MUC5AC and respiratory system disorder: In the specific field of respiratory diseases, e-cigs in particular can express harmful effects related to nicotine and/or flavoured e-liquids which are expressed through mechanisms of cytotoxicity, oxidative stress, inflammation, airway hyper-reactivity, airway remodelling, mucin production, apoptosis and emphysematous changes (evidences 3 and 4), as well as to increased susceptibility to viral infection through an increase in lung ACE2 expression, a mechanism that we have well known with SARS-CoV-2 infection (evidence 5).