Reprinted from the ACJ - November, 1993
We have spoke in the past about acute versus chronic effects of chemical exposure, but we only touched lightly on the interesting relationship the two dangers share. This discussion will try to shed a bit more light on the topic.
You woke up Saturday morning with the drum solo from "In-a-gadda-da-vida" pounding in your head. The sunlight lances your eyeballs like hot pokers and it feels like elephants sat on you before leaving a special something in your mouth.
But the day goes on and by nightfall you're shining up your shoes for another episode of "Playboy After Dark." That is an acute exposure.
Conversely, you pick up this article and begin to read it. You feel slightly dizzy and your senses dull. It gives you a headache that seems to grow every time you read this column. It feels like the life is being sucked out of you paragraph by paragraph. That's chronic poisoning.
How are these two scenes related? They're not, but acute and chronic exposures are actually two ends of a theory called the dose-time relationship. Simply put, it is how much how often. Harmful acute exposures usually involve large doses in a single episode. Chronic exposures are small repeated doses. You would think that anything acutely dangerous would also be chronically dangerous and vice versa. That is not always true. Often they affect different systems in your body.
Taking lead as an example, a harmful, large, one-time dose damages your gastrointestinal system while smaller doses over time tend to gnaw away at your liver, kidneys and nervous system. Alcohol as an acute poison overloads your nervous system, while extended alcohol consumption tends to destroy the liver. Again it's a case of how much how often.
A quick definition to help us along with this discussion: The acute toxicity of substances are described by something called an LD50. This is the amount of the substance that, when given to a group of subjects, will kill 1/2 of the group. It is usually listed with the subject type.
An example would be the LD50 (rabbit), meaning not the amount of substance needed to kill 1/2 of a rabbit, but the dose that killed 1/2 of the test group of rabbits. I won't bother to get into the moral dilemma of animal testing other than to say better them than us.
The lower the number of the LD50, the more toxic the substance. The amount is usually given as the amount of toxin per unit of body weight, such as grams toxin/pound of victim. Now back to our regularly scheduled discussion.
Sometimes a substance can be acutely toxic but not at all chronically dangerous and possibly even beneficial. On the flip side, exposure over time may be dangerous, but a one-time shot causes no ill effects. The idea that something that can kill you in a single dose but is necessary for life in small repeated doses is an odd concept. Take vitamin D for example. The LD50 is listed as 10mg/kg (milligram toxin per kilogram of body weight). That is low enough that it should carry a poison label if it were not a food additive. But we all have been drinking vitamin D fortified milk without overdosing. Rickets is the result of too little vitamin D in our systems.
Table salt is another example. The LD50 is 3g/kg. That translates into a lethal dose for a small child of about two tablespoons. But again, low levels are necessary for the body to function.
Turning the subject over, some chronic toxins are safe in large single doses. Take mercury for example. Because it absorbs very slowly into your body as it passes through, acute exposure is not very dangerous. Eating a thermometer is not a mercury poisoning danger (though the glass may be unpleasant), but eating mercury-contaminated fish throughout your life can cause death if enough accumulates in your body over time. To explain these oddities, we go back to the dose-time relationship, how much how often. It really is the key to understanding toxicity.
Here are some more interesting examples. A lethal dose of caffeine is contained in about 100 cups of coffee (or two cups of my coworker Dan Thompson's espresso). A lethal dose of solanine is hidden in 100 to 400 pounds of potatoes. Ten to 20 pounds of spinach has enough oxalic acid in it to kill you. A fifth of scotch contains a deadly amount of ethanol, and 100 tablets of aspirin will cure your headache permanently. Now it might be difficult to consume all of this at one sitting, but it does sound a little like my diet last week. Why aren't these things harmful to us? Because the doses are spread out over time so as to not tax our systems.
A final thought for you to digest (no pun intended) is the theory of "Sufficient Challenge." It proposes that small non-damaging amounts of a toxic substance may benefit us over time by allowing our bodies to develop ways of dealing with that toxin. Give the body just enough poison to develop a sort of immune response. Sort of a "what doesn't kill us makes us stronger" approach. An interesting subject.
I would be remiss if I didn't mention a great source of information on this subject (and the text that I stole most of this from). It is a book called "The Dose Makes the Poison" by Dr. M. Alice Ottoboni. It's easy reading and has some great trivia to astound your friends at your next session of acute alcohol exposure.
The above article was written by David M. Brown, Chief Engineer of Johnson Manufacturing Company, Inc.
and is published by JOHNSON with the expressed approval of the National Automotive Radiator Service
Association and the Automotive Cooling Journal. Other reproduction or distribution of this
information is forbidden without the written consent of JOHNSON and NARSA/ACJ. All rights reserved.
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