Routes of Exposure to Pesticides and the Dose-Response Relationship
Pesticides are widely used in agriculture mainly to increase crop yields to cater for the huge supply of food products for increasing world population as well as to protect crops from pests and control insect- borne diseases.
Increased use of pesticides results in contamination of the environment and the excess accumulation of pesticide residues in food products, which has always been a matter of serious concern.
Pesticide residues in food and crops are directly related to the irrational application of pesticides to the growing crops. Accumulated pesticide residues in food products have been associated with a broad variety of human health hazards, ranging from short-term effects to long-term toxic effects.
Routes of Exposure to Pesticides
There are four routes by which a substance can enter the body: inhalation, skin (or eye) absorption, ingestion, and injection.
Inhalation: For most chemicals in the form of vapors, gases, mists, or particulates, inhalation is the major route of entry. Once inhaled, chemicals are either exhaled or deposited in the respiratory tract.
If deposited, damage can occur through direct contact with tissue or the chemical may diffuse into the blood through the lung-blood interface. Upon contact with tissue in the upper respiratory tract or lungs, chemicals may cause health effects ranging from simple irritation to severe tissue destruction.
Substances absorbed into the blood are circulated and distributed to organs that have an affinity for that particular chemical. Health effects can then occur in the organs, which are sensitive to the toxicant.
Skin (or eye) absorption: Skin (dermal) contact can cause effects that are relatively innocuous such as redness or mild dermatitis; more severe effects include destruction of skin tissue or other debilitating conditions.
Many chemicals can also cross the skin barrier and be absorbed into the blood system. Once absorbed, they may produce systemic damage to internal organs. The eyes are particularly sensitive to chemicals.
Even a short exposure can cause severe effects to the eyes or the substance can be absorbed through the eyes and be transported to other parts of the body causing harmful effects.
Ingestion: Chemicals that inadvertently get into the mouth and are swallowed do not generally harm the gastrointestinal tract itself unless they are irritating or corrosive.
Chemicals that are insoluble in the fluids of the gastrointestinal tract (stomach, small, and large intestines) are generally excreted. Others that are soluble are absorbed through the lining of the gastrointestinal tract. They are then transported by the blood to internal organs where they can cause damage.
Injection: Substances may enter the body if the skin is penetrated or punctured by contaminated objects. Effects can then occur as the substance is circulated in the blood and deposited in the target organs.
The Dose-Response Relationship
In general, a given amount of a toxic agent will elicit a given type and intensity of response. The dose-response relationship is a fundamental concept in toxicology and the basis for measurement of the relative harmfulness of a chemical.
A dose-response relationship is defined as a consistent mathematical and biologically plausible correlation between the number of individuals responding and a given dose over an exposure period.
In toxicology, studies of the dose given to test organisms is expressed in terms of the quantity administered:
Quantity per unit mass (or weight). Usually expressed as milligram per kilogram of body weight (mg/kg).
Read Also : Factors Influencing Pesticide Degradation in Soils
Quantity perunitarea of skin surface. Usually expressed as milligram per square Centimeter (mg/cm2).
Volume of substance in air per unit volume of air. Usually given as microliters of vapor or gas per liter of air by volume (ppm). Particulates and gases are also given as milligrams of material per cubic meter of air (mg/m3).
The period of time over which a dose has been administered is generally specified. For example, 5 mg/kg/3 D is 5 milligrams of chemical per kilogram of the subject’s body weight administered over a period of three days.
For dose to be meaningful, it must be related to the effect it causes. For example, 50 mg/kg of chemical “X” administered orally to female rats has no relevancy unless the effect of the dose, say sterility in all test subjects, was reported.
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