Although cigarettes cost as much as $2.45 a pack these days, two thin dimes are all it takes to get the nicotine monkey off 16-year-old Javier Rodriguez's back.

That is because Rodriguez buys his smokes one at a time, from mom-and-pop stores near the San Fernando Gardens housing project in Pacoima where he lives.

Buying 20 "singles" or "loosies" is more expensive in the end than purchasing a 20-cigarette pack, but financial logic goes up in smoke when the craving hits. "It's a habit, and you do what you can to get hold of it," Rodriguez said.

The \o7 bodegas \f7 that feed his habit--and similar neighborhood and convenience stores throughout Southern California--routinely violate an obscure state law that bars the sale of loose cigarettes. And when they sell to people as young as Rodriguez, the stores are also violating California's law against tobacco sales to anyone under 18, a ban that exists in 46 other states.

Store owners and health authorities say the sale of single cigarettes has become commonplace, even in the handful of places like California where such sales are banned.

The practice reflects the shifting demographics of smoking, increasingly a habit of lower-income Americans. Although single sales are widespread, they are most prevalent in poor neighborhoods--from Pacoima to the barrios of New York to Chicago's South Side--where the rising cost of cigarettes may be a significant deterrent to smoking.

"They come in here with 20 pennies, and if you don't give them a single cigarette, they get crazy because they don't have the money for a pack," said Paul Sing, operator of the 7 Seas Mini Market in Arleta, in the northeast San Fernando Valley.

Health officials are most concerned about the effect on cash-short adolescents, likening single sales to a starter kit that helps the young get hooked. They also see it as a symptom of a larger problem: illegal tobacco sales to minors that go virtually unchecked throughout the country.

"Access of minors to tobacco is a major problem in every state of the nation," according to a report by the U.S. Department of Health and Human Services. Hundreds of thousands of stores "ignore the laws of their states because enforcement is almost nonexistent."

According to health officials, smoking is the nation's leading preventable cause of disease and death, contributing to more than 400,000 deaths per year. And teen-agers constitute the largest group of new smokers, replacing most of the 1.7 million smokers who quit or die each year.