There are also questions about pot and driving. If pot becomes legal in Illinois, is there a level of THC in the blood that signifies impairment? Would it be fair to ban any level of THC in the blood if a minimal presence doesn't impair driving? One problem here is that, for regular users, THC hangs around in the bloodstream long after the chief impacts on the user fades. That's because, unlike alcohol, THC gets stored in fatty tissue and dissipates at a much slower rate. A regular smoker may be driving to work a day or a week after lighting up and still test positive for THC.