The next-highest ranking counties behind Summit for life expectancy were Cache, Morgan, Washington, Rich and Daggett counties, the study found.
Along with Carbon, Beaver, Duchesne, Juab, Sevier, Emery and Uintah counties had the state's lowest life expectancies, ranging from 76.89 to 77.99 years.
A Carbon County health official said lack of access to health care and work-safety issues in coal mining are thought to be factors in the county's lower-than-average life expectancies, said Bradon Bradford, interim director of the Southeastern Utah Health Department.
"Sometimes these numbers can get you down, especially when we are lagging behind," Bradford said. "But to know people have hope and are working to make their communities better is inspirational."
Health officials, he said, continue to push campaigns to address suicide rates, tobacco use, cancer screenings and infant nutrition — all of which affect longevity.
"In the last several months," Bradford said, "several community coalitions have really grasped onto the idea that we can make a difference in our community."
In Salt Lake County — Utah's most populous — life expectancy was at 79.4 years. In Utah County, it was 80.75, the study found.
Nationwide, Hawaii had the highest life expectancy in the U.S. at 81.15 years, with California and Connecticut close behind, at 80.82 and 80.56 years, respectively. Mississippi residents have the lowest life expectancy at 74.91 years, according to the study, followed by neighboring Alabama, 75.65 years, and Louisiana, 75.82.
The Institute of Health Metrics and Evaluations, at the University of Washington in Seattle, also examined the mortality rates of 21 leading causes of death in every county in the U.S between 1980 and 2014. Causes of death ranged from HIV/AIDs, self harm and interpersonal violence to several forms of cancer, neonatal disorders and transport injuries.
Capturing life expectancy at the county level allowed for a clearer picture of disparities within and between states, said Ali Mokdad, a co-author on the study.
"If you only look at the [statewide] average, you will mask the variation between counties," Mokdad said. "It's important when diagnosing a problem to have a refined picture of it."
Mokdad said he hopes the analysis will help state and regional health departments better identify underlying problems.
Notably, the study found that although Utahns are living longer now than they were 30 years ago, differences in life expectancy between counties in Utah have not changed dramatically.
In 1980, there was a 5.35 year difference between the life expectancies of top-ranked Cache County, at 78.15 years, and bottom-ranked Duchesne County, at 72.8 years. That difference widened by only about half a year by 2014 for Summit and Carbon counties.