A common view is that a toxic chemical is a substance that only needs to be taken in a very small quantity to cause death, for example arsenic. A much broader definition used by the US Environmental Protection Agency states that a toxic substance is any chemical or mixture that may be harmful to the environment and to human health if inhaled, swallowed or absorbed through the skin.
A key concept is that such a harmful effect is only produced if the substance is present in the body in a sufficient quantity or concentration; the higher the “toxicity”, the smaller the amount that is required. For example, a surprising and very rare case is where drinking large, excessive amounts of water has led to death as a result of extreme dilution of the electrolyte in the human body.
The literature on safety and toxicity uses terms which often appear to be interchangeable or the same word is used in more than one sense. Hence a chemical may be described as toxic, harmful, or hazardous. The terms “toxic”, “non-toxic” have perhaps suffered from overuse; “harm”, “toxicity” and “risk” are less ambiguous terms. The introduction of a new Globally Harmonised System of Classification and Labelling of Chemicals , “GHS” which encompasses physical, health and environmental “intrinsic” hazards is currently underway. Up to date information, available from national environmental or government health and safety at work organisations should be consulted for lists, classifications, regulations and advice on working with harmful materials and their safe disposal.
The classification of a substance allows a subsequent “risk assessment”. Generally risk is proportional to the severity of harm multiplied by the likelihood of occurrence. So, in this context, the toxicity is a characteristic of a particular substance while the associated risk is related to the manner and quantity in which the substance is present in relation to the environment or human contact. A material may have a high toxicity but be a low risk if it is handled properly and not absorbed by the body: an open container of acid is a much greater risk than a closed container of the same material.
The oral toxicity of a chemical is generally assessed from experiments on laboratory animals and the results extrapolated to humans with a suitable safety factor included. A commonly used expression is “LD50” – lethal dose – the amount of a substance which is sufficient to kill 50% of the test animal population in a given trial within a specified time, often expressed as milligrams of chemical per kilogramme of body weight of the animal. A related parameter is LC50 – lethal concentration – which is the lethal concentration of the chemical in air when inhaled by the animal –expressed as milligrams per metre3 – or for environmental studies may represent the concentration in water – effects on aquatic life.
Text © A Crujera and R M Perkin