As well as controlling exposure to substances hazardous to health, you need to be aware that legal limits have been set on the amounts of many of the substances that can be present in workplace air. These are known as workplace exposure limits (WELs). They are listed in HSE’s booklet EH40 Workplace exposure limits.
If the substance is known to cause cancer or asthma (check the label/safety data sheet), you must control exposure to as far below the level as is reasonably practicable This means balancing the level of risk against the measures needed to control the real risk in terms of money, time or trouble. However, you do not need to take action if it would be grossly disproportionate to the level of risk. .