Inclusion Criteria:  Living in Beijing for more than one consecutive years; College students between 18-28 years;  Exclusion Criteria:  Current or past smokers; Body mass index (BMI) over 28.0 kg/m2. Alcohol addict; Having thoracic surgery history; History of diagnosed chronic disease; Hepatitis B / hepatitis C virus patient / carrier; Unwilling or unable to provide informed consent or cooperate with all research related procedures.