You could also ask why lead was used for gasoline when it was known that lead was toxic.<p>The reason is simple. People knew it was toxic and caused brain damage (&#x27;madness&#x27; in Roman terms) at high levels of exposure. What they didn&#x27;t realise is that very small levels of exposure could cause an internal buildup over time and cause steady mental degradation. If someone is exposed to massive heavy metal poisoning and immediately goes crazy, the correlation is obvious. If an entire generation slowly gets more violent and crime rates go up, then it&#x27;s much, much harder to spot the issue because everyone is changing at the same time.