2.4.2. Clinical visits were conducted at the district's community health service center
All participants arrived at the center before 8 a.m. on the day of their interview. At each visit, a standard physical examination was conducted by physicians and trained nurses. All results were recorded, and all participants received a copy of the report at the end of examinations. The physical examination included the follow- ing: (1) basic measurements: height (cm), weight (kg), body temperature (1C) and blood pressure (mmHg). For the blood pressure measurement, a standard protocol was followed by the research staff throughout the three study periods. All participants were required to sit for at least five minutes to rest before the measurements. During the measurements, they were seated in a chair with their backs being supported, feet flat on the floor and their arms being supported at the heart level. (2) pulmonary function: respiration rate (breath/min) and peak expiratory flow (PEF). Peak expiratory flow was measured by a pre-calibrated peak flow meter. To ensure that the maneuvers were performed correctly, a trained nurse supervised each participant. In total, three correct maneuvers were per- formed. For each maneuver, the subject waited for at least 10s between blows. If the two largest values differed by greater than 40 L/min, participants performed two extra maneuvers. The highest value of the three maneuvers was recorded as the peak expiratory flow for this person.