The China franchise of McDonald's and Burger King stated on Monday that there are no toxic PFAS substances in food wrappers in China after their overseas counterparts were questioned on the issue.

A report released by environmental advocacy groups based in the U.S. revealed on Thursday that the two food chains use toxic PFAS substances, namely human-made perfluoroalkyl and polyfluoroalkyl chemicals, in the packaging of some of their food.

According to the report, testing included 38 food packaging samples from six fast-food chains shows that nearly half of all samples tested positive for fluorine above the screening level, meaning they likely contain PFAS substances.

The topic became trending on China's social media. Both McDonald's and Burger King assured Chinese customers by saying on their official Weibo accounts that the food packaging used in China fully complies with national standards.

China prohibits the use of PFAS as additives in the food packaging material so that any qualified packaging does not contain them.

Business Insider reported that scientists have uncovered links between PFAS and cancer, liver damage, thyroid disease, and developmental issues. About 99 percent of Americans have the chemicals in their bloodstreams, which could stay in the body for life.