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.