The first thing to understand about mold is that there is a little mold everywhere indoors and outdoors. It's in the air and can be found on plants, foods, dry leaves, and other organic materials.
It's very common to find molds in homes and buildings. After all, molds grow naturally indoors. And mold spores enter the home through doorways, windows, and heating and air conditioning systems. Spores also enter the home on animals, clothing, shoes, bags and people.
When mold spores drop where there is excessive moisture in your home, they will grow. Common problem sites include humidifiers, leaky roofs and pipes, overflowing sinks, bath tubs and plant pots, steam from cooking, wet clothes drying indoors, dryers exhausting indoors, or where there has been flooding.
Many of the building materials for homes provide suitable nutrients for mold, helping it to grow. Such materials include paper and paper products, cardboard, ceiling tiles, wood and wood products, dust, paints, wallpaper, insulation materials, drywall, carpet, fabric, and upholstery.