As someone who vapes and has tried out some different products. Atleast here in Europe Juul was the only "pre-filled" cartridges product which contains nicotine

As someone who vapes and has tried out some different products. Atleast here in Europe Juul was the only "pre-filled" cartridges product which contains nicotine salt instead of freebase nicotine. Nicotine salt is much easier to vape and feels less harsh. I.e I can easily vape 50mg/ml nicotine salt, while above 10mg/ml freebase gets uncomfortable for my throat. That might have been the reason for your dad coughing. But I still agree, besides the salt/freebase aspect there can be different aspects of the ingrefients, each affecting health differently.