I wonder if there is some general irritant in the environment that they are all exposed to, to some degree or another? Some cleaning material...incense...candles...room fresheners...cigarette smoke...or outside pollutant that we can't avoid letting into our homes....?


It might be a bit pricey to get one that really works, but maybe you could get an air purifier for the room(s) they are in? See if that helps to clear things up? And of course, get rid of any possible source that you have control over, such as candles or whatnot.