They were asked if they had experienced any of four symptoms of depression in the previous two weeks, such as problems sleeping, low self-esteem, poor appetite, and/or a decreased interest in activities. Those with three or more symptoms were deemed to be depressed.
Residents were also asked to assess their living conditions, while, at the same time, the researchers conducted visual inspections to calculate the levels and location of any dampness and mold in each home.
Finally, each study participant was asked whether or not they felt in control of their home environment, as well as whether they had any of six conditions that can be associated with exposure to mold, including: cold or throat problems; wheezing; asthmatic attacks or other respiratory problems, fatigue; or headaches.
Housing characteristics -- such as light, ventilation, size, crowding and heating conditions -- were also noted, as were basic demographic information such as employment status. The researchers pointed out that such factors, as well as general health, are sometimes associated with depression.
Shenassa and his colleagues found that 57 percent of all the residents lived in homes that were free of dampness or mold, although the prevalence of mold varied greatly depending on region -- ranging from more than 80 percent in Portugal to a little more than 25 percent in Slovakia.
Meanwhile, nine percent of all residents were determined to be depressed. Women, the elderly and the unemployed were most likely to have depressive symptoms, while those living in crowded conditions also appeared to run a higher risk for depression.
But, even after accounting for such key mitigating factors, the researchers connected the dots and found that having mold in the home appeared to be associated with depression.
"Basically, the risk for depression went up about 40 percent among people who lived in moldy homes," said Shenassa. "And to the extent that there are the same types of mold in Europe as they are in the U.S., the results should also apply to U.S. households."
"But although we saw that there is more depression among people who live in moldy homes, we don't know which came first," Shenassa cautioned. "We think there are multiple pathways to depression So, we need to do more work."
Kelly A. Reynolds, a research microbiologist with the University of Arizona, described the study as "very interesting" but agreed that further research is needed.
"Knowing that the mold-health effects are long-term and chronic and sometimes cumulative means they [the study authors] might be very far from determining which is the chicken and the egg," she said. "So, although there's a lot of speculation, it's difficult to prove a mental health connection. But what we always tell people is that if you can smell or see mold in your house, there's really no reason to not get rid of it."
For more on the health risks of mold, visit the U.S. Centers for Disease Control and Prevention.
SOURCES: Edmond D. Shenassa, division of epidemiology, department of community health, Brown University School of Medicine, Providence, R.I.; Kelly A. Reynolds, Ph.D., research microbiologist, University of Arizona, Tucson; October 2007, American Journal of Public Health