Module 1, Sub-module 1D, Page 1
Dose, Chemical units.
Here is a review of some common chemical units as they are
applied to toxicology.
Dose per Body Weight
Test are done on batches of animals of the same strain and
species and similar age and weight. But, animals within a batch
often differ in body weight. When toxicologists poison lab animals,
we frequently express the dose as mg/kg. That means milligrams
of the poison per kilogram of animals body weight.
So if we are testing at 100 mg/ kg, how much do I feed a 300
Click here if you want to see that calculated.
Use your browser's BACK button to return here. The measure here is mass of
chemical per mass of animal.
Often we want to express the amount of something in solution. For example,
the maximum contaminant level of arsenic in drinking water is 50 micrograms
per liter. The symbol for microgram is the lower case Greek mu followed by
g like this but most
browsers do not support the mu. You will sometimes see a lower case "u"
used for this, ug, which is what I will have to do. Now we have 50 ug of arsenic
per liter, but what is the liter made of. You are tempted to say "water."
But when we are discussion concentrations, we mean liter of solution. If we wanted to make a test solution of 50 ug arsenic
per liter, we start by weighing 50 ug of arsenic into a dry beaker (volumetric
flask would be better), then adding water until the solution is exactly one
liter. You recognize the the true volume of water is slightly less than one
liter. The measure here is mass of contaminant per volume.
Often risk are related to contaminated soil - dirty dirt. We
call the soil the "matrix" and the chemical (or chemicals)
the contaminant. Here we start by weighing the combination, matrix
plus contaminant, and determine its mass. Then we somehow extract
the contaminant, and weigh it. We then have mass of contaminant
per mass of contaminated soil, often expressed as ug/kg or mg/kg.
Since there are one million milligrams in a kilogram, you will
sometimes here these solid mixture described in "parts per
million" or "ppm" instead of mg/kg, this is not
good usage, but it is so common you should know about it; ppm
Contaminants in air
Solid contaminants in air may be measured thus: Weigh a filter, pump a known
volume of contaminated air through the filter, say 1 cubic meter or (Again most browsers do not support
the superscript, so I'm forced to use m^3.) Then weigh the filter and subtract
the original weight of the filter and you have weight of solid contaminant
per volume of air , typically mg/m^3. Many contaminants in air are not solid,
they are gases or vapors. A vapor is the gas phase of a substance that is
liquid at room temperature and pressure. But the part that is in air is a
gas, so that is what we measure. (You can have droplets of liquid suspend
in air, a fog or mist, but we won't go there now.) The amount of contaminant
gases in air are expressed either as ppm, which means volume of contaminant
per volume of contaminated air, or in mg/m^3, which means mass of contaminant
gas per volume of contaminated air. (Here's how to convert from ppm to mg/m^3
but it's not necessary for this module)
Here things get a little hairy. Laboratory animals have fur
over their skin, so it is necessary to shave a patch of skin.
Then some gauze is affixed (don't ask how) and the chemical is
placed under the gauze. The amount placed is expressed in ml or
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