Researchers at the Yale School of Forestry & Environmental Studies have found that drought-prone conditions such as low rainfall may increase the risk of mortality from cardiovascular and respiratory illnesses during the onset of high-severity drought.

They also concluded that residents of areas which have experienced fewer drought events in the past may be at greater risk of both mortality and cardiovascular disease when those areas experience drought. The study, which claims to be the largest investigation of the relationship between drought and health in terms of both population and geographical area, was published in the journal Lancet Planetary Health earlier this month.

“When you consider how natural disasters impact health, there has been a lot of research focusing on forest fires, heat waves or floods. But actually, the natural disaster that impacts the most people across the whole globe is drought,” said lead author and F&ES postdoctoral fellow Jesse Berman. “In 2011–12, drought covered 65.5 percent of the continental U.S., and it is expected to affect over 150 million people.”

The researchers conducted a retrospective study that compared the Medicare claims of 8.6 million people living in 618 counties in the western United States with data from the U.S. Drought Monitor for those same counties between 2000 and 2013. The Drought Monitor ranks drought events on a five-point scale, with the most extreme called “Exceptional Drought” and the least extreme called “Abnormally Dry.”

According to Berman, the researchers used the five-point scale to characterize time periods as either drought or nondrought periods. The study further specifies that the drought periods were then split into high-severity and low-severity drought events.

The researchers observed a correlation between the onset of drought events and cardiovascular admissions. They also noted that higher mortality rates were seen for high-severity drought periods, when compared to nondrought periods.

This correlation prompts the follow-up question: How exactly does drought affect health? Berman added that the researchers “do not know the biological mechanism for how drought affects health,” though he suggested two potential explanations.

First, because drought causes psychosocial stressors, especially in rural populations that depend upon water for irrigation, the conditions may lead to higher rates of high blood pressure and heart disease. Berman said a study in Australia found increased rates of suicide among rural and agricultural populations during periods of drought.

The other hypothesis, Berman said, is that drought may lead to respiratory distress. Even though plant growth is suppressed during a drought, there is an increase in particulate matter such as dust or ash from wildfires. This air quality change can have a significant impact on health. He noted, however, that more research will be needed to determine precisely how drought psychologically and physiologically affects the human population.

In the future, Berman said he hopes that the study of drought and its relationship to human health can be applied to public policy. He points out that in contrast to other natural events, drought has a slower onset, and would provide an opportunity for preventative treatments to be put in place in order to reduce cardiovascular and respiratory stress during the drought periods.

The paper concluded that “preventative care could potentially have a crucial role in the reduction of adverse health effects, especially in areas with new and severe drought exposure.” Since drought has historically affected such a large number of individuals, Berman noted that it will be important to further develop this research.

The study was funded by the Yale Institute for Biospheric Sciences, the National Institute of Environmental Health Sciences and the U.S. Environmental Protection Agency.

According to the National Integrated Drought Information System, drought impacts 52.4 million U.S. citizens daily.