Recently, it seems like airlines are picking on puppies.

Both Delta and United have banned cats and dogs of less than 4 months of age from flying in the cabin, regardless of whether the pet is an emotional support animal or not.

Puppy owners may be bummed by these new restrictions, but Nicole Ellis, a certified professional dog trainer with Rover.com, says there is a good reason for these rules: airplane cabins simply are not a safe place for young pets.

Airplane cleanliness is one of the top reasons for concern.

“Airplanes aren’t the cleanest of places and hence can easily carry many germs and illnesses that can easily be picked up by our pets such as Parvo, which can be deadly and easily transmitted,” Ellis tells PEOPLE.

Young puppies who have not had their vaccinations are especially at risk. If you do find yourself flying with a dog less than 14 weeks of age, Ellis recommends keeping the pup in a carrier at all times. She uses the

, since it is the longest carrier that meets airline requirements.

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Also because of their age, puppies can have trouble controlling their bladders on flights. Ellis says the muscles that control a puppy’s bladder don’t mature until a puppy is 6 months of age. Younger dogs can only control their bladders for a few hours at a time, and that’s without a “flight full of new stimulation,” Ellis adds.

It’s best to keep puppies off of flights, especially longer trips, to prevent them from having an uncomfortable accident that is out of their control.

Whatever your pet’s age, Ellis reminds humans that the key to keeping your pet safe during air travel is acting as your “pet’s biggest advocate,” and keeping them in their carrier as much as possible.