Not all hazardous substances are assigned a WEL and unions are campaigning for a lower occupational exposure limit for substances including silica, for example (see page 81). Unions support the “precautionary principle” which means that if there is a reasonable possibility that a substance may cause harm, there should be a presumption that it will do so, and therefore it should be controlled. Exposure to a WEL is not acceptable when removal or substitution is an option.