Dry air can be uncomfortable to breathe and dry out wooden furniture excessively. It also causes an increase in static levels which is especially bad if you like tinkering with electronic devices. Dry air can also cost you more in heating bills because it feels colder than humid air at the same temperature. When you add humidity to your home you can actually feel warmer at 68 degrees than you do at 75 degrees, saving as much as 15% in heating costs.
