A statutory odour nuisance is something which is offensive and prolonged and which significantly interferes with the enjoyment and use of the affected property.
Many things can affect whether an odour would be considered a statutory nuisance including how long it is a problem for, the type of smell and its effects and the character of the area. For example, in the countryside it is reasonable to expect odour from farming activities.
Odour is not something which can be measured meaning judgement of whether or not odour constitutes a statutory nuisance is subjective and has to be witnessed. This can be problematic if the occurrence of the odour is unpredictable and only evident for short periods of time.