Wu , Tashkin , Djahed & Rose , 1988 , p . 350 ). This study was performed with men who had been smoking for the past five years ; all fifteen of them had been consuming marijuana and tobacco . With the help of other researchers , Dr . Wu measured the carboxyhemoglobin level ( basically the level of carbon monoxide in red blood cells ) of each smoker prior to and after smoking both tobacco and marijuana separately , setting the basis for the research .