What is the proper amount of indoor humidity?

Indoor Air Quality Experts recommend a home's humidity level be between 30 and 60 percent. However, during extremely cold weather, a home loses humidity to the outdoors and the level may drop to as low as 10 percent. When humidity levels drop, we suggest you have a whole-home humidifier to help ensure that the proper level of moisture is present throughout the house. Not having enough moisture in the air can cause dryness in the membranes of the nose, throat and bronchial tubes and can be the source of several health-related problems. Relative humidity also has a significant effect on controlling the occurrence of airborne infections.
In addition to the health benefits, humidifiers are an easy way to help your entire home be more comfortable. Whole-house humidifiers work like old-fashioned room humidifiers. They put moisture into the air, making harsh, dry air easier to breathe, however, they work on a larger and more efficient scale. A humidifier uses from 1.5 to 12 gallons of water per day — just enough to raise the humidity in the home to the desired level, but not enough to make a difference on the water bill. And because a humidifier is installed into the ductwork, there is no need to carry one from room to room.