DEBATE over e-cigarettes — battery-powered cigarette look-alikes that heat liquid nicotine but emit a harmless vapor — is raging. New York City and Chicago are considering adding e-cigarettes to their bans on smoking in bars, restaurants and parks, and Los Angeles is moving to restrict e-cigarette sales, even though e-cigarettes don’t generate smoke and, while not proved to be entirely safe for users, are undoubtedly less hazardous than tobacco cigarettes.