Once it expires, it become more poisonous and that has not been studied that is why it is advise not to use once it expires. Poison are used to damage something or to diminish something. Even it is on a bad sides, manufacturing companies benefit from it. Even with poison, it has an expiry dates. Meaning, you can use it before the expiry dates and not use it after it, because some chemicals breakdown and you don't what will happen once you still continue to use it. Some things are not in our control, it could be more dangerous, it could explode before your eyes or you would not feel it but you already inhale some chemical particles breaking down, directly goes to lungs and will give you illness in the long run. Once a poison is expired, stop using it, it could be more dangerous.