Life expectancy is one of the most important and commonly cited indicators of population health -- and in the United States, life expectancy is falling at a historic rate. According to the Centers for Disease Control and Prevention, life expectancy at birth declined by 1.5 years in 2020, the largest one-year decline since World War II.

The CDC attributes the decline to the COVID-19 pandemic and 93,000 drug overdose deaths -- an all-time one-year high. Homicide, diabetes, and liver disease were also contributing factors. Here is a look at the states with the most drug overdose deaths in 2020

While the national trend is alarming, there are parts of the country where life expectancy is far higher than the national average. In Lake County, Colorado, for example, life expectancy at birth is an estimated 85.5 years -- about six years longer than the comparable national average of 79.2 years.

Variations in life expectancy across populations are closely linked to several factors -- and one of the strongest predictors of life expectancy is income level. Poverty presents challenges and stressors that can take a cumulative toll on both physical and mental health. Additionally, lower-income Americans are less able to afford adequate health care and healthy options related to diet and lifestyle. Recent studies have shown that life expectancy among the wealthiest 1% of Americans exceeds that of the poorest 1% by well over a decade.

Lake County is a notable exception to the pattern, however, as 14.6% of the local population live on poverty-level income, compared to 13.4% of Americans nationwide.

All data on life expectancy came from the Robert Wood Johnson Foundation and the University of Wisconsin Population Health Institute joint program's 2021 County Health Rankings & Roadmaps report. All life expectancy figures are averages for the years 2017 through 2019 -- the most recent period for which county-level data is available. Poverty rates are five-year estimates from the U.S. Census Bureau's 2019 American Community Survey.

Counties were excluded if life expectancy data was unavailable, if the confidence interval for the estimate was deemed too large, or if the county's 2019 five-year population estimate from the Census Bureau was less than 1,000.