Risk, Human Health, and Toxicology - Environmental Science - page 337

have put into the environment during the past few
decades.
Chemists are able to detect increasingly small
amounts of potentially toxic chemicals in air, water,
and food. This is good news, but it can give the false
impression that dangers from toxic chemicals are in-
creasing. In reality, we may simply be uncovering lev-
els of chemicals that have been around for a long time.
Some people also have the mistaken idea that nat-
ural chemicals are safe and synthetic chemicals are
harmful. In fact, many synthetic chemicals are quite
safe if used as intended, and many natural chemicals
are deadly.

100

75

50

25

LD50

Science: Estimating the Toxicity
of a Chemical
Chemicals vary widely in their toxicity to humans
and other animals.
A poison or toxin is a chemical that adversely affects
the health of a living human or animal by causing in-
jury, illness, or death. One method for determining the
relative toxicities of various chemicals is to measure
their effects on test animals. For example, we can deter-
mine a chemical's lethal dose (LD). A chemical's median
lethal dose (LD50) means that the amount received in
that dose kills 50% of the animals (usually rats and
mice) in a test population within a 14-day period (Fig-
ure 14-9).
Chemicals vary widely in their toxicity (Table 14-1).
Some poisons can cause serious harm or death after a
single acute exposure at very low dosages. Others cause
such harm only at dosages so huge that it is nearly im-
possible to get enough into the body to cause injury or
death. Most chemicals fall between these two extremes.

0 2 4 6 8 10 12 14 16

Dose (hypothetical units)

Figure 14-9 Science: hypothetical dose-response curve show-
ing determination of the LD50, the dosage of a specific chemi-
cal that kills 50% of the animals in a test group. Toxicologists
use this method to compare the toxicities of different chemicals.

Science: Using Case Reports and
Epidemiological Studies to Estimate
Toxicity
We can estimate toxicity by using case reports about
the harmful effects of chemicals on human health and
by comparing the health of a group of people exposed
to a chemical with the health of a similar group not
exposed to the chemical.
Scientists use several methods to get information about
the harmful effects of chemicals on human health. For

Table 14-1 Toxicity Ratings and Average Lethal Doses for Humans

LD50 (milligrams per

Toxicity Rating kilogram of body weight)* Average Lethal Dose † Examples

Supertoxic Less than 0.01 Less than 1 drop Nerve gases, botulism toxin, mushroom toxins,

dioxin (TCDD)

Extremely toxic Less than 5 Less than 7 drops Potassium cyanide, heroin, atropine, parathion,

nicotine

Very toxic 5-50 7 drops to 1 teaspoon Mercury salts, morphine, codeine

Toxic 50-500 1 teaspoon to 1 ounce Lead salts, DDT, sodium hydroxide, sodium

fluoride, sulfuric acid, caffeine, carbon
tetrachloride

Moderately toxic 500-5,000 1 ounce to 1 pint Methyl (wood) alcohol, ether, phenobarbital,

amphetamines (speed), kerosene, aspirin

Slightly toxic 5,000-15,000 1 pint to 1 quart Ethyl alcohol, Lysol, soaps

Essentially nontoxic 15,000 or greater More than 1 quart Water, glycerin, table sugar

*Dosage that kills 50% of individuals exposed
† Amounts of substances in liquid form at room temperature that are lethal when given to a 70.4-kilogram (155-pound) human