You’ve encountered Cladosporium before. Worldwide, indoors and out, it is ever-present. Along with Penicillium, Aspergillus and Alternaria, it is one of the four most common molds found in homes, but you may know it as the black, lumpy mold that forms on the water line of your toilet.

Cladosporium is aptly classified as a Black Mold, though colonies can be olive or dark brown. Though it is more commonly found outdoors, it can thrive in any damp, dark, areas and particularly on nonporous surfaces such as fuel tanks or inside refrigerators. It can be found almost anywhere in the home from basement to attic.

Though Cladosporium is not considered toxic, it is an allergen and large colonies can cause severe reactions and those with asthma, allergies, or sensitive immune systems. Those who have reactions often find relief from over-the-counter antihistamines but those with more severe reactions will need to consult a physician.

Cladosporium will be easier to control during the cold, dry winter months but be forewarned, when the warm humid days of summer come, Cladosporium will likely make a strong comeback.

Because Cladosporium is an airborne mold, it can be very difficult to eliminate. Surfaces with Cladosporium should be scrubbed vigorously with bleach and wiped dry. However, as a mold native to the outdoors, its original source may not be your home. Even when defeated indoors, reinforcement spores can enter your home through windows and vents. The best way to gain an advantage over Cladosporium is to deal with the environment. Reducing the moisture in the air in problem areas with dehumidifiers, eliminating leaks, and filling cracks might go a long way but if growth persists, professional help may be needed.