Harmful to the soil or garden is a little hard to understand from a permaculture perspective. Poisonous to humans is quite understandable, and there are many. A few are even deadly. Harmful to the ecology doesn't make sense when you think about the overall balance of nature, as Rachel Carson would say.
Oh the stink of it! Smell my tiny ad!
Grow Half Your Food - Free Viewing March 20 - From The Grow Network