Most nicotine is destroyed in combustion.
Conversely its very bioavailable by skin absorption or drinking the stuff.
Back when I grew it I would get anywhere from a very strong buzz to mild 'green tobacco sickness' just from handling the leaves and flowers of N. tabacum, N. rustica, or N. gossei and that was while I was a smoker.
Two drops of pure nicotine or one cigar has enough nicotine to kill a non-smoker if taken orally. Its one of the poisons the body can build a tolerance to, though.
You'll know if your being exposed to too much if it feels like you drank too much coffee, more exposure will result in increased restlessness, nervousness, as well as nausea. Thats as far as I ever got but things only get worse from there according to the literature. 'Green tobacco sickness' is the actual name of the illness caused by excessive skin exposure to tobacco plants, it happens a lot in growers and harvesters. It takes some minutes for the nicotine to fully soak in. If it feels like you drank coffee when you didnt, go wash with soap and stay away from the tobacco plants for a few hours.