Assessing Safety
The list of synthetic chemicals manufactured and used is very long, and it is vital that potential effects on human health are understood. Each substance must be assessed for risk.

When assessing the safety of a chemical, three questions need to be asked:
· How much exposure can people tolerate?
· What are the effects of being exposed?
· Will the effects wear off, or are changes permanent?

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

diagram: relative lethal doses for some solids

diagram: relative lethal doeses for some liquids

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