Calpol vapour plug in

Ej has a bit of a cold and the main problem is a snuffly nose. I got a calpol plug in ages ago and wondering if anyone has used one before? It says on the box not to use if you have asthma, anyone know why? My oh has asthma and he can sleep in another room but don't know if Ej will develop it so am a bit worried. Anyone have any alternatives?
Thanks

Percy constantly snotty (DS small sinuses apparently)
I've found saline spray the most useful thing. Even when no actual shots, it's antiinflammatiry, especially the hypertonic one.
We also use snuffle babes on a damp cloth on the radiator, which is safe from birth. It can go in a vaporiser dummy too, but percy hates dummies so no good for us but have friends who swear by it.
Hope EJ is feeling better soon