It’s a testament to the power and importance of preventing smoking – an interesting “what if” to mark the 50th anniversary of the landmark 1964 U.S. Surgeon General’s report on the ills of smoking. A study the Journal of the American Medical Association finds that half a century of anti-smoking campaigning has prevented 8 million premature deaths in the United States, giving those people an average of nearly 20 additional years of life.

In 1964, almost 60 percent of American men and more than one-third of women smoked. Today, less than 20 percent of adults smoke.

The researchers figured that without the alarms over smoking the percentage of smokers would have stayed fairly stable or declined among men and grown or stayed fairly stable among women. Then they calculated what these changes would have meant for the rates of death from smoking and compared them to actual figures.

The researchers estimated that tobacco-control efforts prevented 5.3 million deaths among U.S. men and 2.7 million deaths among women between 1964 and 2012. During that time, life expectancy at age 40 grew by 7.8 years for men and 5.4 years for women. According to the researchers’ calculations, the decline in smoking accounted for 2.3 of those additional years for men and 1.6 years for women.

It’s great news, with a caveat: Smoking still claims millions of lives a year around the world.