The one thing you should never do on a plane, according to a doctor.

Along with applauding when the plane lands, there is something else you should never do while you're in the sky. And chances are, you're not going to like it.

Speaking on The Mel Robbins Podcast, physician Dr William Li — who is known for his research on metabolism — shared a rather unsettling insight about one of our most common in-flight habits.

"To do my research, I do travel quite a bit," he said. "So I do know a lot about sitting on planes. And one of the things about planes that's kind of crazy is that we're instinctively served beverages and foods.

"If you're like me, and I'm working on my laptop, and somebody comes by and hands me something, my instinct is to take it. But there's one or two things that I don't do."

Watch: Flight attendant drops her 20-hour makeup routine. Post continues after video.

He continued, "I don't drink coffee or tea on an airplane."

His reasoning, you ask? Well, caffeine lovers, you might want to look away.

"A lot of people don't know this, but the water from your coffee or tea comes from a tank in the airplane, right? And they don't clean that tank but once or twice a year."

Um, ew? Think about the state of a container that just… doesn't get washed. Gross.

But if that mental image isn't enough to put you off, his next description might do the trick.

"I don't even want to go into the chassis of an airplane to take a look at the rusty old tank with all the crud that's in the bottom that they are actually using and pouring out," he said.

It seems Dr Li isn't alone in his concerns. 

A flight attendant with 20 years of experience told Time Magazine "I won't drink the tap water. I just don't always trust the cleanliness of the aircraft and the testing of it."

Another former flight attendant, Kat Kamalani, seconded this, as per Green Matters.

Kamalani said in a TikTok video: "Those water tanks [on the aircraft] are never cleaned and they are disgusting." The coffee machines, too, are apparently "rarely cleaned unless they are broken".

Yikes.

And it's not just personal anecdotes; studies and regulations also suggest that the tanks aren't, um, the cleanest of vessels.

In 2004, the US Environmental Protection Agency (EPA) found coliform bacteria (which is naturally found in digestive tracts and faeses, ew) in 15 per cent of water samples from more than 300 planes. This led to the implementation of the Aircraft Drinking Water Rule in 2011, as per Time.

"The primary purpose of the Aircraft Drinking Water Rule (ADWR) is to ensure that safe and reliable drinking water is provided to aircraft passengers and crew," reads the official EPA website.

Two years after this rule was implemented, EPA data showed that 12 per cent of commercial planes in the States still tested positive for coliform bacteria at least once, per Time.

A 2015 study published in the International Journal of Environmental Research and Public Health also found that airplane water tanks are "conducive for microbial growth".

Coliform bacteria in your plane coffee or tea doesn't necessarily mean the liquid is unsafe to drink, but it does raise concerns about water quality and safety, as per the Washington State Department of Health.

But before you panic about that time you downed three cups of plane coffee on your last long-haul flight, it's worth noting that not everyone agrees with this assessment.

Many airlines maintain that their water is safe and complies with EPA regulations, with some claiming to disinfect tanks up to four times a year.

Vance Hilderman, chief technical officer of Afuzion — an aviation certification company — also stated that "it's 100 per cent safe to drink tea, coffee and even water straight from the taps on modern airplanes in developed and well-regulated countries," as per Green Matters.

"There's a chemical rinse they put in (the tanks). It's not toxic. It's not industrial hospital quality, but it's actually quite good. It's very doubtful that bacteria would survive. It's equivalent to air filtering systems, HEPA systems on aircraft."

Still, Dr Li says he would rather be safe than sorry.

"Listen, the place where the flight attendants are actually making the coffee looks pretty clean. The coffee looks okay. I can't taste anything," the physician said.

"But I know enough that I care about what I put into my body."

Okay, all of this is very interesting, but what's the solution? Because oftentimes, coffee is a matter of 'need' not want, especially 20 hours deep in a flight.

Well, according to Dr Li, you should "bring your own thermos, metal thermos or glass thermos. Fill up your own coffee."

So, next time you're settling into your seat and the drink cart comes around, maybe consider reaching for that sealed bottle of water instead. Your gut might thank you later.

Feature Image: Getty.