Molds can reproduce by producing spores (2 - 100 microns [µm] in diameter), similar to the seeds produced by plants. These spores are so small they easily float through the air and can be carried for great distances by even the gentlest breezes. The number of mold spores suspended in indoor and outdoor air fluctuates by seasons, daily and even hourly.

Mold spores cannot be eliminated from indoor environments. Some mold spores will be found floating through the air and in settled dust; however, they will not grow if moisture is not present. Molds are ubiquitous in nature, and mold spores are a common component of household dust. The term toxic mold is sometimes used to refer to mold-related indoor air quality problems.

Exposure to significant quantities of mold spores can cause allergic reactions. Under proper growing conditions, some species of molds may generate molecular compounds called mycotoxins. In large quantities or with chronic exposure, mycotoxins can be toxic to humans or animals.

The main problem with the presence of mold in buildings arises from the inhalation of mold spores. Some molds are particularly serious in this respect, because their spores are important causes of allergies also, the spores of some fungi like Stachybotrys release potent toxins into the lungs when inhaled; Stachybotrys spores can cause very severe symptoms and lung lesions in children. In the home, the presence of mold can mean that something is wrong. Mold doesn’t require sunlight; often, sunlight will kill it. Mold does require moisture and food. In houses and buildings, this food source is usually some form of cellulose such as wood or cardboard.

• If an entire house has problems with mold year-round, there is probably too much moisture in the house, possibly from a dirt basement or a leaking roof.
   
• If there are mold problems in one specific part of a house year-round, then that part of the house probably needs more sunlight and ventilation. Mold often forms on perimeter walls, because they are coolest, and where ventilation is poorest (in corners, around furniture).

If there are mold problems in a house only during certain times of the year, then it is probably either too air-tight, or too drafty. Mold problems occur in air-tight homes more frequently in the warmer months when humidity reaches high levels inside the house, and moisture is trapped. And it occurs in drafty homes more frequently in the colder months when warm air escapes from the living area into unconditioned space, and condenses. If a house is humidified artificially during the winter, this can create conditions favorable to mold.