Airport travel is bound to be stressful this week, and United Airlines is bringing in four-legged backup to help with the problem.

The airline will have more than 230 trained comfort dogs, mostly golden retrievers, to seven airports across the country to greet travelers.

"Research shows that just petting a dog releases oxytocin, the hormone associated with bonding and affection, and also decreases levels of stress, helps us breathe easier and lowers our blood pressure," said veterinary consultant Dr. Walter Woolf in a press release.

The dogs will be scattered throughout the airports. But many will be located at the gates to interact with travelers after they've made it through security.

The United Paws program runs December 21 through December 23 at Chicago, Cleveland, Denver, Houston, Los Angeles, Newark and Washington/Dulles airports.

United (UAL) launched the program last holiday season.

But airplane travel is expected to be heavier this holiday season: 38 million passengers are expected to fly from December 18 through January 3, according to Airlines for America.