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Sexual harassment aboard flights happens often, but the misconduct often goes unreported and unpunished, airline industry experts say.

The problem made headlines this week after Randi Zuckerberg, a media executive and the sister of Facebook founder Mark Zuckerberg, wrote a letter to Alaska Airlines complaining that a male passenger next to her made "explicit, lewd, and highly offensive sexual comments," and that the flight crew dismissed her concerns.

"When I brought it to the flight attendants' attention, their response was that this guy was a frequent Alaska Airlines traveler on this exact route, and they have had to talk to him about his behavior in the past, but oh well, don't take it personally, this guy just doesn't have a filter," Zuckerberg, who was flying from Los Angeles to Mazatlan, Mexico, wrote.

Randi Zuckerberg, sister of Facebook founder Mark Zuckerberg, participates in the BUILD Speaker Series Be Fierce tech panel on Oct. 17, 2017 in New York. Andy Kropa / Invision/AP

Alaska Airlines later said it was conducting an investigation and had temporarily suspended the passenger's travel privileges. Two of the airline's executive also reached out to Zuckerberg personally.

"We want our guests to feel safe," the airline's vice president of people, Andrea Schneider, said in a blog post on Thursday. "As a company, we have zero tolerance for any type of misconduct that creates an unsafe environment for our guests and our employees."

The incident comes amid a watershed moment on sexual harassment, with a spotlight being brought on abusive behavior in Hollywood, media, Congress and elsewhere.

Sara Nelson, the international president of the Association of Flight Attendants-CWA, called sexual harassment on jetliners an "epidemic" — and said she hoped the national attention would lead to changes in the airline industry, too.

In addition to travelers getting sexually harassed, flight attendants are subject to inappropriate behavior from passengers "every day," Nelson said, citing examples from her own 21-year career as a flight attendant.

Related: Airline launches probe after Randi Zuckerberg describes sex harasser passenger

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She recalled one time when she was pregnant and a passenger followed her throughout the cabin, telling her "the thing that turns him on more than anything else is pregnant women."

"Our planes are fuller than ever. The passenger-on-passenger assault is more likely in crowded planes."

Nelson said Alaska Airlines did the right thing in response to Zuckerberg by ultimately suspending the passenger's ability to travel, but added that airlines need to do more to stop sexual harassment in the skies.

"Up until a couple weeks ago, this was entirely acceptable behavior from all of society, not just the airline industry," she said. "We are rejoicing in that [change], but we don't have the tools and the training and the clear direction from our management [to stop it]."

The problem has become particularly challenging to deal with as airlines have reduced their flight attendant staffs to the bare minimum, while increasing their passenger loads, Nelson said.

"Our planes are fuller than ever," she said. "The passenger-on-passenger assault is more likely in crowded planes."

Related: Strangers on a Plane: Delta Passengers Arrested for Midflight Tryst

Paul Hudson, president of FlyersRights.org, said Zuckerberg is far from the only person who has been victimized by a fellow passenger — but because there is no running database of such incidents, it's hard to know how frequently they happen. In 2016, his group wrote a letter to President Barack Obama calling on the FAA to track sexual assaults on planes and to require that airlines institute guidelines for their staff on how to respond to midflight sexual misconduct.

"It's generally ignored or downplayed," Hudson said, adding that the blasé response Zuckerberg said she got from the flight attendants "seems to be typical."

Last year, Nelson's flight attendant group conducted a member survey that found that one out of five flight attendants had experienced a report of passenger-on-passenger sexual assault, but that when law enforcement was contacted, authorities met the plane only about half of the time.

That kind of response, combined with what she says is a dearth of training on how to handle sexual harassment claims, has paralyzed flight attendants when it comes to reporting sexual misconduct, Nelson said.

"I think what the culture has inspired is the response that Zuckerberg got," she added.

There are several factors that experts say make sexual harassment or assault more likely on planes, such as long fights, like overnight or international flights, and alcohol. Zuckerberg said that the man harassing her was drinking and that at one point he had three drinks on his tray table.

While alcohol is often free in first class, Nelson said it's not just at the front of the cabin where inappropriate behavior is seen: "It's the whole plane," she said.

According to the trade group Airlines for America, carriers "have well-defined processes and procedures in place" for onboard criminal activity.

"Employees receive extensive customer service training to ensure the safety and well-being of all our passengers and crew," a spokesman for the group, Vaughn Jennings, said via email.

But Nelson said airline CEOs "would be shocked to hear" what flight attendants experience.

"There are flights attendants on the front lines who are experiencing sexual harassment and varying degrees of assault every single day," she said. "If they are going to be able to have the proper tools to keep themselves and passengers safe, they have to have the backing of airline management."