EO. Ethylene Oxide.
It’s a naturally-occurring substance. (Actually, it’s a gas: ripening bananas release it and it tells the surrounding bananas to ripen too).
But it kills germs, and it is known to cause cancer.
So the swabs are sterilized with EO gas, and little trace amounts may be left on the swabs when they’re done. I don’t know the concentration.
Thanks.