arnika[.]org/en/news/the-sound-of-contamination-all-analysed-headphones-on-the-central-european-market-found-to-contain-hormone-disrupting-chemicals
The_sound_of_contamination_Briefing_Paper_ToxFreeLife.pdf
arnika[.]org/en/publications/download/2128_f40ae4eb2e63e4dc3205035fb376d8e3
"Arnika is a Czech non-profit organization that has been uniting people striving for a better environment since 2001." Researchers for or at Arnika tested 81 brands/models of headphones for known toxic chemicals, bisphenols, finding it in 98% of samples. While you probably wouldn't be overly concerned, or surprised with something like a headphone cable being potentially toxic, you might not want that with the parts of the headsets that touch your skin. The linked PDF shows brands/models & results of their testing.
"These chemicals are not just additives; they may be migrating from the headphones into our body," said Karolina Brabcová, chemical expert at Arnika. "Daily use—especially during exercise when heat and sweat are present—accelerates this migration directly to the skin. Although there is no immediate health risk, long-term exposures, especially vulnerable groups like teenagers, are of great concern. There is no 'safe' level for endocrine disruptors that mimic our natural hormones."