Aim for a reading between 30% and 50% relative humidity, and never let it sit above 60%. In a cold snap, stay toward the low end of that range so your windows don’t fog up and drip. And before you trust the number, check your hygrometer, because a cheap hygrometer can be off by a few points in either direction.
I’m not a doctor, a nurse or a pharmacist (the site’s name is just its name), and this isn’t medical advice. If you have asthma, allergies or a lung condition, ask your doctor what indoor humidity is right for you before you run a humidifier all winter.
The Number to Aim For: 30% to 50%
Keep indoor relative humidity below 60%, and ideally between 30% and 50%. That middle band is comfortable for most people and is less friendly to mold.
If you run a humidifier, stop at 50%. Humidifying past 50% can feed the growth of mold and bacteria, so switch it off once the display hits that number.
“Relative humidity” just means how much water vapor the air is holding compared with the most it could hold at that temperature. That’s why the display on a hygrometer shows a percentage, and why the same room can read much lower in January than in October without anyone removing a drop of water.
Why Your Reading Drops When the Heat Comes On
Cold air can’t hold much water. When that cold outdoor air leaks into your home and the furnace warms it up, the amount of water stays the same but the air’s capacity jumps. The reading falls, and it can drop well under 30%.
That’s why winter is when I hear about this the most. Your skin feels tight, your lips crack, and you get a little zap every time you touch a doorknob. The heat is doing its job. It’s just drying the air out as it goes, which is kinda annoying when you’re the one with the chapped lips.
Dry air can irritate the lining of your nose and throat, and a humidifier can help ease a stuffy nose and loosen mucus when you have a cold. But a humidifier treats the air, not an illness. A cough or a fever that hangs on is something for your doctor, whatever the reading says.
When It’s Freezing Outside, Aim Lower
Here’s the part I wish every humidifier box printed on the front. The colder it gets outside, the colder your window glass gets, and a cold pane pulls water out of damp indoor air. A room at 45% can look fine on the hygrometer while the windows stream.
Condensation on your windows in winter is a sign of too much moisture, and somewhere around 25% in winter to 50% in summer is healthy and comfortable for most homes.
The window industry’s trade group publishes a handy chart for a 70°F room. When it’s 20 to 40°F outside, keep indoor humidity at 40% or less. At 10 to 20°F, 35% or less. From 0 to 10°F, 30% or less. From -10 to 0°F, 25% or less. From -20 to -10°F, 20% or less, and below -20°F, 15% or less.
Those numbers vary with your windows. Old single panes fog sooner than new double panes, and closed curtains make it worse because they keep the glass cold. Your windows get the final word. If they fog up, turn the humidity down a notch.
How Far Off Can a Cheap Hygrometer Be?
Further than the display makes it look, it turns out. A digital hygrometer shows a crisp whole number, like 41%, and that number feels exact. It isn’t.
Take the ThermoPro TP50, a popular budget model. It’s good to within 2 points either way between 30% and 80%, and 3 points outside that range. So when yours shows 41%, the real number is somewhere from 39% to 43%. A dry winter 25% could be anything from 22% to 28%. That’s close enough for keeping a room in the 30% to 50% band, just not worth fussing over a single point.
Put two cheap hygrometers side by side and you’ll often see two different numbers. That doesn’t mean either one is broken. It means you should check yours against something known before you trust it, and that’s what the salt test is for.
READ: Forehead vs Ear Thermometer: Which Reads Truer at Home
The Salt Test: Check Yours in About a Day
This trick works because of plain table salt, and it’s kinda brilliant. A paste of salt with a little water, sealed in a small space, settles the air above it at about 75% humidity. It’s steady enough that engineers use it as a fixed point, about 75.5% at 68°F.
Put a few spoonfuls of salt in a bottle cap or a small dish and add just enough water to make a damp paste. It should look like wet sand, not salty water. Set the dish and your hygrometer inside a zip-top bag or an airtight container, seal it, and leave it at room temperature, out of the sun.
Wait about 24 hours, then read the display without opening the bag (opening it lets room air back in and spoils the test). If it says 72%, yours reads about 3 points low, so add 3 to every reading from now on. If it says 78%, subtract 3.
A single test checks just one point on the scale, so a hygrometer can still drift at the dry end. It’s a home check, not a lab calibration. Still, I’d rather know my 35% is really 35% than guess.
Signs Your Air Is Too Damp
It’s easy to overdo a humidifier in winter, especially one running all night in a small bedroom. The first warning is usually on the glass. Condensation collecting on windows, walls or pipes means it’s time to dry the surface and cut back the moisture.
Other clues are a musty smell, black spots in window corners, damp sills and peeling paint. If you see any of them, turn the humidifier down or off and recheck the reading in a day.
Mold is the real worry. Small patches you can clean yourself, but if mold covers more than 10 square feet, or there’s been a lot of water damage, it’s time for a contractor who handles mold cleanup.
Cool Mist or Warm Mist?
Both add moisture to the air about equally well. The difference that matters to me is safety. If you have kids in the house, pick a cool mist humidifier, because a warm mist vaporizer can burn a child who gets too close to the steam or knocks it over.
Cool mist comes with its own catch. Ultrasonic and impeller models, the ones usually sold as “cool mist,” can throw minerals and germs from the tank into the air. Steam and evaporative models aren’t expected to spread much in the way of minerals.
That’s where the “white dust” comes from. If you see a fine white film on the furniture near an ultrasonic humidifier, those are minerals from your tap water. Bottled water labeled “distilled” fixes most of it. It still has a few minerals, but far fewer than most tap water, so I’d keep a jug next to the humidifier.
Keep the Tank Clean, Every Third Day
I’ll be blunt here: a dirty humidifier can do more harm than dry air. Standing water grows bacteria and mold, and the machine’s whole job is to send that water into the room you’re breathing in.
For a portable unit, empty and refill the water every day, with the unit unplugged, and clean it every third day. Follow the cleaning steps in the manual that came with yours, and rinse the tank well so no cleaner ends up in the mist. When winter’s over, clean it and dry it out before you store it.
If you use a CPAP with a heated humidifier, the same idea applies to its water chamber. Your machine’s manual has its own steps.
READ: CPAP Machine Cleaning and Disinfecting
Where to Put the Hygrometer
Placement changes the reading more than you’d expect, and it’s a total pain to chase a number that’s really just measuring the wrong spot. Right next to a humidifier, the number will be too high. Next to a window, a heating vent or in a sunny spot, it’ll swing all day. The kitchen and bathroom spike every time someone cooks or showers.
Set it on a shelf or a dresser in the room you care about most, usually the bedroom, a few feet from the humidifier and away from outside walls. After you move it, give it a little while to settle before you trust the number.
My advice is to check it at the same time each day, like when you get up. A morning reading is a fair picture of the air you slept in, and it’s the one to compare from day to day.
When to Call Someone
A hygrometer measures the air. It can’t tell you what’s going on in your lungs or your sinuses. Call your doctor if you have nosebleeds that keep coming back, a cough that won’t quit, or asthma or allergy symptoms that get worse when the humidifier runs.
Call someone about the house if the reading stays above 60% even with the humidifier off, if windows stream every morning, or if you find mold bigger than a small patch. A heating and cooling technician can look at venting and air flow, and a mold contractor can handle the rest.
Most homes just need an inexpensive hygrometer you’ve checked with salt, a clean humidifier and a little attention to the windows. Get the reading into the 30% to 50% range, lower when it’s bitter cold, and your skin and your house will both be happier.
