Southwest Airlines will bring its policy for large passengers to AirTran Airways starting in March.

The new policy will require those passengers -- whom Southwest delicately calls "customers of size" -- to buy a second seat if they are flying in AirTran's coach section.

As of March 1, AirTran will require the purchase of more than one seat for a passenger who, "in the carrier's sole discretion," can't sit in just one seat with the armrest lowered.

Those who dislike the policy say it's not just about money.

Peggy Howell, a spokeswoman for the National Association to Advance Fat Acceptance, said some passengers end up "embarrassed and humiliated" when they're asked to buy a second seat at the airport.

"I’m sorry to hear that they’re now going to be passing on their bad policy to another airline," Howell said. "We are paying customers."

Before this, AirTran has not had a stated "customers of size" policy, but AirTran employees at the gate typically offer to sell a second seat or an upgrade for a fee, Southwest said.

It is an AirTran policy change that's coming as Southwest integrates the airline. Southwest estimates the policy, which it has had in place for about 30 years, affects "far less" than one-half of 1 percent of its customers.

According to SeatGuru.com, AirTran's seats are 18 inches wide in coach class and 22 inches wide in business class.

Southwest does not have business class and has open seating.

"Customers of size," according to Southwest, are those "who encroach upon any part of the neighboring seat[s]. ... The armrest is considered to be the definitive boundary between seats."

Atlanta-based Delta Air Lines does not have a specific written policy for larger customers.

"Our flight crews and customer service agents work as a team to ensure all our customers, regardless of their circumstances, are comfortable when flying," Delta spokeswoman Susan Elliott said.

She said if a passenger raises an issue about a customer of size at departure time and the flight isn't full, the passenger will be moved to another seat. If the flight is full, "we work with both passengers to re-accommodate one of them on another flight based on availability."

The Southwest policy on its website says if the flight does not oversell, passengers may contact Southwest for a refund of the cost of additional seating after travel. AirTran will have the same refund policy.

The frequently asked questions section of Southwest's website dedicated to the "customers of size" issue notes that Southwest "became more vigilant regarding the additional purchase when we began seeing an increase in the number of valid complaints from passengers who traveled without full access to their seat because a large customer infringed upon the adjacent seating space."

"We could no longer ignore complaints from customers who traveled without full access to their seat due to encroachment by a large seatmate whose body extended into the neighboring seat. These customers had uncomfortable [and sometimes painful] travel experiences," Southwest said.

Howell said one issue is that there are no clearly defined parameters for enforcement of the policy, and that "they leave it up to the judgment of their staff."

The Southwest policy notes that having broad shoulders would not necessarily require the purchase of a second seat. "The upper body can be adjusted, but the portion of the body in the actual seating and armrest area doesn't have this flexibility."

Separately, AirTran will maintain its fees for first and second checked bags, but excess baggage fees on AirTran will increase from $50 to $110 per item each way effective April 10, among other changes. AirTran will continue flying under its own name as Southwest begins flying to Atlanta and integrating AirTran operations into its own over the next couple of years.