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Midlatitude Mildew: Indoor Mold Growth Is Influenced More by Location Than Building Type

In a surprising study result, more numerous and diverse varieties of fungi inhabit temperate zone households than dwellings in tropical climes



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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.

Other species the researchers identified were those that were likely carried in from the outdoors, including aquatic fungi and other species that are known only to survive in association with plants. What the researchers found "wasn't just the typical mold that you think of growing on walls," Amend says.

According to the U.S. Environmental Protection Agency, mold growth on walls of homes and other buildings is primarily caused by excessive moisture; reducing humidity and preventing condensation are the recommended courses of action.

The researcher's biggest surprise was that, globally, fewer fungal species were found in dwellings located in tropical regions. In fact, fungal diversity was much greater farther from the equator in temperate climates. "This global pattern is actually the reverse of most other organisms," Bruns says. He added that for most species, greater diversity is found in equatorial regions.

The researchers suspect that these global variations may be partly due to differences in seasons and weather. For example, cooler months in temperate regions might enable the growth of organisms that cannot thrive in tropical climes. Their results, however, could also be due to global differences in socioeconomics and lifestyle or other factors not yet explored.

In the future the researchers would like to be able to detect in their samples microbes that are alive. "The sampling technique tells us that DNA is there but doesn't tell us whether the organism is alive," Bruns says. Furthermore, their samples of settled dust meant that they examined organisms that likely collected over a very broad time period. "We're definitely going to follow up with shorter timescale views of fungal diversity."

Amend notes that this study revealed some patterns of fungal colonization, and the next step would be to examine the global processes that shape those patterns. For example, he would like to know why some fungal species are widespread whereas others seem to be restricted to a single country or hemisphere. "Is this because some fungi disperse better? Is everything dispersing equally, but some are better at surviving in different environments?" Amend asks.

"This is really important," says James Scott, professor of occupational and environmental health at the University of Toronto who wasn't involved in the study. "Over the last couple of decades we've really started to appreciate how important the indoor environment is to our health, particularly the health of kids." Scott noted that the study used a fairly small sample set, and only included three locations sampled between the tropics of Cancer and Capricorn compared with nine locations sampled in temperate regions. "But it's the first look at something that is of huge interest from an environmental health standpoint," Scott says.

Scott noted that there are a lot of mixed findings about exposures to indoor fungi and their links to disease. "For the last 50 to 60 years, people interested in indoor environment and health have collected samples from the indoor environment in a certain manner. What this study highlights is that those sampling methods that have been used for quite some time were probably missing 90 to 95 percent of the biodiversity," Scott says.

"It's shocking, really. We need to start rethinking how we measure," Scott says.

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