A lot of us would, according to a study reported in the Spanish-language journal Archives of Bronconeumology.
The Servicio de Neumologia of the Hospital de Sant Boi in Barcelona, Spain, asked 125 patients if they smoked. Then researchers got the same patients to take a breath test, and measured the amount of exhaled carbon dioxide.
They found that 21 percent of patients who claimed not to smoke were using tobacco. Even worse, among patients who said they used to smoke but had given it up, 27 percent were found to have lied.