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An Interesting Resource For Toxins



The toxicology unit from

United States Naval Flight Surgeon Handbook: 2nd Edition 1998


Toxicologic evaluation involves several concepts which must be defined. Risk denotes the probability (expected frequency) that a chemical will produce undesirable effects under specified conditions. NEL (no effect level) is the maximum dose that has not induced any sign of toxicity in the most susceptible species of animals tested and using the most sensitive indicator of toxicity (not applied to carcinogens). There is no threshold defined for carcinogens because cancer cells can be induced by a single change in the cellular genetic material and they are self-replicating. The dose-response relationship is graphically displayed by plotting the frequency of an event vs. the dose on a log scale, which results in a sigmoid-shaped curve. The portion of this curve between 16-84% response is nearly linear and represents one standard deviation each direction from the mean.

The LD50 is also utilized to classify the toxicity of substances, as demonstrated in the following:





Extremely toxic

5-50 mg/kg

Highly toxic

50-500 mg/kg

Moderately toxic

0.5-5 g/kg

Slightly toxic

5-15 g/kg

Practically non-toxic

15 g/kg

Duration and frequency of exposure are also important parameters:

acute exposure - an exposure of 24 hours or less.

subacute exposure - repeated exposure over one month.

subchronic exposure - exposure occurring over 1-3 months.

chronic exposure - over 3 months


Chemical nature -- a generic term for naturally occurring fibrous minerals. The fibers are made up of fibrils which are not visible to the optical microscope, but are to the electron microscope.

Method of absorption -- serious toxicity is from inhalation. Visible fibers break into fibrils in the lung.

Associated clinical syndrome -- Three major distinct syndromes: (1) Asbestosis, (2) Lung cancer, and (3) Mesothelioma. Asbestosis is diffuse interstitial fibrosis with a small tight lung. This is due to massive exposure to the airborne fibers. This is the most common disease state associated with exposure. Lung cancer is the most common cause of death, however. This is induced by imbedded fibrils in the lung. Asbestos workers have ten times the lung cancer death rate of non-asbestos workers from lung cancer, all other things being equal. Smokers add a multiplier of eight to this. Mesothelioma was previously extremely rare and has increased to fairly common in asbestos workers. This is what is called a signal neoplasm in preventive medicine. Pulmonary plaques are also to be found after exposure. Although they are not associated directly with illness, they are yet another indicator of the reactivity of lung tissue with asbestos exposure.

Association with USN -- previously ships were full of asbestos, however, new ships have very little. Ripout and breakup of old ships is quite hazardous.

A very common mineral fiber. Detailed safety procedures are necessary when working with asbestos. Latency of disease make cause and effect difficult to measure.

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