Last year we had our bathroom gutted and re-fitted and had the boiler changed at the same time. One side effect of this was that we no longer have open tanks in our 'loft' but the whole system is sealed and operating at mains pressure. Since then my asthma has been a lot better and I have only had 2 chest infections.
I don't think this is a coincidence as there is some evidence (although I can't find it at the moment) linking showering with asthma due to a build up of bacteria in the shower head. This would line up with my experience as my asthma started after we moved to our current house and I started showering after a run instead of having a bath. Getting rid of the header tank has, as far as I can tell, removed a source of infection.