You need to understand how epidemiology and statistics work. For example, it is impossible to "prove" that a specific disease (i.e., lung cancer) affecting a specific person was caused by a specific insult (i.e., tobacco smoke). However, it is statistically highly probably that people who smoke tobacco have a vastly elevated rate of lung cancer. Similarly, we know that this class of chemicals have particular effects and it is highly probable that those effects will manifest in human populations exposed to these chemicals at a higher rate than those populations not so exposed.