Unfortunately, we are right in the middle of cold season and at this point, we're desperately looking for anything that will help ease a nasty cold.
Increasing the air moisture is a common way to help you (or your child) feel better when congested -- The added moisture in the air can relieve symptoms such as sore throat and congestion. A cool mist vaporizer or humidifier is preferred over a warm mist one because of the risk of your child (or someone else's child) getting accidentally burned by the outflow of hot steam.
The problem is that a cool mist vaporizer or humidifier gets dirty faster because the heat (in a hot vaporizer) kills mold and germs gathering in the vaporizer. To avoid this, clean your cool mist vaporizer frequently (at least once a week) to deter growth of bacteria and other pathogens.
Fit's Tip: I like Vicks Natural Mist Humidifier ($32.84) because it is quiet and has a wicking filter with built-in bactericide, which helps ward off germs. Buy one on Walmart.com.