Before smoking was banned on flights, the air in the fuselage was changed every ten minutes or so, as smoke was visible this could be seen to be done.
However since smoking was banned airlines have be decreasing the regularity of air replacement as a cost saving measure (air is stored on board in very expensive super cooled liquid form and heated before being placed in to the Ventilation system).
Some airline only change the air once every two hours, leading to a large ratio of methane and carbon dioxide building up in the air, leading to headaches, jet lag, air rage and nausea.
I think this might be the case here