The Air Quality Index is used to measure pollution in urban centres. When the Manchester clubs were in Beijing it hovered around the 200 mark. At 100, the recommendation is that people with heart or breathing problems reduce outdoor exercise. A reading of 201 is considered heavy pollution; six hours in smog of that thickness is the rough equivalent of smoking more than a pack of cigarettes.
As managers who expect their players to be in peak form, one imagines Pep Guardiola and Jose Mourinho would not approve of extreme nicotine inhalation. So why they want to be hanging around Beijing pre-season is anybody's guess.