Staying hydrated on flights and at altitude

2026-10-30

Airplane cabin air is very dry, usually 10–20% humidity, but for a healthy adult that mostly dries out your eyes, nose and skin rather than seriously dehydrating your body. The practical advice is simple: drink normally during the flight, with a glass of water (8 fl oz, 250 ml) every hour or two on long-haul flights, go easy on alcohol, and keep moving. At real altitude, in the mountains, your water losses do rise, but drinking extra won't prevent altitude sickness; ascending slowly does.

How dry is cabin air?

Very. The CDC's travel guidance notes that aircraft cabin air is typically 10–20% humidity, far drier than a typical home or office. Air at cruising altitude holds almost no moisture, and the cabin is constantly refreshed with that outside air.

The same CDC page says cabins are pressurized to the equivalent of at most about 2,440 m (8,000 ft) above sea level. That means slightly less oxygen in your blood, which healthy travelers rarely notice but which matters for people with heart or lung disease.

Does flying actually dehydrate you?

Less than you'd think. Dry air increases the water you lose through breathing and through your skin, but over a few hours the extra amount is modest. The World Health Organization's travel guidance has long stated that low cabin humidity causes discomfort of the eyes, nose and skin, but that there's no evidence it causes internal dehydration, and no need to drink more than usual.

What most people feel on a long flight (dry eyes, a scratchy throat, a stuffy nose, tight skin) is local dryness, not a sign that the whole body is short of water. That's why the most effective fixes are local too:

Eyes: wear glasses instead of contact lenses on long flights, or use lubricating eye drops.
Nose: a saline nasal spray helps.
Skin and lips: moisturizer and lip balm.

That said, plenty of people do end up underhydrated after a travel day, for reasons that have little to do with humidity:

They skip drinks to avoid climbing over a neighbor to reach the toilet.
They drink little during the hours of airport security, queues and delays.
They drink alcohol and coffee, and sleep through drink service.
They arrive somewhere hot.

So the goal isn't to drink extra liters; it's to drink normally despite the circumstances.

A simple plan for flight days

Before

Drink as usual on the day; arriving well hydrated is easier than catching up on board.
Bring an empty reusable bottle through security and fill it at a fountain or café airside.
Book an aisle seat on long flights if you can; you'll drink and move more freely.

On board

Drink regularly, roughly a glass of water every hour or two on long flights, plus water with meals. Accept water every time the cart comes by.
Watch the alcohol. Alcohol increases urine output by suppressing the hormone that helps your kidneys hold on to water (more in alcohol, dehydration and hangovers). It may also affect you more in the air: in a 2024 study by the German Aerospace Center, healthy young adults who drank alcohol and then slept at simulated cabin pressure had noticeably lower blood oxygen levels and higher heart rates than those who didn't. The study was small and in a lab, but it's another reason to keep it to one drink, or skip it.
Coffee and tea are fine in moderate amounts and count toward your fluids (see does coffee dehydrate you?). Just remember caffeine can make sleeping on an overnight flight harder.
Move. Long flights increase the risk of blood clots in the legs (deep vein thrombosis). For journeys of four hours or more, the NHS advises drinking plenty of water, avoiding alcohol, wearing loose clothing and walking around when possible. Drinking water won't prevent clots on its own, but it does give you a reason to walk to the galley and back.

After

Drink with your first meal on arrival, and keep a bottle with you through the first day, especially if you've landed somewhere hot (see hot weather hydration).
Jet lag and dehydration both cause tiredness and headaches, so it's worth ruling out the easier one.

Hydration at real altitude

Mountain trips are a different story. Above roughly 2,500 m (8,000 ft), you breathe faster and deeper, and cold, dry mountain air takes more water out of your lungs with every breath. Many people also urinate more during the first days at altitude, as part of normal acclimatization. Add hiking or skiing, and fluid losses can be significant.

But one widespread belief is wrong: drinking large amounts of water does not prevent altitude sickness. A summary of the Wilderness Medical Society's guidelines states that forced overhydration is not effective for preventing altitude illness and increases the risk of low blood sodium. Staying adequately hydrated still matters, because dehydration causes headache, fatigue and nausea that are easily confused with acute mountain sickness.

What actually prevents altitude illness is going up gradually. The CDC advises that any unacclimatized traveler sleeping at 2,450 m (8,000 ft) or higher is at risk, recommends avoiding alcohol for the first 48 hours at high altitude, and suggests talking to a doctor about preventive medicine for higher-risk trips.

A practical approach at altitude:

Drink regularly and enough to keep your urine pale; thirst can be blunted in the cold.
Carry water on hikes and keep bottles from freezing (insulated bottles, or upside down inside your pack).
Eat regular meals; food provides water and salt.
Treat headache plus nausea, unusual fatigue or poor sleep as possible altitude sickness, not just "thirst". Stop ascending, and go down if symptoms worsen.

Who should be more careful

Talk to your doctor before flying or traveling to altitude if you're pregnant, have heart or lung disease, kidney disease, a history of blood clots, or take diuretics. For pregnancy-specific advice, see water intake during pregnancy.

Make it automatic

Travel days scramble routines, which is exactly when a nudge helps. Water Tracker works on its own on your Apple Watch, so you can log a cup of water from the cart with a turn of the Digital Crown even with your phone in flight mode, and its reminders stop once you've reached your goal for the day. Everything is stored on your devices, with no account needed.

The bottom line

Cabin air is extremely dry, but for healthy travelers it mainly irritates eyes, nose and skin; it doesn't cause major dehydration. Drink normally on flight days, about a glass of water every hour or two on long flights, limit alcohol, and get up and move. At real altitude, keep your intake adequate but don't force extra water; ascending slowly is what prevents altitude sickness.

This article is general information, not medical advice. If you have a health condition, are pregnant or take medication, ask your doctor how much you should drink.