Matthew E. Hermes
Kennesaw State University

Fuels and Society C: Toxicity

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A material is toxic if it is capable of causing injury or death. All materials have some level of toxicity. The air we breath consists of 79% nitrogen and 20% oxygen. So nitrogen cannot be toxic, can it? But of course if we were placed in a pure nitrogen atmosphere we would die rapidly for lack of oxygen!

Nevertheless, the toxicity of chemical substances is an important consideration in their use and abuse. We have a number of ways of measuring and reporting toxicity. Sometimes these methods lead to guidelines for exposure to the materials, sometimes they do not.

LD50:

This is the amount of a substance, given at one time that will cause the death of 50% of the tested subjects. LD50 is a measure of acute or immediate toxicity and varies with the animal that is tested. You can see a few LD50 values posted by the University of Oregon in a unit on laboratory safety..

Animal values of LD50 vary depending on the experiment and the experimenter. Compare ethanol at about 2100mg/kg in rats as posted by Oregon with a value of 7000mg/kg found in a second tabulation.Of course we have no data from controlled tests on humans. But we can get some idea of the range of LD50 in humans for ethyl alcohol where we know that individuals whose blood alcohol level gets to above about 0.4% are at risk of death and we can determine the amount of alcohol required to reach thatn blood alcohol level..

The Canadian Center for Occupational Health and Safety gives us some definitions of LD50.

LD50 - MTBE

The LD50 is at best a guideline for toxicity. We seldom wish to expose ourselves or others at the level that will cause death so we must try to determine the level at which certain elements of harm or risk will be present. In addition, a one-time, acute exposure to a chemical substance is far different than a long term exposure at low concentrations.

The LD50 of MTBE, the gasoline additive is 4g/kg or 4,000ppm! Yet because of a drive to regulate the most insignificant of effects of potential trace MTBE in water, , California will reduce the level of MTBE allowable in drinking water to 5 parts per billion. This seems to be an extremely rigid position but we need to judge this standard by considering the story of vinyl chloride.

LD50 - Vinyl Chloride:

The LD50 of vinyl chloride, a gas that was once used as an aerosol propellant and continues to be a critically important monomer for the production of polyvinylchloride plastics is 500mg/kg in rats. However, since before 1978 we have known that long term exposure to quite low levels of vinyl chloride results in human carcinomas. As a result, most industrialized nations regulate vinyl chloride to levels below 10ppm for workplace exposure. The US Permitted Exposure Limit (PEL), the average exposure permitted across an eight hour day is 1ppm. So we regulate vinyl chloride to a limit of a fraction of one percent of the fatal dose because of the chronic effects of exposure to this material. And these chronic diseases took decades to develop.

It is often the potential for harm rather than the identification of harm that drives regulation towards zero exposure to synthetic chemicals as a standard.

TLV:

Government and industry cooperate to identify hazards of common chemicals and , at least for the workplace, agree on a Threshold Limit Value (TLV) - a concentration that will not be exceeded. In the US, the American Conference of Governmental Industrial Hygienists (ACGIH) manages the development and dissemination of TLV's