Your Heated Home Is Drier Than Phoenix

In the winter of 2014 to 2015, two researchers put small temperature and humidity loggers in 34 occupied apartments in New York City and left them running until spring. The apartments stayed warm, about 74°F on average, and that barely changed from one month to the next. They were also very dry. The study reports humidity as vapor pressure (a measure of how much water is in the air), so we converted its numbers into relative humidity, the percentage you see on a weather app. Relative humidity is how much water vapor the air is holding compared with the most it could hold at that temperature, so the lower the number, the drier the air. By our math the apartments averaged roughly 24 percent relative humidity over the winter, meaning the air indoors was holding less than a quarter of the water it could. They spent close to a month below about 18 percent, and about a week below about 10.

For comparison, the National Weather Service's climate record for Phoenix put the city's average relative humidity at 34 to 37 percent in the late 1980s and early 1990s (averaged from early-morning and late-afternoon readings, so not a perfect match). Even allowing for that, a winter in these apartments was drier than an average day outside in Phoenix.

That winter air is also a long way from how humid homes are the rest of the year. Measurements from German homes, compiled in a paper by building physicist Hartwig Künzel, show indoor relative humidity running around 40 to 50 percent in spring and fall and up to about 60 percent in summer, then dropping to around 30 percent in winter when it is near freezing outside. A 2015 study that logged one home in each of eight cities for a full year found a house in Boston averaging about 50 percent across the year and one in Atlanta about 62. The New York apartments, at 24 percent, were drier still than the German winter figure.

Why Heating Dries the Air

Warm air can hold far more water than cold air. Heating warms the air in your home without adding any water to it, so the amount of water stays about the same while the most the air could hold goes up, and the relative humidity falls.

Take a damp November day, 35°F outside with 70 percent humidity. Warm that same air to 70°F indoors and, using the standard formula, it lands at about 19 percent. Start from a 20°F day at the same 70 percent and the indoor figure is closer to 10. The people living in a home add some water back (the New York apartments held slightly more water than the outdoor air), but not nearly enough to make up the difference. The researchers also found that apartments in big buildings (100 units or more) were significantly drier than the rest, so if you live in one, expect your air to run drier still.

What Dry Air Does to Skin

The outer layer of skin (dermatologists call it the stratum corneum) holds water, and it gives some of that water up to the air around it, faster when the air is dry. In a 2002 study, researchers sat healthy volunteers in a low-humidity room and measured their forearms and cheeks. After three hours the water content of the outer layer had dropped significantly, and the fine pattern of lines on the skin had changed with it.

A 1996 study looked at the same outer layer across the seasons, sampling the face, hands and legs of women of different ages, and found the lipids in it (the fats between skin cells that slow water loss) were "dramatically depleted in winter compared with spring and summer." A 2016 review of the research on climate and skin concluded that low humidity and cold weaken the skin's barrier function and are linked to more flares in people with atopic dermatitis (eczema). If you have eczema, the start of winter and heating season is a good time to see your dermatologist before a flare starts.

Skin does seem to adjust to dry air given time, at least in mice. In a separate 2002 study, hairless mice moved from very humid air into very dry air lost six to seven times more water through their skin within two days, and were back to normal within a week. We could not find the same experiment done in people, but it is one more reason to change your routine early rather than late.

What to Put on Your Skin This Winter

The time to change your skincare is the week the heat comes on, rather than the week your skin starts to feel tight. The first change is texture. The American Academy of Dermatology (AAD), the professional association for dermatologists in the United States, says ointments and creams "add more moisture to skin and are more effective than lotions," so if a light lotion got you through the summer, this is the time to move to a cream for your face and body, and to something richer at night. The ingredients the AAD suggests looking for include glycerin, hyaluronic acid, shea butter, jojoba oil, lanolin, dimethicone, mineral oil and petrolatum (a post next month will get into how these ingredients differ from one another).

The second change is how often. The AAD's advice is to moisturize several times a day: after a shower while your skin is still damp, and whenever your skin feels dry. Keep a lip balm within reach as well, and if a lip balm stings or tingles, switch to a different one.

Last, go easier on anything that strips oil from your skin. The AAD recommends a gentle cleanser that does not work up a thick lather, and for people whose skin is already dry, it suggests steering clear of products containing, among other things, alcohol (hand sanitizer excepted) and retinoids. We wrote more about choosing a cleanser in Why Your Skin Hates Your Cleanser, and it is worth a second read this time of year.

If your home is anything like the ones in these studies, the air indoors is now far drier than it was in summer, and it will stay that way until spring. Slight changes to your routine through a heavier cream, a gentle cleanser, and consistent hydration are all small changes, and the sooner they start, the less catching up your skin has to do.

Cheers,
The Relterra Team