Environment Bill

I am most grateful, and I thank my noble friend for his answer. He may have said this in his reply, but I ask again because I could not pick it up. When authorisations are given for substances, is the mixture—the toxic cocktails, if you like—actually checked? I am no scientist, but I do know that when you mix certain chemicals together, they have a different effect from what they have when they are on their own. I am just wondering whether that is checked to make sure that the effects are not harmful.