A yak's undercoat, known as 'khullu,' is warmer than sheep's wool and as soft as premium cashmere.

The Tibetan yak (Bos grunniens) survives at altitudes between 10,000 and 18,000 feet, where winter temperatures regularly plummet below -40°F. To endure these extremes, the animal grows a dense secondary coat of downy fiber called khullu. This fiber typically measures between 16 and 20 microns in diameter, placing it in the same fineness category as high-grade cashmere. Unlike sheep wool, yak fiber is hollow, which significantly increases its thermal insulation properties by trapping air within the fiber itself.Research conducted by the International Yak Association and various textile laboratories shows that yak down is approximately 10% to 15% warmer than Merino wool of the same weight. Because yaks do not produce lanolin, the waxy substance found in sheep wool, the fiber does not require harsh chemical scouring during processing. This makes the final product naturally hypoallergenic and better for sensitive skin. The absence of lanolin also means the fiber is less prone to attracting dust and odors compared to traditional wool products.In the early 2000s, companies like Shokay and Tengri began formalizing the supply chain for yak fiber to provide sustainable income for Himalayan nomadic communities. Unlike cashmere goats, which have sharp hooves that contribute to desertification in Mongolia and China, yaks have a lighter environmental footprint on the grasslands. A single adult yak produces only about 100 grams to 500 grams of khullu per year, which is harvested by hand-combing the animal during the spring molting season. This rarity and high performance have earned it the nickname 'soft gold' in the global textile market.