ALBANY - Exposure to second-hand smoke dropped sharply in New York after the state enacted its indoor smoking ban four years ago, according to a study released Thursday.

Researchers studied saliva samples from 1,594 nonsmokers collected before and after New York banned smoking in most places of business in July 2003. They found the levels of cotinine - a nicotine byproduct associated with cigarette smoke - dropped 47 percent in the samples after the ban took effect, according to the study published by the Centers for Disease Control.

“This is very strong evidence that we have a population-level impact for this law, and we certainly expect that it will translate into improved health over time,” said Dr. Ursula Bauer, lead author and director of New York state's Tobacco Control Program.

The U.S. Surgeon General reported last year that nonsmokers exposed to secondhand smoke increase their risk of developing heart disease and lung cancer by up to 30 percent.

The New York researchers said their study was the first “biologically validated” by looking at the chemistry of a large population of nonsmokers after a ban. But there has been a lot of research related to second-hand smoke, including a 2003 study that found heart attacks in Helena, Mont., fell by more than half after voters passed an indoor smoking ban.

Thomas Carr, manager of national policy for the American Lung Association, called this study yet another illustration of the danger of second-hand smoke.

“I think it adds to the overwhelming evidence,” Carr said.

Fifteen states and the District of Columbia have indoor smoking bans and laws have been passed in another seven states, Carr said.

New York City and Nassau County residents were excluded from the New York study because smoking bans were already in effect in those areas before July 2003.

Bauer said the patchwork of municipal and private workplace bans across New York before July 2003 contributed to relatively low cotinine levels in subjects even before the ban, and suggests the big difference came from the subjects' exposure in bars and restaurants.

The samples were collected from respondents to a telephone poll on smoking. Respondents determined to be nonsmokers were invited to send a saliva sample through the mail, with a $10 check as an incentive.

Cotinine is a metabolite created when the body processes nicotine and is not harmful by itself. It typically will remain in the body two to four days.