Smoking Cessation for Fathers of Sick Children

Inclusion criteria for parents： * Parents (mainly fathers) aged ≥18 years and smoke at least one cigarette (including alternative tobacco products) daily in the past seven days * Living with a child aged \<18 years who attended the clinic or were admitted to the pediatric ward * Hong Kong residents able to read and communicate in Cantonese or Putonghua * Owned a smartphone and can use an instant messaging app (e.g., WhatsApp, Facebook) Inclusion criteria for children * Children aged \<18 years who attended the clinic or were admitted to the pediatric ward * Lives with at least one or more parents who smoke at least one cigarette (including alternative tobacco products) daily in the past seven days * Able to provide biochemical samples (e.g. saliva) for purpose of research Exclusion criteria for parents * Smokers having a history of psychiatric/psychological disease or currently on regular psychotropic medications * Smokers using smoking cessation medications or other smoking cessation services or projects Exclusion criteria for children * Children who already have serious health diseases (e.g.chronic disease, genetic disease) * Children who are participating in other clinical trials that may affect the results of this study * Children who live in certain environments, such as those living in highly contaminated areas or areas with other potential disease-causing factors