German consumer group Stiftung Warentest tested 12 brands and found the chemical in glue used to bind the bamboo powder together. All 12 tested positive after being filled with hot liquid and left for two hours a total of seven times. Four of the twelve cups contained high levels of melamine after the third filling, while three reached that point after seven fillings. Formaldehyde, a chemical known to cause cancer, was also detected.
Stiftung Warentest also noted that microwaving the cups destroyed the surface integrity of the material—increasing the rate at which melamine and formaldehyde were released.