedman told me that to the best of his knowledge, neither fiberglass nor cellulose should trigger your clients’ asthma unless it’s mixed with something like cat fur or bug feces.

Blown cellulose is usually the best choice for old-house retrofits because it blocks airflow and insulates at the same time. Insulation placed within the walls is unlikely to get into the air in the living space, but while the job’s in progress, things could get dusty, and insulation fibers can irritate your customers’ breathing passages just as...

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