Regulatory risk assessments of products should include friction-induced chemical release, according to a Chinese study of people using yoga mats.
With traditional risk assessment mainly covering emissions to the air or skin, friction-mediated chemical transfer is "a critical yet frequently overlooked" dermal exposure pathway, according to a team led by Eddy Zeng from the South China University of Technology.
Their study found that during exercise, sweaty clothing rubbing against mats continually picks up chemicals which can then pass to the skin, causing a "non-negligible exposure risk", according to an article in the journal Environmental Science & Technology.
The team tested mats for the release of four key additives:
- formamide;
- benzothiazole;
- p-bis(2-hydroxyisopropyl)benzene (pHPB); and
- bis(2-ethylhexyl)terephthalate (DEHT).
They used machines to rub the mats with fabrics, some of which were moistened with artificial human sweat and oil (sebum). They also tested chemical release from mats and fabrics in emission chambers.
They then ran tests in which participants completed 30-minute training sessions comprising four sets of shoulder-tap push-ups on a mat. The participants wore polyethylene gloves with fabric palm sections that were either dry or with added artificial sweat and sebum.
In all cases, friction significantly enhanced the chemical transfer from mats to fabrics, possibly due to surface disruption and elevated temperature.
In particular, sebum, rich in triglycerides and free fatty acids, increased the release of high molecular weight chemicals such as DEHT. Sweat was better at facilitating the transfer of low molecular weight, hydrophilic chemicals such as formamide.
Periods of high friction increased transfer levels. For example, during a shoulder-tap push-up, the friction-induced transfer rate of formamide from a mat was about 140 times the transfer rate for gas-phase emission.
The prolonged, high-humidity friction created "far more aggressive chemical transfer" than typical daily contact with plastics such as phone cases, they said.
The contribution of frictional transfer to dermal loading warrants further health risk assessment, the researchers said. As part of this, integrating friction-induced transfer data with established dermal sorption models is "essential", they said.
The authors recommended standardised friction testing be built into product safety assessments.
"Our research team will build on the published work, part of the ongoing efforts on evaluating human exposure to hazardous chemicals through various pathways," Professor Zeng said.