WASHINGTON — Rarely has the concept of happiness caused so much consternation in public health circles.

Buried deep in the federal government’s voluminous new tobacco regulations is a little-known cost-benefit calculation that public health experts see as potentially poisonous: the happiness quotient. It assumes that the benefits from reducing smoking — fewer early deaths and diseases of the lungs and heart — have to be discounted by 70 percent to offset the loss in pleasure that smokers suffer when they give up their habit.

Experts say that calculation wipes out most of the economic benefits from the regulations and could make them far more vulnerable to legal challenges from the tobacco industry. And it could have a perverse effect, experts said. The more successful regulators are at reducing smoking, the more it hurts them in the final economic accounting.

“This threatens the F.D.A.’s ability to take strong actions against tobacco,” Frank J. Chaloupka, an economist at the University of Illinois at Chicago, said of the Food and Drug Administration. “If they can’t demonstrate that there is a significant economic benefit to doing it, then it makes their job much harder.”