This is true in principle for many heavy workers in forestry,haulage and so on who can reach levels of 22000-30000 kJ for a few days without ill effects.This limit of 20000 kJ per working day is therefore a yearly average which applies only to a heavy worker in a good state of health. Clearly there are individual variations both above and below this mean value,depending particularly upon such factors as constitution,level of training,age and gender.