When you board a commercial airliner, you know there are certain risks that go with this mode of transportation.

You may be placed next to someone with a squalling infant, or behind a traveler who insists on reclining, or alongside an overly talkative stranger — hopefully wearing a tightfitting mask. The plane may sit on the tarmac for hours waiting for takeoff or hit stomach-churning turbulence once it does. Most air travelers learn take these possible outcomes in stride.

But miniature horses? Turtles? Monkeys? Potbellied pigs? Humans have brought (or tried to bring) all sorts of animals on flights, claiming they are needed for psychological purposes. Between 2016 and 2018 alone, the number of "emotional support animals" on flights rose by a whopping 81%.

Airlines have tried to get control. Delta Air Lines drew up a voluminous list of critters that are not allowed, which — you will be relieved to know — include goats, pigs, spiders, rats, snakes and lizards. A United Airlines customer was refused when she tried to bring aboard a peacock.

But plenty of beasts are allowed on board, and the problems associated with them have multiplied. The Association of Flight Attendants attests that these companions "have been known to bite passengers and Flight Attendants, urinate, defecate, cause allergic reactions and encroach on the space and comfort zone of other passengers who have purchased tickets." One flight attendant suffered facial cuts from a pit bull.

Emotional support animals, it should be emphasized, are distinct from service animals — which are usually canines and are thoroughly trained to perform crucial tasks for people with disabilities, such as guiding blind people around obstacles and alerting deaf people to important sounds, such as smoke alarms or doorbells.

The Americans with Disabilities Act specifically mandates that these dogs be permitted to accompany their owners in places "where members of the public are allowed to go." Emotional support animals are supposed to provide comfort and reassurance to people who need it, a function the ADA doesn't recognize as essential.

Earlier this month, though, the U.S. Department of Transportation issued new rules, noting that the proliferation of these creatures has led to "incidents of misbehavior by emotional support animals" and "eroded the public trust in legitimate service animals."

Going forward, airlines will be obligated to let passengers bring their service dogs — and only dogs — without paying a fee. Those include "psychiatric service animals."

You want to bring Tabby the cat in a carrier? Fine, if she will fit under a seat, though it will cost you. All non-service animals may be classified as pets, which their owners may be able to ship as cargo for a fee.

But for the most part, the new rule will help ensure the great majority of travelers a safer, more sensible flying experience. As Sara Nelson, president of the flight attendants union, said in January when DOT announced it would revise the rules, "The days of Noah's Ark in the air are hopefully coming to an end."

FROM AN EDITORIAL IN THE CHICAGO TRIBUNE