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Q.I'm building a custom home for clients who have asthma and allergies. What building details should I pay particular attention to?

A.Ellen Tohn of ERT Associates, an environmental consultant and a senior advisor to the Asthma Regional Council, replies: Asthma and allergies are related -- asthma is a breathing difficulty often brought on by an allergic reaction to certain triggers in the environment. There are some things you can do as a builder to help reduce asthma triggers, which can reduce the frequency and severity of attacks.

A few main allergens are responsible for most asthma attacks -- chiefly, dust allergens, pet dander, pests (especially cockroaches), and mold. You should try to build in a way that helps to reduce all of those. But many asthma sufferers are particularly sensitive to specific allergens, and you should ask clients with asthma to get their doctor's recommendation for which triggers to concentrate on. The doctor can test them to identify their individual sensitivities.

If dust is the main issue, here's how you target it: You want smooth, hard surfaces that are easy to keep clean. So go for hardwood, tile, or vinyl flooring, not carpet. Keep trimwork simple so it doesn't hide dust. A central vac system can help if the exhaust is directed outdoors. Unless they have effective high-efficiency filters, regular vacuums tend to spread dust around and put it in the air, making things worse instead of better (mopping is better than vacuuming, in any case). Forced-air heating also may keep dust airborne, so radiant heat might be a preferred choice.

Two thirds of home dust is tracked in from outside. Give your clients a place to take off shoes and boots before entering the house. If they don't want to do that, a wood rack or bristly welcome mat will at least help them knock dirt off their shoes.

Humidity control also helps with dust allergens. Dust mites, tiny insects that live on the flakes of skin that make up a part of indoor dust, thrive and grow in relative humidities of 50% or higher, and are a major asthma trigger. Relative humidity of 30% or 40% should help suppress dust mite activity. Indoor RH should be kept as low as your clients can tolerate: Give them a prominent humidity gauge for monitoring indoor conditions, and install humidistat controls on the heating and cooling systems.

You can't do much about pets -- that's your clients' business. But you may be able to help with pests like cockroaches. Build the envelope tight and eliminate cracks, voids, and other "roach highways" that let bugs in and help them hide. Make it easy to keep kitchen and dining areas clean -- no hard-to-sweep or hard-to-mop nooks and crannies. Low-toxicity borate insecticides are very effective against roaches, and you can treat framing and wall cavities with borate powders or sprays before enclosing them.

As for mold, it needs moisture to grow and spread. Ordinary good building practices are effective against that. Use good drainage plane and flashing details for roofs, walls, windows, and doors; that will prevent leaks. Make certain that plumbing does not leak, and place plumbing runs where they can be accessed and repaired. Avoid any thermal shortcuts or insulation defects that create cold zones on exterior walls -- condensation at cold spots can harbor mold. Use good foundation drainage and put a moisture barrier between your foundation and the ground -- ground moisture is a major contributor to moldy home environments. And again, control indoor air humidity -- keep it as dry as your clients can stand it.

.Ventilation is the final piece. Install a balanced ventilation system that filters incoming air and keeps humidity low, summer and winter. When you install a bath exhaust fan, it's worth running side ducts to closets. That helps flush out the mustiness and keeps any allergens that develop in the closet from spreading to living areas. And don't cut corners on fans -- install quiet, effective fans (less than 2 sones) that your clients will not mind operating.