Some of the sensors make contact with a person’s skin to measure electrical signals. It is very important that precautions have been taken in the design to protect the person who is connected to the instrument from electrical shock. The FDA (Food and Drug Administration) regulates professional medical devices and sets rules for safety. If they registered with the FDA then the manufacturer would have to follow safety rules. They would be subject to inspections by the FDA which would check to make sure that the company was doing what is required to keep the people using their biofeedback equipment safe from injury. You must make sure that the instrument you are considering buying is registered with the FDA.