AURORA | Americans are snuffing out their cigarettes at a faster rate than they have in two decades, according to a new report.

And that steady stream of people ditching their smokes means the adult smoking rate is just 15 percent — a figure celebrated by experts but still higher than they’d like it to be.

FILE - In this July 16, 2015 file photo, Bruce Schillin, 32, exhales vapor from an e-cigarette in Sacramento, Calif. In 2015, the rate of smoking among adults in the U.S. fell to 15 percent due to the biggest one-year decline in more than 20 years, according to a report by the Centers for Disease Control and Prevention released on Tuesday, May 24, 2016. The increased popularity of electronic cigarettes has likely played a role, but it is not yet clear whether this will help further propel the decline in smoking, or contribute to an increase in smoking in years to come. (AP Photo/Rich Pedroncelli, File)

Arnold Levinson, associate professor at the University of Colorado School of Public Health, said that while the population as a whole has largely kicked the smoking habit, the problem persists among poorer people at a higher rate than their wealthier counterparts. And experts struggle to pinpoint why that is, he said.

“It’s not because of lack of awareness and it’s not because of lack of trying,” he said. “That half of the population tries just as often as the other half who smoke, the problem is that they don’t succeed in their attempts.”

According to the Centers for Disease Control and Prevention, the smoking rate fell 2 percentage points from 2014 when it usually falls only about 1 point or less. The last time there was a drop nearly as big was from 1992 to 1993, when the smoking rate fell 1.5 percentage points, according to Brian King of the CDC.

The CDC reported the new statistic last month. It’s based on a large national survey that is the government’s primary measuring stick for many health-related trends.

Smoking is the nation’s leading cause of preventable illness, causing more than 480,000 deaths each year in the United States, the CDC estimates.

Why the smoking rate fell so much in 2015 — and whether it will fall as fast again — is not quite clear.

About 50 years ago, roughly 42 percent of U.S. adults smoked. It was common nearly everywhere — in office buildings, restaurants, airplanes and even hospitals. The smoking rate’s gradual decline has coincided with an increased public understanding that smoking is a cause of cancer, heart disease and other lethal health problems.

Experts attribute recent declines to the mounting impact of anti-smoking advertising campaigns, cigarette taxes and smoking bans.

Still, other CDC data points to roughly 30 percent of people living below the poverty line using tobacco products.

Levinson said there is no hard proof about why folks on the lower end of the socioeconomic scale use tobacco at a higher rate. One theory, he said, is that when people are economically strapped or socially isolated, adding another stress factor like quitting might be too much.

“Those kinds of stressors are difficult to deal with, and quitting smoking is a hugely stressful in and of itself,” he said.

The increased marketing of electronic cigarettes and their growing popularity has also likely played a role in the decreasing smoking rate overall. But it is not yet clear whether this will help further propel the decline in smoking, or contribute to an increase in smoking in years to come.

E-cigarettes heat liquid nicotine into a vapor, delivering the chemical that smokers crave without the harmful by-products generated from burning tobacco.

FILE - In this Dec. 13, 2005, file photo, an unidentified man smokes a cigarette in Sacramento, Calif. The adult smoking rate fell in 2015, with its largest annual decline in at least 20 years, according to a new government report. (AP Photo/Rich Pedroncelli, File)

That makes them a potentially useful tool to help smokers quit, but experts fear it also creates a new way for people to get addicted to nicotine.

Some CDC surveys have shown a boom in e-cigarette use among teenagers, and health officials fear many of those kids will get hooked on nicotine and later become smokers.

As today’s teenage e-cigarette users become adults in the next few years, “we may see 18-, 19- and 20-year olds pick up the habit,” said Dr. Jonathan Whiteson, a smoking cessation specialist at NYU Langone Medical Center in New York.

Still, he and others are optimistic in part because regulators are turning their attention to the potential dangers of e-cigarettes. Earlier this month, the Food and Drug Administration announced sweeping new rules that will, for the first time, apply long-standing rules covering traditional cigarettes to e-cigarettes, hookah tobacco, pipe tobacco and nicotine gels. Minors would be banned from buying the products.

“We’d expect continued declines in smoking, as we’ve seen in the past 50 years. But it’s hard to say what the future holds,” King said.

— The Associated Press contributed to this report.