Back now with the growing concern facing America's airlines and passengers. Service and emotional support animals on crowded flights, sometimes causing problems. Now, a major airline is changing its rules as three other airlines are reviewing tighter restrictions on which pets can come aboard. Reporter: Your flying experience may soon start to change. More and more airlines now looking to tighten the leash when it comes to bringing animals into the main cabin. Delta airlines leading the way. By tremendous tending their ordinary animals are emotional support animals. The carrier reports an 84% increase in unsanitary or dangerous pet incidents since 2016, including cases of biting. One passenger even mauled by a 70-pound dog aboard a flight. American airlines, united and jetblue saying they'll review their policies. Three years ago we exposed this problem on "20/20." First, showing companies that provide leashes, vests and I.D.S and certificates for emotional support animals without first verifying any mental health issue. Then, how some airlines don't ask to see the documents. I boarded a flight with no pet carrier. Oh, my goodness. Reporter: Later, getting on another flight with a bunny named Leo. One company even provided us with documentation for an albino African phedgehog. The new regulations are aimed at prohibiting this from happening in the future. A letter prepared and signed by a doctor or licensed mental health professional and a signed document confirming the animal can behave. The airline feels these new steps will improve the flying experience for all passengers including those who truly need service and emotional animals. Traveling with a service or support animal will be free as required by law. As for those companies we contacted to obtain those certificates at the time of our investigation, they either didn't comment or said the document was a valuable supplement to the doctor's letters.