Wood,leather, paper, cloth, although they feel dry to the touch, contain water. Not a fixed amount of water, but an amount that will vary greatly with the relative humidity level of the surrounding air. Take for example a cubic foot of wood with a bone dry weight of 30 lbs. At 60% relative humidity the wood hold over 3 pints of water. Now, if the relative humidity is lowered to 10%, the water held by the wood will not fill even one pint measure. So we have, in effect, withdrawn 2 1/2 pints of water from the wood by lowering the relative humidity from 60% to 10% (1 pint equals about 1 lb.)

Now, what are the effects of this constantly changing or constantly low moisture content of the air?
They are damaging, floors Separate!

Electric heat is extremely dry, as are steam radiators and hot water baseboard - they introduce no moisture into the home, as does gas forced air. Remember that very dry heat and long winters followed by humid summers result in a large swing in humidity extremes that will cause excessive expansion/shrinkage in the wood.

Wood is a hygroscopic material. When exposed to air it will dry or pick-up moisture until it is in equilibrium with the humidity and temperature of the air. Moisture absorption causes wood to swell. Moisture loss causes wood to shrink. Wood does not shrink or swell equally in all directions. The use of a humidifier or dehumidifier (depending on the season) is highly recommend to lessen the stress on your wood floor.

Wood changes approximately: 0.1% along the grain and 2% to 8% across the grain.