Percy McDonald was diagnosed as suffering from mesothelioma in July 2012. Sadly, at the beginning of February 2014, just before the appeal in his case was due to be heard by this court, Mr McDonald died. His widow, Edna McDonald, has been substituted as respondent in the appeal. The period between diagnosis and death in Mr McDonald's case is entirely consistent with experience of this insidious disease. Survival for no more than a period of months after diagnosis is the almost invariable outcome.

Mesothelioma is a form of cancer that develops from cells of the mesothelium, the protective lining that covers many of the internal organs of the body. It usually affects the pleura, the outer lining of the lungs and the internal chest wall. It is most commonly caused by exposure to asbestos. Symptoms or signs of mesothelioma may not appear until 50 years (or more) after exposure.

Mr McDonald was employed by a firm known as Building Research Establishment, operated by the government. Between 1954 and March 1959 he attended Battersea power station in the course of his employment. This was for the purpose of collecting pulverised fuel ash. Between 1954 and January 1957 he was at the power station approximately twice a month. Between January 1957 and March 1959 he was there about twice every three months. The plant where the ash was collected did not contain asbestos. But Mr McDonald, while visiting the power station, went into other areas where asbestos dust was generated by lagging work. This happened particularly in the boiler house. It is suggested by the appellant that his visits to these areas took place because of curiosity on his part or because he was on friendly terms with workers employed there. At the times he was exposed to asbestos, Mr McDonald was, the appellant's counsel, Mr Nolan QC, suggested, a "sightseer" or an "interested visitor".

The lagging work involved mixing asbestos powder with water in large drums in order to make a paste. It also included the sawing of preformed asbestos sections and the stripping off of old asbestos lagging. On occasions Mr McDonald walked through dried asbestos paste. The trial judge found that his exposure to asbestos was "of a modest level on a limited number of occasions over a relatively short period of time … [and] … was not greater than those levels thought of in the 1950s and 1960s as being unlikely to pose any real risk to health".

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