The known chemical carcinogens in cigarette smoke can not explain ~96% of lung cancer in smokers.
"However, for conventional cigarettes (i.e., R, Lt, and ULt cigarettes), as they are smoked by populations such as the United States and Canada, the lung carcinogen results obtained here indicate that, currently, it is only possible to account for ≤4% of the observed per pack-year risk for lung cancer." 2007
"the majority of the PO-210 in tobacco plants likely comes from high-phosphate fertilizers applied to the tobacco crop. ... PO-210 has been estimated to be responsible for 1% of all US lung cancers."2008
So that only leaves 95% of lung cancer caused by smoking cigarettes unexplained by smoking cigarettes.