Figure 1: Viability of human middle ear epithelial cells (HMEECs) following formaldehyde (FA) exposure. (a) Nonviable cells take up trypan blue and their numbers increase to greater extents in cultures exposed to FA for 24 h versus 4 h and to 2 mM versus 1 mM FA (×200, trypan blue assay without green filter). (b) The cell counting kit- (CCK-) 8 assay revealed that cell viability decreased after cells were exposed to 1.5 mM FA for 4 h and 0.2 mM FA for 24 h. Error bars represent standard deviation (SD) of the mean.