The percentage of adult smokers in the United States declined last year to 18 percent, from 18.9 percent the previous year, according to the Centers for Disease Control and Prevention (CDC).

Almost one-quarter of adults smoked in 1997. The percentage of adult smokers declined to 20.9 percent in 2005, and dropped again to 20.6 percent in 2009.

Health officials have not yet determined why the smoking rate has declined, the Associated Press reports. The findings come from a survey of about 35,000 adults. Current smokers were defined as those who said they smoked more than 100 cigarettes in their lifetime and now smoke every day or on some days.

The survey found smoking rates were lower among adults ages 65 and over (8.9 percent) than among those ages 18 to 44 (20.3 percent) and 45 to 64 (19.5 percent). The percentage of current smokers was higher for men (20.4 percent) than for women (15.8 percent).

In April, the CDC unveiled a new series of anti-smoking ads designed to appeal to people’s emotions. They follow anti-smoking ads released last year, which the CDC said had a strong impact across the country.

Last year’s graphic ad campaign featured the health consequences of smoking. According to the CDC, call volume to its national toll-free quit line, 800-QUIT-NOW, more than doubled while the ads ran. The hotline received an additional 192,000 calls, while the government’s smoking cessation website, www.smokefree.gov, received 417,000 new visitors—triple its previous traffic.