Tobacco use is the leading cause of preventable illnesses and death in the nation. Most adults start smoking when they are teenagers, making prevention the best public strategy for reducing deaths and disabilities. That's why the Massachusetts Department of Public Health has decided to target adolescents with the launch of a new Web site, the84.org. The site seeks to keep teens from smoking and encourage those who do to quit; its name was taken from the 84 percent of Massachusetts teenagers who choose not to smoke, and it features antismoking ads made by teens, a forum to share thoughts and even merchandise for sale. The Web site, which cost $200,000 to develop, was created with input from an advisory panel of youth from across the state, and is part of a broader antismoking campaign that includes awarding about $200,000 in grants to organizations committed to fighting teen smoking.