While access to better healthcare increased the overall average life expectancy in the United States from 73.8 years in 1980 to 79.1 years in 2014, that improvement was far from uniform across the country, a new study has found. The disparities in life expectancy show a sharp geographical divide, with some counties faring poorer now than they were 35 years ago.

The life expectancy gap between the best and the worst performing counties was of a massive 20.1 years, the study by the Institute for Health Metrics and Evaluation (IHME), at the University of Washington in Seattle, found.

Summit County, Colorado, had the highest life expectancy in the U.S., at 86.8 years, followed by Pitkin County at 86.5 years and Eagle County at 85.9 years, both of which are also in Colorado. Those numbers are higher than the highest national average life expectancy of 84.8 years in Andorra.

Read: US Life Expectancy Fell 0.1% In 2015

In contrast, “Oglala Lakota County, South Dakota — a county that includes the Pine Ridge Native American reservation — revealed the lowest life expectancy in the country in 2014 at 66.8 years, comparable to countries like Sudan (67.2), India (66.9), and Iraq (67.7),” according to a statement Monday by IHME.

A large number of counties, predominantly in the states of Alabama, West Virginia and Kentucky, and some other states along the Mississippi river, had low life expectancies. People born in 1980 in Owsley County, Kentucky, are likely to live 2.2 years longer than those born in 2014 (72.4 years, down to 70.2 years). There are 12 other counties where expected longevity has dropped in the last 35 years, of which most are in Kentucky.

In a statement, lead author of the study, Laura Dwyer-Lindgren, a researcher at IHME, explained: “Looking at life expectancy on a national level masks the massive differences that exist at the local level, especially in a country as diverse as the United States. Risk factors like obesity, lack of exercise, high blood pressure, and smoking explain a large portion of the variation in lifespans, but so do socioeconomic factors like race, education, and income.”

Read: Healthiest Places To Live In The World

Researchers also analyzed the data — taken from the National Center for Health Statistics, the Census Bureau and the Human Mortality Database — for those factors. They found a drop in under-5 mortality in all counties, and a reduction in the gap between counties for this age group. The 25-45 age group saw 11.5 percent of counties with an increased risk of death, and the probability of dying has gone up for the 45-85 group since 1980.

In addition, risk factors — obesity, smoking, diabetes, hypertension and such — explained about three-quarters of the variations in life expectancy, while the remaining was explained by socioeconomic factors, such as poverty, income, race, education and race.

“These findings demonstrate an urgent imperative, that policy changes at all levels are gravely needed to reduce inequality in the health of Americans. Federal, state, and local health departments need to invest in programs that work and engage their communities in disease prevention and health promotion.” Ali Mokdad, a co-author on the study who also leads U.S. county health research at IHME, said in the statement.