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 Centers for Disease Control and Prevention.

The CDC’s National Center for Health Statistics reported the new statistic Tuesday. 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.