Having tested outdoor gear as my main gig for over a decade, I am used to finding myself on the same side as the experts when it comes to gear advice. We agree that cotton doesn’t always kill; waterproof running shoes can actually make your feet wetter on runs; and yes, you should always pack a headlamp. One area where I heartily disagree? Socks. I am a big proponent of wearing synthetic (rather than wool) socks for winter pursuits.

　　I have tested over 150 socks—ranging from ski socks to running socks—for Outside and interviewed professional athletes, thru-hikers, and product designers in the process. Most of these folks believe that wool socks are the best choice for winter wear because of the fabrics natural thermoregulating and odor-mitigating properties.

　　There’s no arguing that wool is a better insulator from the cold. It remains warm when wet,  so your sweaty feet are less likely to freeze in your ski or hiking boots. Textile manufacturers have experimented with brilliant weaves and fancy chemicals to make synthetics (like polyester, nylon, rayon, and spandex, to name a few) as effective as wool at regulating temperature and mitigating odor, but most haven’t been able to match mother nature in those departments.

　　Still, I swear by socks made from synthetics and here’s why: I have brutally sweaty feet that blister like the dickens. Synthetic materials are better than wool at wicking moisture.

　　The right synthetics will actively grab moisture off of your skin and move it away to the exterior of the fabric, mitigating friction that can lead to painful blisters. When well made (from reputable brands like my personal favorite, Polartec), synthetics make the most of the fibers’ hydrophobic properties.

　　That said, every body is different, as are feet. Even two feet on the same body can even be different (my left foot blisters more than my right). So instead of letting you know what socks work best for me in the winter, let’s dive into the pros and cons of each.

　　Wool vs. Synthetic Socks: Pros and Cons

　　Are you concerned about odor? Go with wool.  One of the biggest benefits of wool is its natural odor-resistance. While it is not technically antimicrobial, the keratin protein that wool is made from and the waxy lanolin that many wools contain do not allow our stinky bacteria to thrive on it the way it does on synthetics, like polypropylene (or polypoopaline, as I like to call it).

　　“One of the things that I like least about the synthetic socks is that theyre embarrassing on a multi-day trip when your sock is dried and you can hold it out like a flag in a gale force wind,” said Saylor Flett, an Outside gear tester and program director for Feather River College’s outdoor program. “And its actually just a rigid shape that you have to reconform to your foot every morning until it gets wet again.”

　　If you plan to spend multiple, consecutive days in the same pair of socks this winter, socks with high wool content are the best option.

　　Do you have sweaty feet that are prone to blistering? Choose a sock made from synthetic materials or a synthetic-blend.  “Wool just doesnt dry very quickly. So once you get it wet, it stays wet,” said Tyler Maheu, a textile guru who has worked on connecting brands with Polartec products for over 20 years. “You get a little bit of grit or whatever in your shoes in that moisture and it forms blisters.”

　　Since synthetic moves moisture off the skin so efficiently, it can significantly mitigate the chances of getting painful blisters. My personal favorites for skiing are the Dissent Ski Pro Fit Compression Nano Tour socks. For running, my go-tos are Swiftwick synthetic socks.

　　Do your feet get cold in the winter? Wool’s your answer.  While wool doesn’t dry as quickly as synthetic materials, it stays warm when wet—it’s like neoprene, explained Robert Fry, an expert in material sourcing who was previously in charge of wool product at Mountain Hardwear and The North Face. So if moisture is to blame for your cold feet (and your feet aren’t prone to blistering), then wool might actually help keep your feet warmer because holding the moisture close to your feet allows your body to warm it up.

　　On the other hand, if the moisture is moved away from your feet (i.e. by a moisture-wicking synthetic sock) and gets trapped under the plastic shell of a ski boot or the vapor barrier of mountaineering boots, the cold from the outside can really bring down your boot temp.

　　Do you like drying your feet by a fire on hut trips or in a ski resort parking lot? Choose wool.  “I like drying my feet around a fire,” said Flett. “I dont have to worry about wool melting.” While this one might seem obnoxiously specific, wool’s fire-resistant qualities can be very important to those who use fires to get warm for much of the winter.

　　Are you freaked out by microplastics getting into waterways? Wool’s the only option.  “Id be remiss if I didnt mention microplastics,” said Fry. “Synthetics like air-spun or ring-spun polyester shed a lot of microplastics every time you wash those socks,” said Fry. “Wool doesnt shed the microplastics. Micro-wool isnt harmful the way microplastic is.”

　　Charles Ross, performance sportswear and design lecturer at the Royal College of Art in London, has closely watched the effect of microplastics as well as treated cotton on London’s aging infrastructure and had a particularly interesting story about researching outflow pipes with hundreds of years of waste built up. “When we dug up the outflow pipe, we had synthetic waste, we had cellulose waste, but we had zero wool waste,” said Ross.

　　Still not sure whether to go with wool or synthetic? Go with a wool/synthetic blend.  “The best compromise is by blending wool with a synthetic yarn,” said Ross early in our conversation. When done correctly, there is an opportunity for the materials to deliver the best of both worlds. This blending has to be done very carefully, though, to make sure that the warmth of wool and sturdiness and stretch properties of synthetic are being fully utilized.

　　One way companies achieve this is by putting the bulk of the synthetic materials in the heel and toe to help the socks move and minimize wear, while using wool elsewhere to allow the bulk of the foot to stay warm. Another method manufacturers use  is to place soft wool content next to skin to enhance warmth and synthetic away from the skin to increase the sock’s durability.

　　On the flipside, poorly blended fibers can offer the worst of both worlds. Some of the original wool/synthetic blends are pure garbage because their only goal was to cut down on the amount of wool going into the sock. They were blended in such a way that they held onto moisture and odor.

　　Be wary of socks that use an “intimate blend,” a process in which wool and synthetic fibers are woven together equally, sometimes strand by strand, which makes the end product take on some of the worst properties of both ingredients.