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As well as the dose of a chemical, its toxicity also depends on how long exposure lasts, the duration of exposure. This duration is one way that the toxicity of a chemical can be categorised. Single exposures are referred to as acute exposure, and repeated exposure over a longer time as chronic exposure. Duration of exposure should be reported with toxicity data in safety data sheets, etc. The other way toxicity is categorised is according to the chemical's target, the organ or body system which it damages.

The terms 'acute' and 'chronic' are also used to describe how long it takes for the effect of a chemical to occur, and it is important to be aware of these two uses. An acute effect is one which happens immediately on exposure, whereas a chronic effect does not. It may take years to appear. The time between exposure and the onset of disease is referred to as the latency period. Because there is such a long latency between certain chemical exposures and diseases such as cancer, it is often difficult to link them to occupational exposures. This is one reason why workers should insist that the chemicals they work with are recorded on their medical records.

These two uses of the terms 'acute' and 'chronic' are not connected in the sense that an acute exposure only leads to an acute effect. In fact, an acute exposure can lead to either an acute or a chronic effect.

When talking about toxicity, the easiest way to divide chemicals is by the organ or system they damage. These target organs or systems are often referred to in safety data sheets and toxicology books. The commonest are: the lungs, the skin, the gut, the liver, the kidneys, the nervous system, the blood, the cardiovascular system, the immune system, and the reproductive system. There are even chemicals which can affect hearing.

Chemicals causing liver damage are sometimes called 'hepatotoxins', those which damage the kidneys 'renal toxins', and those harming the nervous system 'neurotoxins'. Chemicals that cause cancer, although they may affect either one or several organs, are lumped together and described as 'carcinogens'. Those that cause birth defects are called 'teratogens'.

"A toxin is generally understood to be a substance that is harmful to biological systems, but within this simple concept lies a great deal of variability. A substance that is harmful at a high dose may be innocuous or even essential at a lower dose. A toxin may damage a specific body system, or it may exert a general effect on an organism. A substance that is toxic to one species may not be toxic to another because of different metabolic pathways or protective mechanisms. And the biologic damage may be temporary, permanent over the organism's lifetime, or expressed over subsequent generations" (H. Frumkin in Levy and Wegman).

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