Estimates range from the tens of thousands to more than 300,000, according to the CDC. The most common types of mold found in the home that cause allergic reactions include Cladosporium, Penicillium, Alternaria, Aspergillus, and Mucor. Other molds that can cause an allergic reaction are Helmin thosporium, Epicoccum, Fusarium, Rhizopus, and Aureobasidium.

Like all fungi, molds reproduce by releasing spores. The spores can be carried off by air, water, or insects. People sensitive to mold spores react to their presence with nasal stuffiness, eye irritation, or wheezing. More severe reactions may include fever and shortness of breath. Mold allergies can cause asthma attacks in some people with asthma.

Molds grow better in warm, humid conditions such as the basement or the shower, the CDC says. Other common indoor sites include pipes, damp walls, and plant pots. Cellulose materials that have become wet are especially good places for molds. These include ceiling tiles, cardboard, and wood. Molds also grow outdoors in shady, damp areas or in places where plants are decomposing. If you are sensitive to molds, be aware that molds are often found in antique shops, greenhouses, saunas, flower shops, and summer cottages. On farms, mold is often found in hay, straw, or grain.

Mold spores are constantly drifting through the air, the CDC says. Once the spores are in your house and drop on places that have excessive moisture, they will grow. Some places might surprise you: dust, paints, wallpaper, drywall, carpet, fabric, and upholstery.

This type of mold, really a greenish-black color, can produce a potent toxin, the CDC says. It only grows on wood or paper products that have become water-logged for several days. Some research has suggested that the toxin can cause serious health problems in infants. This type of mold does not cause an allergic reaction; its effects are caused by the toxin it produces.

It doesn't matter what kind of mold is growing. The most important thing is to get rid of it, the Environmental Protection Agency (EPA) says. Professional testers are expensive. Whether you or someone else does the testing, no standards have been established for acceptable levels of mold.

You can remove mold growth with commercial mold-killing products or with a weak bleach solution (one cup of bleach for each gallon of water), the EPA says. After cleaning, dry the area completely.

The main way to control mold growth is to eliminate moisture, the CDC says. Other tips: Be sure your home has adequate ventilation, including exhaust fans in the kitchen and bathrooms. Fix leaky plumbing. Use mold-killing products when you clean the bathrooms. Remove or replace any carpets or upholstery that has gotten wet.