By Lisa Rapaport

(Reuters Health) - Even as life expectancy is rising in many places across the U.S., there are some places where lifespans are getting shorter and geographical inequalities are becoming more pronounced, a new study suggests.

Nationwide in 2014, the average life expectancy was about 79.1 years, up 5.3 years from 1980, the study found. For men, life expectancy climbed from 70 years to 76.7 years, while for women it increased from 77.5 years to 81.5 years.

But the study also highlighted stark disparities: a baby born in Oglala Lakota County, South Dakota, can expect to live just 66.8 years, while a child born in Summit County, Colorado, can expect to live 86.8 years, on average.

“For both of these geographies, the drastically different life expectancies are likely the result of a combination of risk factors, socioeconomics and access and quality of health care in those areas,” said senior study author Dr. Christopher Murray, director of the Institute for Health Metrics and Evaluation at the University of Washington in Seattle.

“We found that risk factors - obesity, lack of exercise, smoking, hypertension, and diabetes - explained 74 percent of the variation in longevity in the U.S.,” Murray said by email. “Socioeconomic factors - a combination of poverty, income, education, unemployment and race - were independently related to 60 percent of the inequality, and access to and quality of health care explained 27 percent.”

To examine changes in life expectancy over time, researchers looked at death certificates from each county in the country.

Several counties in South and North Dakota, typically with Native American reservations, had the lowest life expectancy, the study found. Counties along the lower half of the Mississippi and in eastern Kentucky and southwestern West Virginia also had very low life expectancy compared with the rest of the country.

In contrast, counties in central Colorado had the highest life expectancy.

Some of the biggest gains in life expectancy during the study were seen in counties in central Colorado, Alaska and in metropolitan areas around San Francisco and New York.

But there was little, if any, improvement in life expectancy in some southern counties in states stretching from Oklahoma to West Virginia. Many counties where life expectancy dropped the most are in Kentucky.

One limitation of the study is that there might be errors in county death records, the authors note. Researchers also lacked data to explore how much the findings might be explained by migration of certain types of people to certain communities.

“The bottom line is that our life expectancy is increasingly being shaped by where we live within the U.S.,” said Jennifer Karas Montez, a sociology researcher at Syracuse University in New York who wasn’t involved in the study.

“Lifestyle behaviors are not causes, they are symptoms,” Montez said by email. “They are symptoms of the environment and the social and economic deprivation that many parts of the country now endure thanks to decades of policy decisions.”