Human equivalent skin model tissue viability (%) exposed to silver nanoparticles of various sizes and coatings for 1 h. Data represents mean ± SD, N=3–4. The concentration of each nanoparticle sample was 1 mg/mL. DPBS (Dulbecco’s phosphate buffered saline) was the negative control. 5% SDS (sodium dodecyl sulfate) was the positive control. PVP (polyvinyl pyrrolidine) is a coating. Chemicals with a mean value below the dashed line at 50% viability were considered to be dermal irritants. ***, significantly different from negative control, p < 0.001. *, % viability for Ag 50 nm PVP nanoparticle significantly greater than that for Ag 50 nm PVP nanoparticle in Fig 3, p < 0.05.