In some situations a loss is known to have been caused by two or more factors operating together (A + B = loss). A case which illustrates this is Bonnington Castings v Wardlaw [1956] AC 613. The plaintiff claimed compensation in negligence from his employers for a respiratory disease. Whilst it was clear that the cause of the disease was exposure to dust at work, only part of this exposure was due to the defendant’s breach of duty. Some of the exposure was deemed to be a natural consequence of the work being carried out (and therefore non-tortious). Here the tortious and non-tortious factors operated cumulatively to produce the loss. The application of the ‘but for’ test, in this situation, is more complex than when dealing with independent causes. The plaintiff would not have suffered the loss but for the exposure to dust. It was impossible to tell, however, how much of the loss was caused by the tortious exposure to the dust and how much by the non-tortious exposure as they operated together. The House of Lords resolved the issue by introducing the material contribution test. If the defendant’s breach could be proved to have materially contributed to the plaintiff developing the disease then the defendant would be liable for all the loss.