Thesubstances which have adverse effects on living organisms and environment, asknown as toxic substances. A toxicsubstance has the ability to cause
systemic damage to living organism. Toxic substancesare resent in air, soil, water and in other living things. They can enterinside body in many ways, like; - through ingestion- by eating or drinking,through inhalation- by breathing, through contact with the
skin- by absorption,through injection- from syringe or from other poisonous insect or snake bite.
Toxic substancesare mainly of three categories. Chemical, biological and physical. Chemical toxicsubstances include (a) inorganic substances like
lead, mercury, asbestos, hydrofluoricacid, chlorine (gas) and (b) organic substances like methyl alcohol, medicineand poison from living things.
The dosageor concentration of the toxic substances is very important. Or properfunctioning of organism, many substances may be essential at low doses,
butthat particular substance can be dangerous at higher doses. For example,manganese is so important for an pregnant woman, that a deficiency of manganeseduring pregnancy reduce growth and can cause mortality of the offspring, whereas workers exposed to high
levels of manganese (manganese mines) sufferingbrain damage that causes memory impairment, disorientation and acute anxiety.