Humidifiers can help with problems caused by dry air, including dry sinuses, bloody noses and cracked lips. They also can ease symptoms of a cold or other respiratory conditions.

But humidifiers can become a household health hazard if they aren’t maintained properly or if humidity levels stay too high. In fact, dirty humidifiers can breed mold or bacteria. If you have allergies or asthma, talk to your doctor before using a humidifier.

Humidifiers are devices that emit water vapor or steam to increase moisture levels in the air. There are several types:

• Central humidifiers are built into home heating and air conditioning systems.

• Ultrasonic humidifiers produce a cool mist with ultrasonic vibration.

• Impeller humidifiers produce a cool mist with a rotating disk.

• Evaporators use a fan to blow air through a wet wick, filter or belt.

• Steam vaporizers use electricity to create steam that cools before leaving the machine. (Avoid this type of humidifier if you have children; hot water inside this type of humidifier may cause burns if spilled.)

Ideal humidity levels

The amount of humidity varies depending on the season, weather and where you live. Generally, humidity levels are higher in the summer and lower during winter months. Ideally, humidity in your home should be 30 to 50 percent. Humidity that’s too low or too high can cause problems.

Too low humidity can cause dry skin, irritate your nasal passages and throat, and make your eyes itchy.

Too high humidity can make your home feel stuffy and can cause condensation on walls, floors and other surfaces, which triggers the growth of harmful bacteria, dust mites and molds. These allergens can cause respiratory problems and trigger allergy and asthma flare-ups.

The best way to test humidity levels in your house is with a hygrometer. This device, which looks like a thermometer, measures the amount of moisture in the air. You can find a hygrometer at the hardware store. Some humidifiers also have built-in hygrometers (called humidistats), which maintain humidity within a healthy range.

Keep it clean

Dirty reservoirs and filters in humidifiers can quickly breed bacteria and mold. While it’s especially problematic for people with asthma and allergies, it can trigger flu-like symptoms or even lung infections in healthy people.

Steam vaporizers or evaporators may be less likely to release airborne allergens than many cool-mist humidifiers.

To keep humidifiers free of harmful mold, fungi and bacteria, follow the guidelines recommended by the manufacturer. These tips for portable humidifiers also can help:

Use distilled or demineralized water. Tap water contains minerals that can create deposits inside your humidifier, promoting bacterial growth. When released into the air, these minerals also can appear as white dust on your furniture. Distilled or demineralized water has a much lower mineral content. Also use demineralization cartridges or filters if recommended by the manufacturer.

Change the water often. Don’t allow film or deposits to develop inside your humidifier. Empty the tanks, dry the inside surfaces and refill with clean water every day if possible, especially if using cool mist or ultrasonic humidifiers. Be sure to unplug the unit first.

Clean every three days. Remove any mineral deposits or film from the tank or other parts of the humidifier with a 3 percent hydrogen peroxide solution, which is available at pharmacies. Some manufacturers recommend using chlorine bleach or other disinfectants.

Rinse the tank well after cleaning to keep harmful chemicals from becoming airborne and inhaled.

Change humidifier filters regularly. If the humidifier has a filter, change it as often as the manufacturer recommends, more often if it’s dirty. Also regularly change the filter in your central air conditioning and heating system.

Keep the area around humidifiers dry. If the area around a humidifier becomes damp or wet — including windows, carpet, drapes or tablecloths — turn the humidifier down or reduce how frequently you use it.

When winter’s over, prep them for storage. Drain and clean humidifiers before storing them. And then clean them again when you take them out of storage for use. Throw away all used cartridges, cassettes or filters. If you have a humidifier built into your central heating and cooling system, read the instruction manual or ask your heating and cooling specialist about proper maintenance.

Replace old humidifiers. Over time, humidifiers can build up deposits that are difficult or impossible to remove and encourage growth of bacteria. If they can’t be cleaned, they should be replaced.