Dry sinuses, bloody noses and cracked lips: Humidifiers are often used to soothe these familiar problems caused by dry indoor air. And if your child has a cold, a cool-mist humidifier may ease a stuffy nose by adding moisture to the air.

But humidifiers can make you sick if they aren't maintained properly or if humidity levels stay too high. If you use a humidifier, check the humidity levels in the room where it is used and keep your humidifier clean. Mold or bacteria can grow in dirty humidifiers. If you have allergies or asthma, talk to your health care provider before using a humidifier.

Humidifiers are devices that release water vapor or steam. They boost the amount of moisture in the air, also called humidity. Types of humidifiers include:

• Central humidifiers. These are built into home heating and air conditioning systems. They're meant to humidify the whole house.
• Ultrasonic humidifiers. These devices use sound waves to release a cool mist.
• Impeller humidifiers. These humidifiers give off a cool mist with a rotating disk.
• Evaporators. These devices use a fan to blow air through a wet wick, filter or belt.
• Steam vaporizers. These use electricity to create steam that cools before leaving the machine. Don't buy this type of humidifier if you have children. The hot water inside a steam vaporizer may cause burns if spilled.

Humidifiers only add moisture to the air. You can't use them to breathe in products such as essential oils for aromatherapy.

Humidity varies depending on the season, the weather and where your home is. In general, humidity levels are higher in the summer and lower in the winter. It's ideal to keep the humidity in your home between 30% and 50%. Humidity that's too low or too high can cause problems.

• Low humidity can cause dry skin. It also can bother the inside of the nose and the throat. It can make the eyes feel itchy too.
• High humidity can make your home feel stuffy. It also can cause condensation, which is when water vapor in the air turns liquid. Droplets may form on walls, floors and other surfaces. Condensation can trigger the growth of harmful bacteria, dust mites and molds. These allergens can cause breathing problems and trigger allergy and asthma flare-ups.

The best way to test humidity levels in your home is with a hygrometer. This device looks like a thermometer. It measures the amount of moisture in the air. When you buy a humidifier, think about getting one with a built-in hygrometer. This is called a humidistat. It keeps humidity within a healthy range.

If you or your child has asthma or allergies, talk to your health care provider before you use a humidifier. There are some risks.

Cool-mist humidifiers may ease a child's stuffy nose during a cold. But heated humidified air doesn't seem to have the same result.

Also, mist from a dirty humidifier can trigger or worsen asthma and allergy symptoms. So can the growth of allergens caused by high humidity.

Just as air that's dry can be a problem, so can air that's too moist. When humidity gets too high, which can be common during summer months, it's a good idea to take steps to lower indoor moisture. You can lessen humidity by using:

• An air conditioner. Central or window-mounted air conditioning units dry the air.
• A dehumidifier. These devices collect extra moisture from the air. That lowers humidity levels. Dehumidifiers work like air conditioners, without the "cooling" effect. They're often used to help dry out damp basements. If you use a dehumidifier, drain and clean its moisture tray often. This helps prevent bacteria and mold.

Dirty water tanks and filters in humidifiers can grow bacteria and mold. This mainly can cause problems for people with asthma and allergies. But dirty humidifiers can be trouble even for healthy people when they release mist or steam tainted with bacteria or fungi into the air. That may trigger flu-like symptoms or even lung illnesses. Evaporators and steam vaporizers may be less likely to release airborne allergens than may cool-mist humidifiers.

To keep humidifiers free of harmful mold and bacteria, follow the instructions suggested by the device maker. These tips for portable humidifiers also can help:

• Use distilled or demineralized water. Tap water contains minerals that can lead to the growth of bacteria inside your humidifier. When released into the air, these minerals often appear as white dust on your furniture. You also might breathe in some minerals that are spread into the air. Distilled or demineralized water has much fewer minerals than does tap water. Also, use demineralization cartridges or filters if the maker of the humidifier suggests doing so.

  Change humidifier water often too. Don't let film or other types of buildup form inside a humidifier. Unplug the device first. Then empty the water tank, dry the inside surfaces and refill it with clean water. Do this every day if you can. These steps are very important if you use a cool-mist or ultrasonic humidifier.

• Clean humidifiers every three days. Unplug the humidifier before you clean it. Remove any mineral buildup or film from the tank or other parts of the humidifier with a 3% hydrogen peroxide solution. You can buy this at pharmacies. Some humidifier makers suggest you use chlorine bleach or other disinfectants.

  Always rinse the tank after cleaning. This can keep harmful chemicals from getting in the air. If they become airborne, you might breathe them in.

• Change humidifier filters. If the humidifier has a filter, change it at least as often as the device maker suggests. Swap it out more often if it's dirty. Also, change the filter in your central air conditioning and heating system on a regular basis.
• Keep the area around humidifiers dry. Things nearby could get damp or wet, including windows, carpet, drapes or tablecloths. If that happens, turn the humidifier down or use it less often.
• Prepare humidifiers for storage. Drain and clean humidifiers before you store them. Clean them again when you take them out of storage for use. Throw away all used cartridges, cassettes or filters.
• Follow instructions for central humidifiers. If you have a humidifier built into your central heating and cooling system, read the instruction manual. Or ask a heating and cooling repairperson how to keep the unit in good shape.
• Think about replacing old humidifiers. Over time, humidifiers can build up material that is hard or impossible to remove. This buildup also can make bacteria more likely to grow.

If you don't have a humidifier, you can take simple steps to ease some problems tied to dry air:

Ease a dry nose with a saline spray or rinse. Saline is a mixture of salt and water that may help moisturize dry nasal passages. You can buy a bottle of saline nose spray off the shelf. Or you could use a bulb syringe, squeeze bottle or a device called a neti pot to pour saline into your nostril. Some people make their own saltwater mixture at home. But you have to use water that's distilled, sterile, filtered or boiled and then cooled. It's not safe to rinse your nose with water straight from the tap. It can cause a serious brain infection that could lead to death.

Help soothe a sore, dry throat during a cold. Gargle with warm saltwater a few times a day. In general, mix a quarter teaspoon of salt with a half cup of water. Spit it out when you're done gargling. It also helps to sip warm tea and drink plenty of water to stay hydrated.

Keep skin moist. Stay away from taking long, hot showers or baths. Limit them to 5 to 10 minutes and use warm water, not hot. If your skin is often dry, think about washing with soaps that are labeled fragrance-free.

Right after you bathe, pat dry and put on an ointment or cream. This helps lock in moisture. You can use cream or ointment after you wash your hands or face too. If your lips get dry and cracked, use lip balm.