Soap and the Removal of Diethylhexyl Phthalate From Hands: N-of-1 and Cross-Over Designs

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).

In the second N-of-1 trial, which was also performed over three days (22-24 May), the protocol using the same subject remained the same for all three days, except that the subject underwent intervention first then placebo. The water rinsing remained the same.

A randomized 2 × 2 crossover study was subsequently conducted between June and September 2013. The investigators assigned one group of 7 females and 7 males to perform handwashing with soap and water (intervention) and another group of 7 females and 7 males to perform handwashing with water only (placebo). Briefly, on the first day, the two groups washed their DEHP exposed hands using either soap and water or water only and then rinsed them in a PE bag with 200 mL water to collect DEHP. After 24 hrs, the experiment was crossed-over with the same two groups receiving the opposite treatments.