The hazard presented by a chemical is a combination of these three factors, which we can call potency, severity and reversibility. As an example, a small amount of lemon juice splashed in the eye produces a painful sensation - the juice has a high potency. Thankfully the actual tissue damage is minimal, and a few drops of saline tears are enough to reverse the effects. The same cannot be said for concentrated sulphuric acid.

The numbers game

What can you do to find out how potentially dangerous a chemical is? The answer is you carry out tests and establish the levels at which exposure stops being "safe". These will then form the basis of regulations with which companies must comply to protect their employees and the public. Broadly speaking there are two main areas of interest:
· What level of exposure is safe on a regular, day-to-day basis?
· How much must a person ingest or inhale for serious or lethal effects?

The OEL is the Occupational Exposure Limit, and is the maximum level employees may be exposed to on a daily basis.

The LDLo is the Lowest Lethal Dose, which is the lowest published dose ever to have resulted in death. The actual lethal dose will depend on body mass, so it is usually quoted in mg per kg of body mass.

There are many other measurements used. For a more detailed treatment of this topic see the Oxford University Chemistry pages