*Participants were assigned to National Statistics Socio-Economic analytic classes (NS-SEC) provided by the Office of National Statistics on the basis of their occupations. The seven classes used were: 1. higher managerial, administrative and professional occupations; 2. lower managerial, administrative and professional occupations; 3. intermediate occupations; 4. small employers and own account workers; 5. lower supervisory and technical occupations; 6. semi-routine occupations; 7. routine occupations. Further details are provided in the online supplemental file 1.

†mMRC (modified Medical Research Council) dyspnoea scale. 0. not troubled with breathlessness except with strenuous exercise; 1. troubled by shortness of breath when hurrying on the level or walking up a slight hill; 2. walks slower than people of the same age on the level because of breathlessness or has to stop for breath when walking at own pace on the level; 3. stops for breath after walking about 100 yards or after a few minutes on the level; 4. too breathless to leave the house or breathless when dressing or undressing.19