"Penicillium" species are present in the air and dust of indoor environments, such as homes and public buildings. The fungus can be readily transported from the outdoors, and grow indoors using building material or accumulated soil to obtain nutrients for growth. "Penicillium" growth can still occur indoors even if the relative humidity is low, as long as there is sufficient moisture available on a given surface. A British study determined that "Aspergillus"- and "Penicillium"-type spores were the most prevalent in the indoor air of residential properties, and exceeded outdoor levels. Even ceiling tiles can support the growth of "Penicillium"—as one study demonstrated—if the relative humidity is 85% and the moisture content of the tiles is greater than 2.2%.