WASHINGTON, D.C. — Smoking is still prevalent in some populations, although the percentage of U.S. adults overall who smoke has been dropping. It went from 20.9 percent in 2004 to about 17 percent by the end of 2014.

The Centers for Disease Control and Prevention released a study on smoking on Thursday based on data from the 2014 National Health Interview Survey.

By the end of 2014, the overall adult smoking rate hit a new low of 16.8 percent. During the first three months of 2015, that number fell to 15.2 percent — the lowest percentage of American adult smokers ever recorded.

While overall smoking rates declined, the study found that U.S. adults who lack health coverage or who are covered by Medicaid smoke at rates more than two times than that of individuals with private health coverage or Medicare. Specifically, about 28 percent of uninsured adults and 29 percent of Medicaid beneficiaries smoke, compared to just 13 percent of their privately insured counterparts and 12.5 percent of those covered by Medicare.

CDC Director Tom Frieden said that while the report shows “real progress in helping American smokers quit,” more must be done to help eradicate smoking nationwide.

The study also found several other population groups with higher rates, including: Adults who have earned a GED at 43 percent; American Indian/Alaskan natives at 29.2 percent; multiracial adults at 27.9 percent and Individuals living below the federal poverty line at 26.3 percent.

“This study reiterates that even though tobacco is an equal-opportunity killer, our success in convincing these vulnerable populations to kick their habits has been decidedly uneven,” said American Heart Association Chief Executive Nancy Brown. “We must break through to all Americans if we are to end our nation’s tobacco epidemic.”

Frieden urged lawmakers to implement strong smoke-free laws, increase taxes on tobacco products and fully fund and support tobacco cessation programs.