The news broke recently that nail polish can harm your health (again!). This time, it’s triphenyl phosphate, a chemical used in nail polish to increase flexibility – it’s also used as a fire retardant. It’s a potential endocrine disruptor (“it could make you fat!”), and was found to be increased around seven-fold – or so it was reported. In particular, the Environmental Working Group (the people behind the “sunscreen is toxic” story every summer) has been the source for most of these articles.

News sites are notoriously bad at reporting scientific studies, so I went back to the source to read the full text of the original paper from Mendelsohn et al.. Unfortunately it’s behind a paywall, so I can’t link you to the full text, but I’ll provide a summary instead.

What’s TPHP and what are its effects?

Triphenyl phosphate (TPHP) is a chemical used in plastics to improve flexibility and as a flame retardant. A handful of animal studies and in vitro studies have suggested that it could be an endocrine disrupter which affects your hormones – in particular, it could potentially affect reproductive health and lipid metabolism (that’s where the “nail polish makes you fat” headline comes from). A couple of epidemiological studies in humans have also found correlations between TPHP and decreased sperm counts. However, all of this is far from conclusive evidence, and as you’ve probably gathered from being on the internet in the past 5 years, it’s almost like every chemical ever can be linked to endocrine disruption if you try hard enough.

In the body, it’s metabolised quickly to a related chemical, diphenyl phosphate (DPHP), and excreted in the urine.

What studies were performed?

The paper reports two main studies, performed with a nail polish containing 0.97% TPHP by weight.

“Cohort 1”

• 16 female students removed any nail polish they were wearing 72 hours before the experiment.
• They then collected one sample of urine (T1).
• 24 hours later, they painted their nails with two coats of the same clear polish.
• 2 – 6 hours later, they collected another urine sample (T2).
• 10-14 hours later, they collected a third urine sample (T3).

“Cohort 2”

• 10 females removed nail polish they were wearing 72 hours before the experiment.
• All urine was collected in the study, either for 24 hours (6 subjects) or 48 hours (4 subjects).
• The participants first collected urine for 24 hours, during which they didn’t use nail polish (Control Phase).
• The subjects were divided into two groups:
  • Half painted their nails with 2 coats of the polish (Nail Painting Phase).
  • The other half painted synthetic nails attached to latex gloves, then waited an hour before throwing out the gloves (Glove Painting Phase).
• After waiting at least 7 days, the groups swapped over (this is what’s known as a crossover study, where each person essentially acts as their own control).

The urine samples were analysed for DPHP content. To account for dilution effects (e.g. if one person drinks a lot of water), the urine samples were corrected for specific gravity (SG).

Discussion

I don’t have a comprehensive background in designing biological studies or in statistical analysis, but nothing really rings any alarm bells for me (but I’m happy to be corrected if anyone spots any shifty manipulation!).

Does wearing nail polish increase the amount of TPHP in your body significantly?

In terms of concentration, yes, it appears so, in both experiments – to quote the study, “nail polish is a likely source of exposure to TPHP and that use may result in exposures substantially greater than background levels.” (Background means in the absence of nail polish.) It’s worth noting that you have to wear the nail polish, not just inhale it – Glove Painting didn’t cause the same increase. As well as the hard numbers, it’s pretty apparent, visually, from the graphs of urine DPHP from Cohort 2 (which I’ve painstakingly re-graphed to hopefully avoid any awkward debates about Fair Use).

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