We define a lone worker as someone who is working in circumstances where there is not someone else within calling distance who would be able to provide assistance if there were to be an accident.
It is important to understand the difference between lone working and out of hours working. You can be a lone worker in a laboratory during the normal working day if everyone else in the laboratory has gone for coffee. You can also be in a laboratory at 10pm in the evening and you would not be a lone worker if another lab worker were present providing that person would know what to do if there were an accident.
Supervisors are expected to determine whether particular tasks can be undertaken by lone workers or whether accompanied working is required. In general lone working should be limited to carrying out relatively simple low risk operations. In some circumstances, devices such as lone worker alarms can be provided to monitor lone workers and allow them to summon assistance.