Three studies, consisting of two N-of-1 trials and one crossover study, were used to compare the effect of handwashing with soap and water (intervention) and handwashing with water only (placebo) on phthalate removal from hands. The soap was plain soap, consisting of vegetable oil and sodium hydroxide. First, the investigators recruited one healthy female aged 25 yrs to participate in two N-of-1 trials. In each trial, her hands would be exposed to DEHP and then be treated with either soap intervention or placebo for three times over the course of three days. The first trial started on 15 May 2013. On day one, after the subject prewashed her hands, she allowed her hands to be exposed to a designated amount of DEHP. She then washed them with water only and rinsed them sequentially in three separate polyethylene (PE) bags, each containing 200 mL water to collect DEHP residue. One hour later, the same protocol was followed, except the exposed hands were treated with soap and water instead of water only (Figure 1a). The water rinsing in three separate bags remained the same. The pairs of studies were repeated on two subsequent days (16 and 17 May).