Lead poisoning can be much more obvious than that, and we've known for centuries that lead was poisonous.

The Romans knew it, and they still used it in their pipes and even in their drinks.

I'd guess that that is actually also why it is taking us so long to fix the problems caused by lead existing in the environment where it can affect humans. We know now that tetraethyllead in gasoline caused a great deal of harm and we've quantified it pretty well, for example, but people knew how dangerous that chemical was much longer.

People who demand a big obvious poisoning, as you said, before accepting the reduction of obviously dangerous substances in the environment is needed are the worst.