Sandie,
My brother uses them and according to him, they seem to be safe. Bottom line, I'm not sure how well they work.
I haven't used them because regardless of what the claims say, I don't want it to annoy my pets.
In other words, it says it will deter rodents, but it "won't harm any pets".
What if you had a pet hamster, or guinea pig? Not that much different than a target pest. How do they differentiate?
We have two dogs and a parrot and I feel animals may be sensitive to any noise and vibration put out by the transmitters. Not certain about that. JMHO.
Trace