Dr. Bill's Environmental  Toxicology Information Site.

Toxicology is the science that deals with the adverse effects of chemicals on living organisms. There are no safe chemicals, but they can be used safely.

Toxicological studies are used to determine  what effects occur  at differing dosage levels and to predict the effects of chemicals in different situations.

In order to assess the effects of a substance, one must know:     -  the chemical and physical properties of the substance     -  the biological system(s) affected     -  the response to the substance     -  the exposure background (amount, time of exposure,environment)

Much of this information can be obtained by conducting laboratory and/or field tests on a variety of test species. These species can range from one cell to multicell plant and animal forms. However, most of us are interested in what effect will occur in humans exposed to the particular chemical. But let's not get ahead of ourselves.

Two statistical parameters are often used in early toxicological studies. These are LD50 and LC50. - LD50 refers to a dose that will kill fifty percent of the test population. It is measured in milligrams per kilogram of body weight of the test population, and can be used to study exposure by all routes of exposure - except by air or water. - LC50 refers to a dose that is lethal to fifty percent of the test population, when administered by inhalation or in water. LC50 is expressed in milliliters per kilogram of body weight ( when exposure is through inhalation)  or in milligrams per liter (when exposure in through the water the animal lives in).

Now you can see why we don't want to study humans yet! However, LD50's and LC50's on animals are affected by: the strain, the sex, the age, the diet, and much more including which biological system (or organ) is being studied (brain, heart, kidney, liver). For example, the insecticide - MALATHION - has an LD50 for rats of 2800 mg/kg while the LC50 for largemouth bass is 0.285 mg/l.

Toxicologists divide the exposure of animals to chemicals into four study categories: - Acute (exposure to test chemical for less than twenty-four hours) - Subacute (exposure to test chemical for one month or less) - Subchronic (exposure for one to three months) - Chronic (exposure for more than three months).

Again, as with the LD50 (or LC50) values, the toxic effects of a chemical are quite variable when the test animal is exposed to a single dose or multiple doses. For example, ethanol can produce slight or moderate depression with acute use, but more severe damage  can be seen with chronic use. Likewise, a pregnant woman who drinks sporadically  may have no apparent overall ill effects but her baby may be severely affected.

It is important to note that the professional activities of toxicologists fall into three main types: - descriptive toxicologists deal with the safety evaluation of chemicals, using experimental test organisms to determine the risks posed to humans and the environment by exposure to specific chemicals - mechanistic toxicologists identify the mechanisms by which chemicals exert actions on living organisms - regulatory toxicologists decide, on the basis of the data provided by the other toxicologists, whether a particular chemical is safe and meets various standards established by different regulatory laws and organizations.

Toxicology generally is subdivided into three areas of specialization: - forensic  deals with the medical and legal aspects of chemicals on humans and animals (most often interested in the cause of death) - clinical deals with disease caused by or associated with toxic chemicals - environmental  deals with the impacts of chemicals in the environment on biological systems. Most often environmental toxicology deals with studies on  many living forms -  except humans.

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