UNDATED  (AP) - The smoking rate among U.S. adults has been falling steadily for decades but the latest survey from the Centers for Disease Control and Prevention finds the 2015 decline is the biggest in more than 20 years.

The government report says the rate of smoking among adults in the U.S. fell two percent, to 15 percent last year, from 17 percent in 2014.

The usual decline is 1 point or less in a year and it's not clear why last year was different.

Smoking is the nation's leading cause of preventable illness and the CDC estimates it causes more than 480,000 U.S. deaths each year.

About 50 years ago, roughly 42 percent of U.S. adults smoked. Experts attribute recent declines to the anti-smoking advertising campaigns, cigarette taxes and smoking bans.

The role of electronic cigarettes is unclear.