an entertaining read on the BBC web site :Viewpoint: Why do people hate the word 'chemicals'?
but off the top of my head i can see several reasons
(1) the use of poisonous gases in WWI
(2) the use of pesticides in agriculture
(3) the enthusiasm of the food industry for making anything look and taste whatever they want it to look and taste like
i'm sure there's others - the inappropriate use of chemicals is so widespread that "chemical" has become the by-word for something that is bad for your health
"Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
"Someone is WRONG on the internet