The following article outlines the possible causes for those first starting off running into this type of problem. But what about someone who has been vaping for some time? PG is known to dehydrate things a bit as well as the combination of particular hardwares and eliquids.
http://www.ecigarettedirect.co.uk/ashtray-blog/2015/07/vape-e-cig-cough.html
http://www.ecigarettedirect.co.uk/ashtray-blog/2015/07/vape-e-cig-cough.html
http://www.ecigarettedirect.co.uk/ashtray-blog/2015/07/vape-e-cig-cough.html
http://www.ecigarettedirect.co.uk/ashtray-blog/2015/07/vape-e-cig-cough.html