Chemicals may cause harm in many different ways. They may be flammable, explosive, radioactive, corrosive, etc. All chemicals are toxic at some level. The dose, however, determines the hazard. Every chemical has some set of exposure conditions in which it is toxic and, conversely, every chemical has some set of exposure conditions in which it is not toxic. Therefore, before beginning to work with a chemical, we must understand how it can affect us and at what dosage.
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Toxicologists are scientists who study how chemicals cause damage to living tissues. Many methods have been developed to determine the toxicity of chemicals. Acute toxicity, which is defined as the immediate effect of a substance as a result of a single dose, is relatively easy to study. Chronic toxicity, which results from low doses of a chemical repeated over long periods of time, is much more difficult to test.
Toxicity is objectively evaluated on the basis of test dosages made on experimental animals under controlled conditions. The most common measure that toxicologists use to estimate the acute toxicity of chemicals on humans is LD50. LD50 is an abbreviation for “Lethal Dose 50%.” The LD50 value for a chemical is the amount of chemical that can be expected to cause death in half (50%) of a group of a particular animal species when the chemical enters the body by ingestion or skin absorption. The amount required to cause death is normally related to body weight: therefore, the LD50 is expressed in milligrams of chemicals per kilogram of body weight (mg/kg). A typical LD50 statement includes the substance, the route of entry, and the animal species; as shown below:
Aniline LD50 oral-rat: 250mg/kg.
In plain English, this LD50 statement says that a single oral dose consisting of 250 mg of aniline will kill, on average, one-half the population of 1-kg rats.
It should be noted that no LD50 data exists for humans. Data from test animals is used to estimate the possible acute toxicity of a chemical on a human being. Toxicity, data should therefore be used to evaluate the relative toxicity of various chemicals and which chemicals may require greater precautions when handled. The lower the LD50 value, the more toxic the substance. Refer to the chart in the Laboratory Safety Article section of the Flinn Scientific Catalog/Reference Manual for: LD50 values for many common chemicals found in high school chemistry laboratories. LD50 values have not been measured for all chemicals – even some known hazardous chemicals such as lead compounds, do not have known LD50 values.
In general, chemicals with LD50 values less than 300mg/kg are considered highly toxic, those with LD50 values between 300 and 1,000 mg/kg are considered moderately toxic, and those with LD50 values between 1,000 and 5,000 mg/kg are considered slightly toxic. Because LD50 values depend on body weight, however, many chemicals that may not harm an adult may be toxic to a small child.
Never, ever consume any laboratory chemicals. The LD50 value is an approximation, relative acute toxicity value based on statistical calculations.
Some people may become severely ill or even die at much lower dosages than an LD50 may suggest.
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