2.2. Eligibility Criteria
Inclusion criteria included: (1) at least one smoking parent living in the household who smoked at least 10 cigarettes per week (Study 1) or no parents who smoked in the household (Study 2); (2) having a child aged up to 8 years; (3) the parent(s) were willing to provide a hair sample for research purposes; and (4) children had a sufficient hair sample for the laboratory analysis. 