Electronic cigarettes may produce an aerosol vapor instead of smoke, but two new studies raise burning questions about their uses and risks.

E-cigs—as these battery-operated nicotine inhalers are commonly called—are increasingly popular, with a Wells Fargo financial analyst predicting that U.S. sales will double this year, going up to $1.7 billion.

Their visibility is becoming ever greater as well, with television and online marketing campaigns that feature celebrities like Jenny McCarthy and Stephen Dorff touting the pleasures of what they describe as more socially acceptable, "guilt-free" smoking.