Of all the in-flight habits that infuriate passengers, this is the worst

I'm settling into my aisle seat on the plane, quietly thrilled to find the middle seat empty, when the man by the window starts to get comfortable. Off come his shoes. Then - horrifyingly - his socks.

He folds himself up like a cat, lifts the armrest and places his bare white feet - toes splayed and relaxed - on the seat between us.

Could this flight start off any worse? This is not your lounge room, I want to say. This is a metal tube filled with hundreds of strangers. Just because you've paid handsomely for your seat doesn't make it your personal spa.

Instead, I opt for the universal language of disapproval: a pointed look. To his credit, he gets the message - or perhaps realises it's not that comfy after all - and lowers his feet.

But it got me thinking: when did we all start treating planes like our own private spaces? And more importantly, where do you stand - or rather, sit - on the shoes-on or shoes-off debate? Are socks OK? And if they are, can you walk around in them?

There's something about long-haul travel that blurs our usual sense of public decorum. Because it's a place we sleep, people seem to think it's also a place to bare parts of themselves they'd never unveil in public. And yes, I'm talking about toes. Hairy ones.

Former flight attendant Kristy Driessens, who now teaches cabin crew at TAFE NSW, says people are definitely treating planes like their lounge rooms - and shoes aren't the only things people have taken off on her watch.

"[One passenger] took his shirt off and then just went to sleep and I had to physically come and wake him and tell him to put his shirt back on. He just said, 'Oh, it's getting a bit warm'," she recalls.

There's also the issue of smell. Most of us are, understandably, immune to our own personal funk. But that doesn't mean the rest of us are. You may believe your feet are squeaky clean - and perhaps they were when you left the house - but after a few hours, we're all marinating in a pressurised cabin at 30,000 feet. It's not the time to let them breathe.

I can come to terms with you taking your shoes off while you sleep as long as you wear clean socks (and preferably cover them with a blanket).

But please put those shoes back on when you get up and walk around and, god forbid, walk on the cesspit of germs that is the toilet floor.

"I have had to ask people to put their shoes back on because other passengers complained about it," Driessens says.

And she agrees: "If you go to the toilet, your shoes must be back on."

Dr Michael Taylor, adjunct academic in the College of Science and Engineering at Flinders University, is of the same mind. He says when you go into the toilet area on a plane, you're walking into "essentially a public toilet". And we all know what those floors can be like.

But it's not just the gross factor. You can actually catch things from people's feet.