eNO will be measured using a Sievers 280i chemiluminescence analyser (Sievers Instruments, Boulder, CO, USA) sensitive to NO from 1 ppb to 200 ppm and with a resolution of 1 ppb and accuracy of 1 ppb designed for online recording. The eNO measurement will be performed according to ATS standards. Briefly, subjects should remain seated without nose clip during the procedure, with 2-minute rest between eNO measurements. The eNO will be measured online at an expiratory flow rate of 50 ml/sec, a dead space of 10 ml and 2-second plateau duration. Repeated exhalations are performed until three NO plateau values agree at the 10% level or two agree at the 5% level. The mean NO value will then be recorded.