Heat-related illnesses sent a high number of teenagers and young adults to hospital emergency rooms in recent years, especially men living in the hottest, rural counties of Minnesota and Wisconsin.

The trend was reported this week through a unique research partnership between the Minnesota Department of Health and the Wisconsin Department of Health Services, which cross-referenced ER visits in their states from 2006 through 2015 with the ages, hometowns and genders of the patients.

"Over the past few years, our agencies started noticing more and more cases of heat-related illness among younger, working-age people, " said Tess Konen, a Minnesota epidemiologist and leader of the project.

That made them curious because elderly people and young children are thought to be more vulnerable to severe heat.

The rate of heat-related ER visits was 10.7 per 100,000 people aged 15 to 34 in the two states. That was the highest rate for any age group, significantly exceeding the rates of 1.7 for preschool-aged children and 7.9 for senior citizens. Those two groups, however, tend to have higher rates of death and complications because they are more susceptible to the effects of elevated temperature, dizziness, dehydration and other symptoms.

ER visit rates also were higher in counties with above average summer heat index levels. In Minnesota, that corresponded to a band of counties from Faribault on the Iowa border to Clay in the northwest.

Researchers said this doesn't negate the "urban heat island effect" that can create risks for city dwellers.

Because the research was done at the county level, it might not have detected increased risks in extremely urban ZIP codes, where large amounts of concrete and asphalt surfaces can capture and preserve heat.

Men were twice as likely as women to visit the ER for heat-related illnesses, which researchers suspected might be due to gender differences in outdoor occupations. The proliferation of youth sports in the summer also could be a factor.

State officials said the findings will result in new and tailored messages to teenagers and young adults about their risks.

"During hot summer months, we are still concerned about the very young and very old, " Konen said. "But … we can do better to reach other populations who may not be aware of their risk."