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As well as their formula and structure, chemists describe a great many properties of a chemical. Some of these are also important to the toxicologist or industrial hygienist because they tell us how a chemical behaves in various situations.

According to its boiling point and melting point, a chemical will be a solid, liquid or gas at room temperature and pressure. This is known as the chemical's physical state, and this changes with temperature and pressure. Water, for example, changes from a solid to a liquid to a gas between 0 and 100 ^oC. A chemical's physical state, coupled with other properties such as its volatility, will influence the likelihood of its entry into the human body by various routes of exposure.

Several other properties influence the effect of chemicals:

Vapour density: the weight of a volume of a gaseous chemical compared with the weight of the same volume of air. If the vapour density is greater than 1, the gas is heavier than air and will tend to collect at floor level where it can be a fire or explosion hazard. In confined spaces, gases with vapour density less than 1 may replace air in the space, so that there is a danger of suffocation

Vapour pressure: describes how fast a solid or liquid evaporates, and increases with increasing temperature. A liquid which evaporates easily into the air is more likely to be inhaled

Flash point: describes the temperature at which a substance gives off enough vapour to form a mixture with air which can be ignited by a spark or flame

Autoignition temperature: the lowest temperature at which a substance burns without a spark or flame

Explosive or flammability limits: an upper and lower concentration of a gas or vapour in air, between which it may explode if ignited by a spark or flame

The properties of hundreds of chemicals are published in several books [Dangerous Properties of Industrial Materials (known as Sax), Merck Index, The Dictionary of Substances and their Effects (DOSE)], web sites, and on safety data sheets.

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