Poor mental health ranks as one of the costliest forms of sickness for U.S. workers and may sap billions of dollars from the country's income growth, according to a team of researchers.

In an analysis of economic and demographic data from 2008 to 2014, the researchers found that a single extra poor mental health day in a month was associated with a 1.84 percent drop in the per capita real income growth rate, resulting in $53 billion less total income each year, said Stephan Goetz, professor of agricultural and regional economics, Penn State, and director of the Northeast Regional Center for Rural Development.

Poor mental health days refer to days when people describe their mental health as not good and could include conditions such as depression, anxiety, stress and problems with emotions, according to the researchers, who report their findings in a current issue of the Review of Regional Studies.

The researchers added that the global economic cost of mental illness is expected to be more than $16 trillion over the next 20 years, which is more than the cost of any other non-communicable disease. The effect is stronger in rural counties, which tend to be poorer than urban counties. A poor mental health day in rural counties was associated with a reduction of 2.3 percent in income growth, compared with only a .87 percent reduction in urban counties.

The researchers suggest that investing in mental health resources may be one way of lowering the economic costs of poor mental health, particularly in the harder-hit rural counties.