The Science
Your body already knows this
Testes sit outside the body for one reason: sperm production works best at 2 to 4 degrees Celsius below core body temperature. It's the only system in the body with its own external cooling design. Heat is the variable it's most sensitive to.
What the research shows
In a 2013 study published in Human Reproduction (Oxford), Garolla and colleagues followed healthy men through three months of regular Finnish sauna use: two sessions per week, 80 to 90 degrees Celsius, 15 minutes at a time. The results: significant reductions in sperm count and motility. The effects were temporary, with values returning to baseline roughly six months after stopping.
Two honest takeaways: the effect is real and measurable in healthy men, and it's reversible, but only during time away from heat. For someone in the sauna several times a week, there is no recovery window.
The paradox
Sauna is one of the best-evidenced longevity habits there is. The problem was never the sauna. It's that whole-body heat includes one region designed to stay cool.
What KOLDWR does
KOLDWR Pro is a physical solution to a physical problem: boxer briefs with swappable cooling inserts that hold temperatures down in the one area built to run cooler, while the rest of your body gets the full heat. 30+ minutes of active cooling per insert at sauna temperatures.
That's the entire claim: we keep the temperature down where it should be low. What your body does with that is its own business.
Source: Garolla A, et al. "Seminal and molecular evidence that sauna exposure affects human spermatogenesis." Human Reproduction, 2013.