Development of transepithelial electrical resistance (TEER) in cells grown at (ALI). Different primary cells and cell lines were cultured at ALI over 21–28 days. TEER was measured every 2-3 days. Results from three separate experiments are shown for each cell line/donor, six replicates per experiment. HBEC D1 is Donor 1 and HBEC D2 is Donor 2. Error bars show standard deviation.