DIVERSE DIGS: Scientists have discovered greater indoor fungal diversity in temperate climates than in tropical regions; the study suggests that location influences indoor fungal growth more than building type. Image: NIH
In the first-ever global survey of indoor fungi scientists report that geography rather than building design and function has the greatest effect on the fungal species likely to be found indoors. The study suggests that the types of mold and other fungi most likely to be found in a dwelling may be largely unaffected by features like HVAC (heating, ventilating and air conditioning) filters and weather stripping.
The results of the study were published online June 28 in Proceedings of the National Academy of Sciences.
Mold and other types of fungi (including yeast and mushrooms) are naturally found everywhere, indoors or out, and for the most part these organisms cause humans little harm. Some fungal species, however, can spoil food and rot buildings as well as cause allergies, asthma or other ailments in humans. The American College of Occupational and Environmental Medicine estimates that approximately 5 percent of the U.S. population suffers from mold allergies. Symptoms can include sneezing, itchy and watery eyes, nasal congestion, and rash or hives.
Human indoor exposure to microbes is of prime importance because the average person in an industrialized nation spends approximately 90 percent of his or her life inside. Many questions remain as to which fungal species colonize human dwellings as well as how they impact human health.
New research led by Anthony Amend, postdoctoral researcher in the Department of Plant and Microbial Biology at the University of California, Berkeley, surveys what different kinds of fungi are likely to be found indoors, and describes the factors that shape indoor fungal colonization. These findings will help future characterization of what constitutes a normal indoor fungal load, which is needed before an abnormal variance can be defined.
The researchers collected 72 dust samples from typical buildings (including homes, offices, shops and a church) on every continent except Antarctica. They identified the different species of fungi present in the samples by analyzing the microbial DNA and found that, in a given geographic location, the variety of fungi found indoors closely resembled that found outdoors. Surprisingly, different buildings in close proximity to one another had very similar fungal contents, despite vast differences in building materials, architecture and function.
Many previous studies have focused on how air filters, smoking or pets influence the indoor environment. In terms of their effects on fungi found indoors, "We found that these factors are really minor," Amend says.
Earlier research also limited surveys primarily to the relatively few visible fungal species or those that can be easily grown and identified in a lab. Whereas some prior studies concluded that indoor fungi are distinct from the outdoor variety, "not all fungi grow well on artificial medium—it really depends on the medium that you use," Amend says.
The culture-independent identification method used by Amend and his colleagues enabled them to detect fungi that are not easily lab grown and likely overlooked by previous studies. "We found way more diversity than anyone has really found before," Amend says. Altogether, the researchers identified nearly 4,500 different fungal species in their indoor samples.
"The fungi that were dominant [indoors] were not necessarily the ones you think of as indoor fungi," says Tom Bruns, professor of plant and microbial biology at U.C. Berkeley and co-author of the study. Bruns pointed out that two common indoor fungal species are Penicillium and Aspergilus. "They were certainly in our samples but they weren't anywhere near the most abundant types there," Bruns says.