Shawn Kathleen became so annoyed by rude passengers while working as a flight attendant in the U.S. that she started writing about them in a blog.

Many people didn't believe her stories — until she also started posting photographic evidence.

She was fired in 2013 — she believes because her employer discovered she was behind the blog.

Former flight attendant Shawn Kathleen created the site to show how not to behave on a plane.

But the job loss didn't end the Ohio resident's mission to expose bad behaviour on planes. Instead, the blog morphed into an Instagram site called PassengerShaming, which has more than 522,000 followers.

Based on photo contributions from air travellers and flight staff worldwide, the site shows it all: passengers making out in their seats, clipping nails and nose hairs, tossing garbage on the floor, flying shirtless and even watching porn on their electronic devices.

Some passengers apparently think it's OK to fly shirtless. (PassengerShaming)

"It's kind of nice to put it out there for people to see it's not OK to do this," says the former flight attendant who goes by her first names, Shawn Kathleen. She doesn't want her last name published for security reasons: her Instagram site has become wildly popular, and with that comes plenty of internet trolls.

"I've had people say, 'I'm going to cut your head off.' I've had death threats."

But that doesn't stop her from continually updating her site, which isn't difficult to do considering she gets up to 50 photo submissions a day.

"It's totally grown organically. It's like a movement."

PassengerShaming includes plenty of photos of passengers who find it very convenient to rest their feet on someone else's armrest. (PassengerShaming)

One of the site's main themes is wayward feet — resting on the headrest of the seat in front or bare feet propped up on armrests or meal trays.

"That's like the thing we eat off of, so it's not a foot rest."

How appetizing. Feet on a meal tray. (PassengerShaming)

Nor is it a changing table. Shawn Kathleen says during her seven years in the industry, she would often see parents changing their babies' diapers on meal trays.

So perhaps it's no surprise that some of the shaming photos show dirty diapers stuffed in the seat pocket, on the floor, or even shoved in a drinking glass.

Leaving a diaper in a drinking glass is probably something this passenger wouldn't do at home. (PassengerShaming)

"The other fun thing is, when you're going through the cabin, they'll try to hand them to you," she says. "I'm like, 'Let me get some gloves. No I'm not going to just take that, I'm sorry.'"

Photos of other gross objects passengers have left behind at their seats include a used condom, false teeth, urine in a bottle and chewed gum.

Anybody missing false teeth? (PassengerShaming)

Shawn Kathleen says gum stuck in inappropriate places was a daily occurrence during her flight attendant days.

"People put gum in the safety information cards, which are things people need to read."

The photos also highlight people with too much carry-on luggage. Culprits include passengers who leave oversized suitcases in the overhead storage with the compartment's cover still open because the item won't fit.

"Flight attendants have magical powers that can just make bags smaller," jokes Shawn Kathleen, who says she constantly had to hunt down luggage offenders on flights.

Sometimes passengers leave their oversized carry-on in the overhead bin and hope no one will notice the bin's cover won't shut. (PassengerShaming)

When asked for some of her worst experiences with passengers, she tells the story of a man who set off the plane's smoke detector while in the lavatory.

When she confronted him, she discovered that instead of a cigarette, he had been smoking crack cocaine.

"He [had] singed off all hair on the right front portion of his head."

She says the passenger's biggest concern was if he would still make his flight connection. "Like no, you're going to jail."

Shawn Kathleen also recalls people asking for a beverage at highly inappropriate times, such as when she was helping a passenger suffering from a heart attack.

"You literally have a bottle of oxygen under your arm and you're hustling up the aisle and somebody's like, 'Umm, excuse me,' and they'll snap their fingers, 'Can I get a Coke?'"

Former flight attendant Shawn Kathleen says passengers often leave chewed gum in inappropriate places. (PassengerShaming)

Airline analyst Robert Kokonis believes we see more bad behaviour on planes these days because flying is no longer a special occasion. With the onslaught of discount fares, it has become commonplace — to the point where sometimes passengers forget their manners.

"We go back to the '70s and the '60s, people dressed up in a dress, a suit and tie [to fly], and today you're getting ripped jeans," says Kokonis, with Toronto airline consulting firm AirTrav. "It's a bit of the Wild West now."

But he says people need to keep in mind that most air travellers are well-mannered. "Websites, blogs, social media tend to make certain problems look a lot bigger than they actually are."

PassengerShaming offers plenty of photos of passengers making a mess. (PassengerShaming)

Nevertheless, Shawn Kathleen — who has also worked as a paramedic and a police officer — says her toughest gig by far was working as a flight attendant.

"When I was a cop, I had a gun, I have backup," she says.

"[When] you're on that plane at 35,000 feet, you and a couple flight attendants and a couple hundred people, God only knows what the hell could happen up there."