Why Does Traveling Make You Dehydrated?

Flying mostly dries you out rather than truly dehydrating you. At 35,000 feet the cabin air is extremely dry, only about 10-20% relative humidity (drier than most deserts), so it pulls moisture from your eyes, nose and throat. You also lose a little extra water through breathing, and alcohol or coffee in the air can add to it.

Imagine the last time you were on a flight, one that was at least two or three hours in duration. The snacks were too few and far between, the alcohol was overly expensive, and by the time you landed in your destination, your throat was oddly parched. In fact, your eyes were probably rather dry too!

This is not a unique occurrence; most people experience this sort of dehydration when they travel – both in planes and cars. Sometimes, dogs do too!

However, just because it happens to everyone doesn’t make it any less interesting – so what’s the real reason behind this common phenomenon?

Short answer… air circulation systems.

The Air Up There

When you are traveling in a plane, the cabin must be pressurized and fresh oxygen must be provided to the passengers. If people gulped in all the oxygen in the sealed tube and then exhaled carbon dioxide, eventually, there wouldn’t be any air left to breathe!

Therefore, the air circulation system of any aircraft must take in breathable air from the outside of the plane and deliver it to the passengers, as it simultaneously cycles out the old air. However, the air outside of a plane at 35,000 feet tends to be extremely dry, with a relative humidity of only 10-20% (and even lower on some aircraft). That is drier than most deserts, and well below the 30-60% range most people find comfortable indoors. This is why your eyes, nose and throat feel parched long before any other part of you does.

When the air around you is that dry, water evaporates from your nose, eyes, mouth and throat into the drier air, much like a puddle drying up on a warm day. Your lungs add to this: every breath you exhale is fully saturated with water vapor, and the drier the air you breathe in, the more moisture your body has to donate to humidify it. One review estimated that this respiratory water loss can climb from roughly 160 mL (5.4 fl oz) per hour at 60% humidity to about 360 mL (12 fl oz) per hour at 12% humidity. So while the circulated cabin air is good for keeping things clean, it certainly doesn’t help your water balance.

That said, it is worth being honest about scale. Most of what you feel is dryness of the mucous membranes (the parched throat and gritty eyes), not necessarily whole-body dehydration. Scientists who have reviewed the evidence point out that the actual amount of water you lose over a normal flight is fairly modest and has never been precisely measured, so the idea that flying badly dehydrates an otherwise healthy person is more assumed than proven. The dryness is real; the danger, for most short and medium flights, is usually mild.

There is also a second effect that air travelers must deal with; when the air is dry, the moist lining of your nose and airways thins out and the tiny hair-like cilia that sweep mucus along can’t clear airborne pathogens and irritants as effectively. Studies confirm that this mucociliary clearance slows in low-humidity air, which is one reason it can be easier to pick up a cold on a plane, even though the cabin air is being filtered and recirculated throughout the flight.

Dehydrated On The Roads

Although dehydration while flying is quite common, and something that most people can relate to, driving dehydration is not as readily recognized. When we are sitting for long periods of time on a road trip, perhaps in the summer, our bodies are naturally warmer, which means that we tend to sweat more. Furthermore, if you are using the air conditioning in your car, that jet stream of air is more readily removing water from your skin and keeping you cool, even as your body continues to push out more sweat.

When combined with the normal loss of hydration through breathing, it can be very easy to become dehydrated in the car without noticing. You may not be performing any physical exertions, per se, but your body is constantly losing water – particularly at the warmer spots of contact between your body and the seat.

There is even research arguing that driving while dehydrated can be just as dangerous as driving drunk. In a 2015 Loughborough University driving-simulator study, mildly dehydrated drivers made more than twice as many errors (drifting out of lane, late braking, touching the rumble strip) as they did when properly hydrated, roughly the same error rate seen in drivers at the legal blood-alcohol limit. Dehydration can also bring on slow reflexes, exhaustion, blurred vision, light-headedness, dizziness and a lack of focus. All of this can spell disaster in the tenth hour of a cross-country road trip, so you need to ensure that you stay hydrated. A good practice is to grab a bottle of water or two each time you stop to fill up the car, and drink electrolyte-infused drinks consistently throughout the drive.

How To Counter Travel Dehydration

The obvious solution to being dehydrated while traveling is to drink more water, but that can be easier said than done, particularly with stringent liquid restrictions on airplanes, and the lack of physical exertion – which typically works as a trigger to drink more water. As mentioned above, you should always set out for a road trip with plenty of food and water in the car, and again, staying away from salty junk food is essential.

Eating water-rich foods like fruits and vegetables is a good way to keep your hydration levels up between beverages, and avoiding dry, salty snack foods is also a good piece of advice. This might be difficult when all you are offered on airplanes is sodium-packed snacks, but what you can do is go easy on alcohol and coffee. Aside from being wildly expensive on flights, both alcohol and caffeine are mild diuretics, meaning they prompt your kidneys to send more water to your bladder. While it might be tempting to catch a buzz during that transatlantic flight, the hangover and the dryness by the time you land will be much worse.

Dressing appropriately for travel can also help. If you are worried about the effects of dehydration, err on the side of being slightly colder, rather than overdressed, as too many clothes will increase the rate at which you perspire. Dressing in layers and paying close attention to your body temperature is key. Bringing saline drops for your eyes, as well as mints to stimulate saliva production, can also make your trips more comfortable, even if your body feels a bit dried up.

Most importantly, if you are feeling lightheaded, dizzy, confused or simply parched, stop what you’re doing and drink plenty of water. The effects of dehydration can be serious, in some cases, so it’s always worth addressing immediately to keep yourself and others safe.