Join thousands of others in getting the stories that matter to you sent straight to your inbox.Burned gums and dental damage can be caused by some teeth-whitening kits available on online marketplaces.The warning comes from Which? after it found 21 out of 36 kits tested exceeded the legal amount of hydrogen peroxide.One of the kits was found to contain a staggering 300 times too much of the chemical.The consumer group flagged the unsafe products and they were taken off sale.Which? is concerned