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How poisonous - or toxic - a chemical is depends on the nature of the chemical itself, including its structure and properties. But its potential to cause harm to workers also depends on the dose and duration of exposure, as well as other factors.

The word 'toxic' comes from the Greek toxicon or arrow-poison. The word toxic is the root of many terms covered in the following sections, such as toxicity (how toxic a substance is) and toxicology (the study of poisons).

"Toxicology is the study of the harmful effects of chemicals on biological systems. It is a hybrid science built on advances in biochemistry, physiology, pathology, physical chemistry, pharmacology, and public health." (H. Frumkin in Levy and Wegman)

"It is probably safe to say that the mechanism of action of no chemical is understood in every detail. Toxicologists know a great deal about a few chemicals, a little about many, and next to nothing about most" (J.V. Rodricks p. 146).

The following sections describe how chemicals enter the body (routes of exposure), and the effects that they can have on various parts of the body. We will look at the time-scale over which these effects operate, so-called acute and chronic effects, and how certain combinations of chemicals are much more toxic together than would be predicted by adding up the damage they do on their own.

Finally, we will look at the many ways in which scientists study and measure toxicity, so that toxicity information in books, journals and safety data sheets can be interpreted more easily. Many of these issues are also illustrated by case studies.

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