Last year 15 percent of American adults smoked, down from 17 percent in 2014, according to new data from the Centers for Disease Control and Prevention (CDC).

The decrease was the largest one-year decline in more than two decades, the Associated Press reports. The smoking rate has been decreasing for decades, but generally drops only 1 percent or less annually.

About 50 years ago, 42 percent of U.S. adults smoked, the CDC noted. Recent declines in smoking rates are due to anti-smoking ad campaigns, as well as cigarette taxes and smoking bans, experts say.

The increasing popularity of e-cigarettes has also likely been a factor in the decline in smoking rates. It is not yet known whether e-cigarettes will help further reduce smoking rates, or contribute to an increase in smoking in the future. Some public health experts are concerned e-cigarettes could create a new way for people to become addicted to nicotine.

In April, the CDC released a report that found e-cigarettes are now the most widely used tobacco product among teens. E-cigarette use rose among middle school and high school students from 2011 to 2015, the report found.

Three million middle and high school students used e-cigarettes in 2015, an increase of 2.5 percent from the previous year. Among high school students, e-cigarette use rose from 1.5 percent to 16 percent, according to the report. Among middle school students, e-cigarette use increased from 0.6 percent to 5.3 percent during that period.

Earlier this month, the Food and Drug Administration (FDA) announced it is extending its oversight to all tobacco products, including e-cigarettes. The agency will ban sales of e-cigarettes, cigars, pipe tobacco and hookah tobacco to people under age 18.