The Dangers of High Humidity in your Home
Excessive humidity levels in your home can pose health problems for the members of your family, and can also cause expensive problems with your house.
The most immediate threat is to your health, and is caused by mold growth. In the most extreme cases mold can cause serious illness and sometimes even death. Mold will begin to grow as soon as an airborn mold spore hits a moist surface. All known types of mold require moisture to grow, so reducing the moisture in your home can help control mold growth.
Mold growth can be a serious problem even if the air in your basement is not humid. Cold, moist surfaces such as concrete block or poured cement walls have a relative humidity of 100% (even when the relative humidity in the surrounding air is normal). When mold spores come in contact with these moist surfaces, mold growth starts immediately.
In nature mold is very important, but inside your home it is very dangerous. Any type of moisture problem can contribute to mold growth (leaking pipes, moist building materials or carpet, etc.). A whole house dehumidifier is a great way to control the humidity you can and can't see, and that is the first and best step toward controlling mold growth.
Wood Swelling and Rot
Most of the building materials used in homes today (and moreso in older homes) do not react well to elevated humidity levels. When wood absorbs moisture it tends to swell, which can push apart joints. Even after the excess moisture dries up nails or screws might be loose which can cause creaks or worse.
Prolonged exposure to excess moisture can cause untreated lumber to rot. In the US most homes are constructed with untreated lumber (treated lumber tends to shrink over time, and treated lumber emits toxic fumes in a fire).
Damage to Furniture and Metal Equipment
Most furniture, like the wood in your home, is very sensitive to elevated moisture levels. Even non-wood furniture can be a breeding grounds for mold and mildew. Items with iron-rich metal contents (like tools) will tend to rust quickly in high humidity areas.