To Keep Cold Away
Winter is the time when children (especially below 5 years) catch cold. Nasal congestion due to cold is a common phenomenon in winter. If a child is suffering from an upper respiratory infection or related breathing difficulties, other health complications may ensue. A humidifier may offer better comfort to a child as it improves the humidity level; the mucous membranes remain moist, allowing the child to breathe easier. Moist mucous membranes are also far better equipped to fight infection than dry ones. There are three types of humidifiers to choose from:

cool mist, steam vaporizer and ultrasonic.

Cool-Mist Humidifier
These devices work by making water vapor through a rapidly turning disk within the water of the humidifier. Because, the vapor from the machine is not heated, there is no risk of burning the child should the water spill or the child places his/her face close to where the vapor escapes. However, cool-mist humidifier usually includes cleansing of the tank on a daily basis, as the cool water in it can be an excellent breeding ground for mold and bacteria. In addition, only distilled water should be used in cool-mist humidifiers.

Steam Vaporizer
These machines are better and are less likely to have a lot of bacterial growth. However, the risk of burn can be significant. Vapor is made in these devices by using a heating element to cause steam. Normal tap water can often be used with these devices making them much less expensive to operate. However, owing to very high temperature of the water, steam vaporizers should not be used for very young (below 4 years) children.

Ultrasonic Humidifiers
These devices cause vapor by creating ultrasonic vibrations within the water. These humidifiers were originally thought to be better because the risk of dispersing molds, bacteria and minerals were minimal. The safety of these devices is much better than the steam vaporizers, and they do tend to disperse much less bacteria and mold than the cool-mist humidifiers. However, they send minerals into the air quite efficiently, so distilled water is a must with these as well.
With the burn risk to younger children, steer clear of steam vaporizer if you have children. And, with regard to your decision about whether to purchase an ultrasonic or regular cool-mist humidifier ought to be based on cost and how difficult cleaning is going to be since they will need maintenance every day.

Finally, a word of caution about humidifiers! While these appliances can often offer good relief to a child suffering from cold, the humidity in the air allows for the growth of mold within the carpeting or other areas of the house. This increased exposure to mold can often make matters worse for children actually suffering from asthma. So, if your child has asthma or other chronic respiratory difficulties, please consult your pediatrician before spending the money on any humidifier.