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.
You frighten me terribly. I would like to go home now. Here, take this tiny ad:
A rocket mass heater is the most sustainable way to heat a conventional home