It has to give out wireless 'radiations' in order to command it to 'stimulate' the dog. I would think it is not physically harmful, I have read the description at Dr. Fosters and Smith which claims it is perfectly harmless.
Whether this type of negative reinforcement is a proper way to train a dog is something else. I do not feel qualified to comment on this but my first thoughts are that positive reinforcement is a better way to train a dog... they are usually very eager to please their owner. However, if you have behaviour that is extremely obnoxious and need to deter the dog. This product should work.
If you are thinking about buying it I found it at our price comparison [
petsupply.animal-world.com] and found some better prices.