When I was growing up, it was the sound of my father filling the humidifier down the hall that woke me each morning. The cascading whoosh as the bucket of water tilted into the tank was a comforting constant. My parents believed in the humidifier. "It's why you don't get colds," my mother would say with conviction. Yet experts say that while humidifiers offer certain health benefits, they can also cause problems, particularly for people with asthma and other breathing disorders.

A humidifier's job is straightforward: to boost the water content of air. Different types do this job in different ways: Ultrasonic and impeller (also known as cool mist) humidifiers shoot a plume of tiny water droplets into the air. Evaporative models blow air through a moistened wick, filter, or belt. Steam vaporizers release steam. And warm-mist humidifiers, a variation on steam models, cool the steam somewhat before releasing it.

As it turns out, there's no evidence that humidifiers help prevent colds or other respiratory infections. (It was probably my parents' insistence on hand washing that kept me relatively sniffle-free.) If you do get a respiratory infection, however, a humidifier can ease your symptoms by keeping mucus moist so you can expel it more easily, says Stanley Goldstein, MD, a Rockville Centre, NY, allergist and a spokesman for the American Academy of Allergy, Asthma, and Immunology. Using one can also help keep your nose, throat, and lips from feeling dry and uncomfortable.By the same token, a humidifier can keep your skin from drying out and itching, says Jerome Litt, MD, an assistant professor of dermatology at Case Western Reserve University. Moisture can be very soothing if you have psoriasis and the accompanying thick, scaly patches. And while it won't prevent wrinkles--alas, too much to ask--a humidifier can make your skin look younger if you also use a moisturizer.

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The downside: With overly enthusiastic use, humidity can rise too high, promoting the growth of mold and dust mites, both common allergens, says Norman Edelman, MD, consultant to the American Lung Association. If you have asthma, excessive humidity can irritate your breathing passages, as can the hot or warm mist from evaporative and steam humidifiers.[pagebreak]Slipshod humidifier maintenance causes the most trouble. If you don't clean your humidifier properly, bacteria and mold can proliferate in the water. This can be especially hazardous with ultrasonic and impeller models--the types that shoot a mist of water into the air--because the mist will disperse that bacteria and mold. If you breathe them in, you can develop respiratory troubles ranging from flulike symptoms to a chronic pneumonia-like condition dubbed "humidifier lung." Smokers and those with respiratory disorders are most vulnerable.

Here are the keys to finding happiness with a humidifier.

Buy the Right One

Consider these three factors:

Type If someone in your household has asthma, choose something other than a steam humidifier. Ditto if you have small children, since this type of humidifier can cause serious burns.Size and stylePortable humidifiers are small and can usually humidify just a single room. Console humidifiers are enclosed in cabinets and may cover multiple rooms or a whole house.

Central humidifiers are built in to a home's heating and air-conditioning system and are designed to humidify the entire place. Console and central varieties are always evaporative.

PricePlug-in portables start at around $45. Consoles go for about $120 and up. A central humidifier will cost considerably more depending on the size of your home and where you live. Expect to pay a few thousand dollars.

Run It Right

Here's what that means:

Use It SparinglyTurn it on only when your skin, nose, lips, or throat are dry, or when you need relief from a cold or related symptoms, says Edelman.

MonitorIf your humidifier has a built-in "humidistat," set it at 30 to 50 percent to prevent an overdose of moisture. If it doesn't, buy a device called a hygrometer (about $20 at hardware stores). If the dial edges beyond 50 percent, turn down the humidifier's output volume, or use it less often.

Clean Often and Thoroughly If You Have...

A portable humidifier:Dump out old water, and fill with fresh daily. Clean the humidifier at least weekly, following the manufacturer's instructions. Remember to scrub away any crusty mineral buildup, called scale, which is a breeding ground for bacteria. (Using distilled water can help reduce scale buildup, and some humidifiers come with filters designed to do the same.)

A console or central humidifier:Because these are evaporative types, bacteria and mold are less of a problem. However, you still need to follow cleaning instructions carefully.

Other Options

Uncomfortable with a winter cold, and don't want to bother with a humidifier? Try these solutions:

Saline Nasal SprayBuy it at the drugstore, and spritz some into each nostril, following package directions, to help keep mucus moist.

A Steamy RoomStep into the bathroom, close the door, and run a hot shower for 10 minutes. Don't go into the shower; rather, stand nearby and breathe deeply.

The Cheapest and Simplest Put a pan of water in each room, and allow the water to evaporate. Change water daily, and wash the pan weekly.