Chemical substance cause adverse health effects by either entering the body or coming in contact with it. There are four main routes for chemical substances to enter the human body :
- Inhalation (breathing in)
- Absorption (through the skin or eyes)
- Ingestion (eating, swallowing)
- Transfer across the placenta of a pregnant woman to the unborn baby
As already introduced above, the common chemical groups that cause health risks are: dusts, fumes and gases, solvents, acids, bases, heavy metals etc. Many chemicals may be dispersed into the air to form dust, mist, fumes, gas or vapour and can then be inhaled. Skin absorption is, after inhalation, the second most common route through which exposure may take place. Handling chemical substances without proper protection exposes one to the risk of absorbing harmful amounts of chemical through the skin. This usually happens when handling the chemical in liquid form. Dust may also be absorbed through the skin if it is wetted by, for instance, sweat. The capacity of different chemical substances to penetrate the skin varies considerably. Some substances pass through it without creating any feeling [http://www.ilo.org].
The protective external layer of skin may be softened (by toluene, dilute washing soda solution, etc) thus permitting other chemicals to enter readily into the bloodstream (such as aniline, phenol, benzene, etc). Eyes may also absorb chemical substances, either from splashes or from vapours. Dangerous chemicals can enter the body through ingestion as gases, dusts, vapours, fumes, liquids or solids. Inhaled dust may be swallowed, and food or cigarettes may be contaminated by dirty hands.
Whatever the route of entry, chemicals can reach the blood stream and be distributed all over the body. In this way damage can be caused at the site of entry as well as to organs distant from the exposed area. Chemical exposure may also cause adverse impacts at systemic levels: such as nervous and reproductive systems [Refs. 2, 5].