'GMA' Investigates: How Dirty Is Your Humidifier?

— -- The steam from a humidifier is often seen as a way to prevent sore throats and dry sinuses, but many people don't know how to or how often they should properly clean their humidifiers, resulting in steam that could also be spreading germs.

"Good Morning America" collected 10 humidifiers from Philadelphia-area families and brought them to Thomas Jefferson University Hospital in Philadelphia, where they tested for bacteria and mold inside the humidifier’s water tank and in the vapor it emits.

Some of the families that "GMA" spoke with have children with asthma and allergies, and almost all admitted that they don’t clean their humidifiers as often as they should.

“I don’t know how to clean a humidifier,” said Justine Winkler of Bala Cynwyd, Pennsylvania.

"I don’t know what you’re going to find, but I'm curious to see," said Kenny Dubin of Merion, Pennsylvania.

Out of the 10 humidifiers that were tested, all contained some level of bacteria in the water tank. Two contained heavy levels of bacteria that were growing inside the tank and also in the vapor.

And, in three of the humidifiers, there was mold.

Watch the video above to see the families' reactions.

Dr. Gregory Kane, a pulmonologist at Thomas Jefferson University Hospital, said that mold can cause nasal congestion, a runny nose or a sore throat.

“The mold can cause asthma in the particular individual or if the individual has asthma it can cause chest tightening or wheezing,” Dr. Kane told ABC News.

Experts say that consumers need to clean their humidifier on a daily basis by emptying it and rinsing the base and letting it air dry before refilling. They also recommend that it should be cleaned thoroughly every week according to the manufacturer’s instructions.

Consumer Reports is also calling on the Environmental Protection Agency to investigate allegedly misleading claims made by some humidifier models that claim to prevent bacteria growth.