The toxicity of a substance is its capacity to cause injury once inside the body (Carson and Mumford, 2002). Humans are exposed to hazardous wastes and substances through inhalation, ingestion and absorption through the skin.
Gases, vapours, mists, dusts, fumes and aerosols can be inhaled and they can also affect the skin, eyes and mucous membranes. The skin can be affected directly by contact with the chemicals, even when intact, but its permeability to certain substances also offers a route into the body.
Direct ingestion is rare although possible as a result of poor personal hygiene, subconscious hand-to-mouth contact, or accidents. Indirect ingestion through the consumption of plants and animals that have been previously exposed to high concentrations of poisonous metals and chemicals is, however, more prevalent.
The consumption of food crops and fish contaminated with heavy metals is a major food chain route for human exposure.
Heavy metals become toxic when they are not metabolized by the body and accumulate in the soft tissues (Lalor, 2008). Chronic level ingestion of toxic metals has undesirable impacts on humans and the associated harmful impacts become perceptible only after several years of exposure (Singh and Kalamdhad, 2011).Cadmium (Cd), for example, is a heavy metal toxicant that affects the liver, placenta, kidneys, lungs, brain and bones.
Severe exposure may result in pulmonary odema and death. Pulmonary effects (emphysema, bronchiolitis and alveolitis) and renal effects may occur following subchronic inhalation exposure to Cadmium and its compounds (Duruibe et al, 2007).
Exposure of humans to Copper (Cu) occurs primarily from the consumption of food and drinking water that have been polluted by toxic substances.
Excessive human intake of Copper may lead to severe mucosal irritation and corrosion, widespread capillary damage, hepatic and renal damage and central nervous system irritation followed by depression.
Severe gastrointestinal irritation and possible necrotic changes in the liver and kidney can also occur (Singh and Kalamdhad, 2011).
As illustrated in the booklet of health effects of chemical exposure by the United States Agency for Toxic Substances and Disease Registry (ATSDR), the following body systems are directly affected by exposure to hazardous substances and harmful chemicals: respiratory, renal, cardiovascular, reproductive, nervous, immune and hepatic systems.
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The skin is also affected. The functions of these systems and the pathways through which exposures to toxic substances can cause diseases are as follow:
The respiratory system – functions to supply oxygen to the body and remove carbon dioxide. It includes the nasal passages, pharynx, trachea, bronchi, and lungs. Possible health effects of the respiratory system include asbestosis, lung cancer, chronic bronchitis, fibrosis, emphysema, and decreased oxygen supply in blood.
Therenal system– Its function is to rid the body of waste, to regulate the amount of body fluids, and to regulate the amount of salts in the body. It includes the kidneys, the urethra, the bladder, and the ureter.
Possible health effects of the renal system include decreased formation of urine, decreased blood flow to kidney, decreased ability to filter the blood, prevented urine flow, kidney tissue damage, and kidney cancer.
The cardiovascular system – Its function is to move nutrients, gases, and wastes to and from the body, to help stabilize body temperature, and to fight diseases and infections by transporting white blood cells to important areas. It includes the heart, blood, arteries, veins, and capillaries.
Possible health effects include heart failure and the inability of blood to carry the necessary oxygen to the body.
The reproductive system – functions to produce egg and sperm cells, to nurture a developing foetus, and to produce hormones. For males, it includes the testicles, seminal vesicles, prostate glands, and the penis.
For females, it includes the uterus, bladder, vagina, fallopian tubes, ovaries, and the cervix. Possible health effects of the reproductive system include decreased ability to have a baby, increased baby deaths, increased birth defects, and infertility (the inability to have children).
The nervous system– Its function is to transmit messages from one part of the body to another. It includes the central nervous system (the brain and spinal cord) and the peripheral nervous system.
Possible health effects of the nervous system include inability to move, loss of feeling, confusion, and decreased speech, sight, memory, muscle strength, or coordination.
The immune system’s function is to protect the body from tumor cells, environmental substances, and invading viruses or bacteria. It includes the lymph system, bone marrow, white blood cells, and the spleen.
Possible health effects of the immune system include over-reaction to environmental substances (allergy), immune system slow down or failure, and autoimmunity (autoimmunity causes the body to attack itself – which makes it more likely to have an over- reaction or infection).
Theskin– It serves as a barrier to germs and other substances, prevents dehydration, and regulates body temperature. Possible health effects of the skin include irritation, rash, redness or discoloration, dermatitis, and health effect related to other systems and organs due to contamination through the skin.
The hepatic system – Its function is to break down food and store nutrients, to make proteins which are essential for blood to clot, and to purify the body of drugs, contaminants, or chemicals. It includes the liver and its veins. Possible health effects of the hepatic system include liver damage, tumors, accumulation of fat (steatosis), and death of liver cells.