about all we the buyers of the chemicals have is common sense. Seems to me if something can be absorbed in through the skin, then be transported throughout the body, then come out through the skin, and still have enough poison power left to kill either ticks or fleas, then it is a dander to the living tissues of the dog. A similar human experience would be people who smoke. Those chemicals are absorbed into the body at the lungs. And we all know all the cancers which have been linked to smoking chemicals. The dog is totally dependent on the owner. I choose not to subject my animals to those chemicals. 

thanks to all for responding