Toxicity is the capability of a chemical to produce injury. Almost any substance is toxic when taken in doses exceeding "tolerable" limits. Hazard is the probability that an injury will occur or rather the prospect that an individual will receive a toxic dose. The effects of a toxic chemical may be qualified into several categories. Local toxicity is the effect a substance has on the body tissues at the point of contact. Acute toxicity is the effect a substance has after only one or a few short, relatively large exposures. Chronic toxicity is the effect a substance has as a result of many small exposures over a long period of time.
An individual may be exposed to a chemical substance via a number of different routes: inhalation ingestion, contact with skin or eyes. Inhalation of toxic vapors, mists, gases or dusts can result in poisoning by absorption through the mucous membrane of the mouth, throat, and lungs, and can cause serious local effects. Because of the large surface area of the lung (90 square meters total surface) along with its continuous blood flow, inhaled gases or vapor may be very rapidly absorbed and carried into the circulatory system. The rate of absorption will vary with the concentration of the toxic substance, its solubility and the individual inhalation rate. The degree of injury from exposure to a toxic substance depends on the toxicity of the material, its solubility in tissue fluids and the concentration and duration of exposure.
Ingestion of chemicals used in the laboratory may result in significant injury. The relative acute toxicity of a chemical can be determined by its oral LD50, that quantity of material which, when ingested, will cause the death of 50% of test animals. This LD50 is expressed usually in milligrams per kilogram of body weight. To prevent ingestion of chemicals, laboratory workers should wash their hands immediately after using a toxic substance and before leaving the laboratory. Food and drink should not be stored or consumed in areas where chemicals are being used. Chemicals should not be tasted, and pipetting and siphoning of liquids should never be done by mouth.
Skin contact is the most frequent route of exposure to chemical substances. A common result of skin contact is localized irritation, but some materials can be absorbed through the skin sufficiently to produce systemic poisoning. Contact of most chemicals with the eyes will result in pain and irritation. A considerable number of chemical substances are capable of causing burns or loss of vision. Alkaline materials, phenols and strong acids are particularly corrosive and may cause permanent loss of vision. Furthermore, the vascular network of the eyes may permit the rapid absorption of many chemicals.
Before initiating work with a chemical substance, the researcher or laboratory worker should be familiar with the types of toxicity, the toxic dose, and the hazards of the chemical. It is also important to realize that two or more substances may act synergistically to produce a toxic effect than that of either substance alone. Furthermore, chemical reactions involving two or more substances may form products significantly more toxic than the starting materials. Therefore, the entire experimental procedure should be evaluated.