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Humidity is definitely one of those things that you don’t notice unless there’s too much or not enough. Too much humidity can lead to the growth of mold or damage of inanimate objects, but not enough causes more direct problems with your body. The ideal relative humidity level is around 30% to 50%, but if you’re on the road, at the office, or just not sure what relative humidity really means you might need a tip or two to not dry out. 

Let’s take a look at how to think about humidity in whatever room you’re in and the best and safest ways to get it to the right level.

What happens when you dry out?

Your mucous membranes are parts of your body that touch the outside world, like the inside of your mouth or up your nose. They’re coated with a usually thin (if you’re not ill) layer of mucus, which is one of the stickiest substances known. Pathogens or parasites that land in mucus quickly find themselves mired and unable to do anything. Mucus needs water and the right pH balance to be most effective, so when the mucous membranes dry out, it can’t do its job and you might get infected more easily. Dried out membranes are also uncomfortable, leading to redness, irritation, chapped lips, and the occasional bloody nose.

Your skin also contacts the outside world, and is also very reliant on water to keep up its protective barrier to keep out invaders and air pollution. In addition to being more susceptible to damage, dry skin can be uncomfortable and unsightly. The right kinds of lotions can help, but keeping the air just right is another great method for keeping skin healthy. 

Condensation on a window with a city skyline blurred behind it

Tip 1: Relative humidity is relative

As air gets hotter it gets thirstier, as air gets cooler it will leave liquid water around.

Yes, the wording is obvious but it’s still not totally clear how much water is in the air. Humidity is measured as a percentage of relative humidity, or RH%. The percentage represents how much of the air’s water-holding capacity is being taken up by the current level of humidity. What makes it confusing is that air holds more water when it’s warm (and more steam can form) and less when it’s cold (and it might become liquid or even ice). This is important because when the air isn’t holding much water relative to its capacity and is in the 0% to 30% range, air becomes thirsty and will suck moisture out of the surroundings (and you).

The atmosphere’s water-holding capacity is not a straight line. At close to freezing or 32F, it can hold about 5 grams of water per cubic meter, and about 7 grams per cubic meter at 42F, which isn’t a huge difference. But as the temperature goes up, the water capacity of the air goes up faster. It can hold 10 grams per cubic meter at 52F, 14 grams at 62F, about 20 grams at 72F, and 27 grams at 82F. That’s why you can literally feel the amount of water in the air in hot climates.

Hygrometer on a wall

Tip 2: RH meters are very cheap

Spend a few dollars for a battery-power meter.

Temperature and RH combination meters are extremely inexpensive, $5 to $10 for a nice one with a stand and closer to $2 for a small pocket version, though these are so cheap most manufacturers won’t sell you less than a six-pack. Pop one in your suitcase, backpack, or purse so you can check the water content of any room. If you want to splurge you can pay more like $15 to $20 for one with an app that will track RH over time.

An old radiator beneath a window

Tip 3: Adjusting the thermostat lowers the humidity

If it’s too dry, don’t use the heater OR air conditioner.

Unfortunately due to the way the mechanisms work, both heating and air conditioning will reduce relative humidity. Heaters work by simply warming up cool air, and since hot air holds more water than cool air, the air coming out of the heater will be thirsty. Standard air conditioners work by blowing hot air over a cold (around 40F) metal coil which means warm air is going to dump a lot of water when it hits the coil, which is then collected in a drip tray. The 40F air exiting will get warmer as it mixes with the room, so air coming out of the air conditioner will also be thirsty because it’s warmer in addition to leaving most of its water behind.

Caveat: Coolers that don’t lower the humidity

So-called swamp coolers or evaporative coolers take advantage of a little loophole in relative humidity and temperature, specifically that it takes more heat energy to turn liquid water into steam than to just heat up liquid water. This is how our sweat cools us off, airborne water or steam needs to use more heat to fly around than liquid

These coolers will have a lot of liquid water on a porous surface like fabric, and will blow air through it, and all that water evaporating at once will drop the temperature of the exiting air while increasing the humidity. While these won’t dry you out, the reverse could be a problem, with too much humidity that leads to mold growth. And even if it doesn’t lead to mold growth, they don’t work when the RH% is high because the air won’t be thirsty, so it won’t let the water evaporate off the fabric, and the temperature will stay the same.

A towel hanging in a kitschy living room

Tip 4: Increase surface area

Water can only evaporate if it’s in contact with the air. So, soak a big towel.

The water in the drip pan in the air conditioner doesn’t evaporate because it’s in a pool and only the water at the surface has access to the air. On the other side of the coin the water on the fabric of an evaporative cooler turns to steam fast because it can be strung out on all the fibers with a lot of surface area to leave all at once. Placing a few glasses of water out won’t change the humidity a great deal, but hanging a wet towel will add moisture to the air. Particularly if it’s in a breeze or direct sunlight. Boiling water is also very effective because it has extra heat to get into the air, but requires a heat source and a vessel, when a towel on a coat hanger is much easier to come by and doesn’t require supervision.

A well designed bathroom with some condensation here and there

Tip 5: Bathtubs and showers

The bathroom is usually the place to look out for too much humidity, so don’t send it all down the drain if it’s too dry.

A pool of water in a bathtub has a large surface area to let water get into the air, so leaving the tub full can help to increase humidity. The same can be true of the shower, but all that warm water spraying through the air will almost certainly bring the RH to 100%, so unless it’s very dry turning on the fan vent to blow it outside is a better idea.

Header photo by mohamed abdelghaffar, window with condensation photo by Akshay Nayak, hygrometer on wall photo by Sofia Guzeva: window and radiator photo by Jana Al Mubaslat, mantlepiece photo by Marina Leonova, bathroom photo by W O L F Λ R T, all on Pexels.

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