D12   Except in the circumstances specified in regulation 2(d)—   a   chronic bronchitis; or     b   emphysema; or     c   both,    where there is accompanying evidence of a forced expiratory volume in one second (measured from the position of maximum inspiration with the claimant making maximum effort) which is—   i   at least one litre below the mean value predicted in accordance with “Lung Function: Assessment and Application in Medicine” by J E Cotes, 5th Edition 1994 published at Oxford by Blackwell Scientific Publications Limited ( ISBN 0-632-03926-9) for a person of the claimant’s age, height and sex; or     ii   less than one litre.     ,Exposure to coal dust by reason of working underground in a coal mine for a period or periods amounting in aggregate to at least 20 years (whether before or after 5th July 1948) and any such period or periods shall include a period or periods of incapacity while engaged in such an occupation.,