Cotinine is a specific metabolite of nicotine and a gold standard marker of secondhand smoke (SHS) and thirdhand smoke (THS) in nonsmokers. In comparison to nicotine, cotinine is much easier to measure at low levels than nicotine, and more laboratories carry out cotinine analyses. Although cotinine is an analyte detected by many commercial laboratories in biological and environmental samples including surface wipes, there are no assessments on the cotinine levels recovered from human dermis using handwipes. As demonstrated in our previous silicone wristband study, in which the cotinine/nicotine ratio varied significantly between exposure levels, measuring nicotine in handwipes could potentially be used to differentiate exposure to SHS and THS. Also, there is a need to better understand the nicotine-cotinine metabolism and exposure ratios in relation to absorption-desorption of chemicals present in THS among young children. Therefore, we propose to investigate cotinine in handwipes as practical and accurate exposure assessment tool by levering samples that have been previously collected from children known to be exposed to SHS/THS. Our aims are to: 1) evaluate cotinine levels from handwipes in relation to children exposure to THS and SHS by using previously collected samples from 90 children (THS exposed = 30; SHS exposed = 30; control = 30); 2) assess the nicotine/cotinine exposure ratios from collected samples and compare cotinine in handwipes with salivary cotinine and handwipe nicotine levels; and 3) determine exposure cotinine cutoff values from handwipes that can be utilized in future studies and examine socio-demographic factors related to THS/SHS exposure.