Underwear Claims to Be Thin, Warm & Self-Heating, but Tests Show It’s Only Thin

　　Recently, some manufacturers' "self-heating" thermal underwear has been selling well in the market. This type of "thin-warm underwear" claims that its fabric can "self-generate heat" and is much thinner than bulky regular thermal underwear, attracting the attention of many consumers. However, some readers called the lifestyle lab column of our newspaper hotline, expressing doubts about the "self-heating" effect of the underwear. Are "self-heating" underwear high-tech or just a marketing gimmick by manufacturers? Yesterday, reporters conducted a series of experiments and found that the so-called "self-heating" underwear does not actually raise the temperature of objects. Due to its thinness and other factors, these "thin-warm" underwear do not provide as good thermal insulation as ordinary pure cotton underwear.

　　Reporters found that whether in large department stores or online shops, thermal underwear labeled with "self-heating," "Thermolite," and similar tags are selling the best. On the online store of a famous brand, the monthly sales of a women's thermal insulation underwear reached over 20,000 pieces. These "self-heating" thermal underwear claim to have two main advantages over regular thermal underwear: one is "warmth," and the other is "thinness." That is, the so-called "thin-warm."

　　A sales customer service representative from a Taobao online shop recommended a thinner thermal underwear to reporters. "This is high-tech, mainly composed of heating fibers. Despite being thin, it feels light and warm when worn." But when asked about the heating principle, the sales representative was vague, only saying that it would feel a bit warmer than regular underwear. The description of the "heating" function on this underwear states: "The fiber itself absorbs moisture emitted from the body and converts it into heat energy, warming the material itself." In addition, this underwear also offers thermal insulation, moisture retention, and anti-static features.

　　Reader Mr. Wang purchased a set of such thermal underwear online. "It is indeed very thin, very light, and the fabric is comfortable. It even seems to emit a faint warmth. But once the wind blows, this underwear is not very effective," Mr. Wang said of his experience. However, he added that the fabric of this underwear is all synthetic fiber, with no cotton content.

　　So, do "self-heating" underwear really generate heat? Which is warmer between "self-heating" underwear and regular underwear? Reporters conducted a series of experiments.

　　Reporters obtained three types of underwear made of different materials, including this year’s popular "self-heating" underwear, pure cotton underwear, and fleece-lined thick underwear. These three types of underwear come from three different well-known brands. After measurement, reporters found that the single-layer thickness of the "self-heating" underwear is about 0.5 millimeters, the pure cotton underwear is about 1 millimeter, and the fleece-lined thick underwear is about 1.5 millimeters. All three types of underwear felt soft and comfortable to the touch.

　　Reporters then prepared four beakers of the same model. During the experiment, 60 milliliters of hot water were poured into each beaker, allowed to cool naturally to 57°C, and then three of the beakers were placed about 15 centimeters deep into the sleeves of the three types of underwear. The ends of the sleeves wrapped around the beakers were then loosely sealed. The fourth beaker was left unwrapped. Under the same conditions, the four beakers were placed together on a wooden floor. The indoor temperature at that time was 26°C.

　　After 20 minutes, reporters measured the temperature of the water in the beakers inside the sleeves. A metal thermometer with a minimum scale of 0.1°C was used. After measuring, it was found that the water temperature in the unwrapped beaker dropped to 36.3°C, the "self-heating" underwear’s beaker dropped to 40.7°C, the pure cotton underwear’s beaker dropped to 42.5°C, and the fleece-lined thick underwear’s beaker dropped to 42.2°C.

　　It can be concluded that all three types of underwear in the experiment—the "self-heating" type, pure cotton, and fleece-lined thick—provided thermal insulation. However, compared to the others, the "self-heating" underwear did not actually "generate heat," and its thermal insulation performance was not better than that of ordinary pure cotton underwear or fleece-lined thick underwear.

　　An industry insider said that the "thermal insulation" claimed by thermal underwear is actually just heat retention. Fleece and cotton underwear rely on thickness for warmth—the thicker the material, the better the insulation. Fleece underwear provides insulation because the large gaps between fibers create an air layer that prevents heat loss. As for so-called "self-heating" underwear, its principle mainly involves using the body’s own released heat and then reflecting it back to the body. She said this type of fabric appeared several years ago and has been widely used in the thermal underwear field, with "Thermolite" being a common example.

　　"Thermolite," also known as far-infrared emissive acrylic, is a filament that uses advanced jet technology to blend far-infrared emissive ceramics into acrylic fibers. This material can reflect the body’s own heat back through the underwear, achieving a warming effect. Industry insiders say that under the same thickness, this type of underwear provides better insulation than regular fabric, but because it is relatively thin, its insulation is limited. This is why Mr. Wang said it doesn’t feel warm when there’s wind. Therefore, it’s clear that this type of "thin-warm" underwear struggles to keep warm in Beijing’s bitterly cold winter.

　　The industry insider mentioned that commonly used materials in thermal underwear, such as "Thermolite" and polyester fibers, are all chemical fibers. She said wearing synthetic fabrics may cause some discomfort, so some manufacturers apply anti-static treatments or even add moisture retention features to ensure comfort. However, such thermal underwear tends to be more expensive.