Tiny spores, pollen and bacteria by the thousands enter the lungs with each breath. Most of these are breathed out, others are removed or destroyed by the defence mechanisms of our body. Our sense of smell is not always infallible. But there are a large number of chemicals which have no smell. A deadly example is carbon monoxide gas.
Carbon monoxide is produced when a fuel burns where there is a lack of oxygen, and it has no smell.
The skin forms a barrier between the organs of the body and the outside world. But a number of chemicals corrode or burn the skin, others dissolve the outer layer. Once chemicals enter through the skin, they may be carried away in the blood stream, causing harm to the organs or to bodily functions related to breathing, the nervous system, and so on. 1.2 A cross-sectional view of the skin showing different protective layers Heavy exposure to potentially harmful chemicals may occur during such activities as wood polishing if no protective measures are adopted Most alkaline and acids have corrosive action on the skin and can cause burns and boils. Most organic solvents can penetrate through the protective layer of the skin.