To do this, Apple puts Apple Watch straps inside a jar containing artificial sweat it has made and simulates human body temperature. Its analysts then monitor the samples "to ensure that nickel and other allergens and irritants stay where they belong", the company said in its annual Environmental Responsibility Report.
The U.S. technology giant also tests other products that come in contact with the human body.
"We analyze materials that someone might put in their mouth—like an Apple Pencil you might chew on while considering the next line in your drawing—to make sure they're safe," the report said.
Apple did not say how it does this or whether it makes artificial saliva to test it.
"We also evaluate products after they've undergone extensive reliability testing to ensure that they remain safe for our customers, even after years of use and exposure to different environmental conditions," the company added.