THC is a fat soluble substance and tends to hang around where there's a lot of fatty stuff. One place is the brain. (Fat= energy in simplistic terms and the brain uses a lot)
Hence it hangs around the body for a lot longer than alcohol. (Alcohol is metabolised in the liver and broken down by the enzyme alcohol dehydrogenase)
Depending on usage rates, the brain can be affected in decreasing amounts for up to a week. THC can be detected in hair for up to 3 months.
At the moment, there is no specific test for THC in the class 1 but I am sure it will be tested for in the future.