By Don Jurries
Japanese brand Mizuno recently began to globally promote their Breath Thermo fiber in a variety of apparel products, including baselayers, arm warmers, socks, leggings and gloves. It’s been a long-time coming for what appears to be an incredibly effective concept for heat-generation in clothing. The Mizuno brand has always meant athletic shoes to me, being my turf shoe of choice for years. Given a mid-weight baselayer featuring the Breath Thermo fiber at ISPO, I am now a huge fan of their apparel line as well.
I spent numerous days skiing in the Mizuno Breath Thermo mid-weight baselayer during the recent cold snap in Europe, with temperatures down below 0ºF (between -15º and -20ºC). Over top of the baselayer, I wore a 100% merino wool mid-layer and the same decade-old Quechua ski jacket I always use for comparability and consistency in testing. While my gloves and ski socks started giving out at the coldest temperatures, my torso was never cold.
The Breath Thermo fiber has actually been around since 1994. Jointly created by Mizuno and Toyobo, a Japanese textile maker, the heat generating fiber was knitted into the official uniforms worn by the Japanese team at the Lillihammer Olympics.
The Breath Thermo fiber uses your own body moisture, or sweat, to generate heat. This may seem counter-intuitive, as wet clothing normally equates to becoming cold, dangerous in extreme conditions. Water molecules, with a slight positive electrical charge, form hydrogen bonds with many common fibers to then be retained in the fabric. Cotton fiber, for example, can absorb well over 20 times its own weight.
Made from a type of polyacrylate, the Breath Thermo fiber is designed to simply not expand. As the water molecules bump into the fiber, they can’t form hydrogen bonds, thus creating friction instead. Friction creates heat. The calorific value of this heat is about 3x’s greater than wool or down.
The Mizuno team demonstrates this heat generating quality by putting a small clump of the pure Breath Thermo fiber in your hand, sprays water on it, then has you close your hand around it. Immediately I felt a warmth about the equivalent of a heat pack. At ISPO, Mizuno claimed this was about 3-4ºF degrees (2ºC) warmer than your own body temperature.
While apparel with 100% of the Breath Thermo fiber would be too warm for most applications, the Breath Thermo instead gets blended in with other fibers, primarily a quick-drying performance polyester that wicks the moisture away and includes pH control (garments get more alkaline when washed) as well as anti-bacterial properties.
In the US, the Mizuno Breath Thermo line is still labeled as Running apparel. A long-sleeve running crew made from the Breath Thermo fabric retails for around $60. A running jacket up to $125. I hope the labeling changes soon, as Mizuno realizes the full potential of the heat generating fabric for a broader variety of cold weather activities.