That makes no sense. Having traces of THC in your system does not mean that you are still high. Same for nicotine.
They can still fire you. If it is in your system how do they know if you smoked last night or smoked right before you came to work and got into an accident? That is the same as if you had alcohol in your system. Alcohol just flushes out quicker.
There is a hospital in Tennessee that does nicotine tests on employees and if you have it in your system they terminate you.
2015/16 -3- Kirkwood days
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