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Sure, wearing a sheet mask on a plane makes for a good Snapchat pic, but there’s a very practical reason you see your favorite celebrities create a spa for themselves in the air. To put it frankly, weird things happen to your body, especially your skin, while you’re flying, proving that travel beauty goes far beyond those pint-sized and incredibly cute containers of cleanser.

I noticed this myself the other day when I was on a flight home from Los Angeles and realized my hydrated, glowy skin had transformed into a blotchy, dry mess. I also noticed I had two new zits and that my rings suddenly fit way snugger than they did when I boarded. Intrigued, I contacted a dermatologist to find out the truth behind my suspicions.

So why does it seem like all the moisture is being sucked from your face the minute you buckle your seatbelt? You're not crazy, as you're actually prone to dryness in the air.

"The air in planes is not humidified," explains Dr. Heidi Waldorf, the Director of Laser and Cosmetic Dermatology at The Mount Sinai Hospital in New York City. "The drier the air, the more water loss from the skin." Now all the serum-rich sheet masks we see in pics and the age-old tips about slathering a thick cream on your face make more sense.

In addition to dryness, I've always heard people complain that they breakout after flights, but Dr. Waldorf says not to fully blame your acne on the plane. "It's more likely a combination of the stress associated with travel, poor sleep, sweating against travel pillows, not washing your face as your normally do, and touching your face more," she says. "That’s why I recommend carrying pre-moistened makeup remover wipes in a Ziploc in your carry-on so you can wipe your face before sticking on your sleep mask. Carrying some acne medication—either a sample size or a small amount in a contact lens case—is a good idea if you are prone so you can apply it after wiping."

But if you have oily skin and it becomes dry on the plane, board-certified dermatologist Dr. Kenneth Mark explains your skin could overcompensate and produce even more oil, potentially causing blemishes. "There are also times when it becomes hot on a plane and people can sweat, which further clogs pores which flare breakouts," he says.

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Finally, the swelling I mentioned earlier? It's real. "It’s a combination of inactivity—lack of muscle movement and staying in one position with legs hanging down," says Dr. Waldorf.

In addition, she says that because the cabin pressure is low, it can slow down circulation in your extremities, and if you're eating high-sodium food and drinking alcohol, you could be experiencing the effects of dehydration and water retention.

That mid-flight mask selfie has never sounded better, and you can bet next time I'll be well-stocked with lotions, eye creams, and and more before I board.