Oxygen is essential for the health of the skin. Cigarette smoke contains carbon monoxide which eradicates the oxygen in the skin. Cigarette smoke also contains nicotine which stagnates the flow of blood. This leaves the skin dry and discoloured. Smoking also drains many nutrients like vitamin C. These nutrients help protect the skin and repair skin damage.
Alcohol is terribly damaging to the skin, it affects the mucous membranes from the pancreas and liver to the skin. This leads to dehydration as it takes all the fluid out of the skin.
In conclusion, smoking and drinking leave the skin dry, discoloured and incapable of protecting itself. The skin is prone to allergies, inflammations and wrinkles. Scars and pigmentation do not heal.