For some people, extremely dry air can trigger asthma and nasal congestion and make irritated nasal membranes more suscepti­ble to assaults by allergens and irritants. In this case, ideal hu­midity levels would be higher, around 40 to 60 percent.

If you're not allergic to molds and mites, your doctor may recommend a humidifier to relieve the discomfort of dry eyes, nose, lips, mouth, and/or skin. These symptoms particularly plague people who live in warm or desert climates.

Humidifiers can be helpful in keeping the air moist and your airways well hydrated, but if the tanks are not cleaned reli­giously, the accumulated bacteria can cause an inflammation known as "humidifier lung."

Your daily care routine should follow these steps:

1. Empty the humidifier tank and wash carefully with hot, soapy water. Rinse under faucet and fill to appropriate level with tap (or preferably distilled) water.

2. After use, clean tank again, add 2 cups of white vinegar and enough water to enable the machine to create vapor. Run thirty minutes in a well-ventilated area, then rinse again.

3. Fill with water, run for three minutes, rinse twice. Keep dry until next usage.

Ultrasonic humidifiers should be used with caution. Some spew into the air minerals that could irritate the lungs.