It has been suggested in previous studies that there could be a link between impaired lung function and a negative effect on the thinking processes, but it is not clear what the mechanism for that might be.

BBC: NEWS | Health | Smoking 'is bad for your brain'

The researchers suggest a "small but significant" negative effect of 4% linked to the combined effects of smoking and impaired lung function - itself linked to smoking.

BBC: NEWS | Health | Smoking 'is bad for your brain'