(CNN) -- United Airlines is changing its policies on emotional support animals on its flights.
As of Monday, support animals won't be allowed on flights longer than eight hours.
United said it's seeing more on-board animal-related incidents, adding many are not used to spending so much time on a place.
In addition, United won't allow kittens or puppies less than four months old as support animals, in-cabin pets or service animals. The airline said animals that young typically haven't had all the necessary vaccinations.
United joins Delta and Spirit in changing policies on emotional support animals. Delta and Spirit now require at least 48 hours notice to bring service animals on flights, as well as extra documentation.