> You can't just slap a cigarette on your arm and hope the nicotine penetrates. It doesn't work like that.
What about chewing tobacco, and all the people getting centralized tooth, gum and mouth cancer? Clearly it doesn't just enter your stomach for digestion.
And most of your other examples are temporary applications. Shampoo? How long do you keep it on? Sunscreen often stays on for an entire day, since a lot of it is water resistent. Besides, there are many studies about sunscreen chemicals being absorbed. I don't know how you could even deny something like that.