Short answer
For heat, fit matters more than fabric. Roomier underwear is linked with slightly better sperm counts in some studies, but no fibre, merino included, has been shown to improve fertility. On comfort: polyester smelled worse than cotton after a sweaty workout in one study, and fine merino did well for sensitive skin in one small, wool-industry-funded trial. Choose on fit, skin and how you spend your day.
It's a surprisingly heated debate for something nobody else sees. Here's what the evidence says about each fibre, and where fabric marketing runs ahead of it. We sell merino, so we've tried to be extra strict with ourselves.
Does underwear fabric change how hot you get?
Less than you'd think. A review of clothing and exercise found that neither modest amounts of clothing nor fabric type changed body temperature regulation or thermal comfort during exercise in warm conditions (Gavin, 2003). A 2026 review of underwear and fertility found that evidence on fabric composition is limited, and that controlled human studies comparing materials in normal wear are lacking (Zleczewski et al., 2026).
Does fit matter more than fabric?
Mínguez-Alarcón et al., 2018: 656 men at a fertility centre
What the research found: Men who mostly wore boxers had 25% higher sperm concentration, 17% higher total count and 14% lower FSH than men who didn't. The authors suggest the higher FSH may reflect the body compensating for poorer sperm production (Mínguez-Alarcón et al., 2018).
What it doesn't prove: It was a one-off snapshot of self-reported habits, so it can't show that boxers caused the difference. The authors also flagged that they had no information on trouser type or the fabric of the underwear.
Studies of actual pregnancies are calmer. In 501 couples, underwear choice made no significant difference to time to pregnancy (Sapra et al., 2016), and in 3,041 couples tight underwear showed little association with the chance of conceiving (McKinnon et al., 2022). NICE says raised scrotal temperature is linked with reduced semen quality, but it's uncertain whether loose-fitting underwear improves fertility (NICE, 2026). Our boxers vs briefs guide goes deeper.
Is polyester underwear bad for sperm?
Shafik, 1992: the polyester sling
What the research found: Fourteen men wore a polyester scrotal sling, day and night, for a year, as a contraceptive. All of them ended up with no sperm in their semen, after an average of about 140 days, and counts returned to normal around 157 days after the sling came off. The author suggested static electricity from the fabric and disrupted temperature control as causes (Shafik, 1992).
What it doesn't prove: A scrotal sling worn round the clock is not a pair of boxers. The same author reported a similar effect in dogs wearing loose polyester pants (Shafik, 1993), but controlled human studies comparing fabrics in normal wear are lacking (Zleczewski et al., 2026). Our polyester and sperm count guide covers the microplastics side.
Two better-supported points about polyester: after a spinning class, polyester T-shirts smelled significantly less pleasant and more intense than cotton ones, with different bacteria growing on them (Callewaert et al., 2014). And polyester fabric sheds more microfibres in the wash than a polyester-cotton blend (Napper and Thompson, 2016).
Is merino or cotton better for sweat and skin?
Cotton comes out of the odour study well. Merino has more research behind it than you'd expect, with a big caveat: the human trials we found were funded by the wool industry.
- Sleep in the heat. In 36 adults aged 50 to 70 sleeping at 30°C, merino sleepwear gave small benefits, and older participants fell asleep faster in wool (12 minutes) than cotton (27) or polyester (22) (Chow et al., 2019).
- Sensitive skin. In 50 children and adults with mild to moderate eczema, six weeks in fine merino improved eczema scores and quality of life compared with their usual clothing, with no negative skin effects (Fowler et al., 2019).
- The itch question. Fabric prickle comes from stiff fibre ends pressing into skin (Garnsworthy et al., 1988), and the eczema trial's authors note its strength is strongly related to fibre diameter. That's why the eczema trial used fine merino of 17.5 microns or less, not scratchy-jumper wool.
| Fibre | What studies show | Not proven |
|---|---|---|
| Cotton | Smelled less than polyester after exercise | Better for sperm |
| Polyester | Smelled worse after exercise; sheds microfibres in the wash | Everyday boxers lower sperm count |
| Merino | Small industry-funded trials: faster sleep onset, fewer eczema symptoms | Cools you down, or helps sperm |
So which underwear should you choose?
- Fit first: room rather than compression, especially on long sitting days.
- Pick a fibre that suits your skin and your day. There's no wrong answer on fertility grounds.
- Change after sweaty sessions, particularly if you wear synthetics.
- Cut the bigger heat sources first: saunas, hot tubs, long baths and a laptop on your lap.
- Trying for a while? A semen analysis via your GP tells you more than any label.
Full disclosure: NumbNuts makes a 100% merino wool boxer with a centre sleeve for a reusable cooling insert. We chose merino for comfort and sell the kit as comfort. It hasn't been shown to improve sperm count or fertility, and no underwear has. If you're curious, you can reserve a founder kit.
To see where heat really comes from in your week, take the free five-question Heat Check, an educational habits quiz rather than a fertility test. Every source we cite lives in our research hub.
Frequently asked questions
What is the best underwear for male fertility?
No underwear is proven to improve fertility. Looser fits are linked with slightly better sperm counts in some studies, not with more pregnancies.
Is merino underwear too hot for summer?
Not necessarily. In a small, wool-industry-funded study at 30°C, merino sleepwear slightly improved sleep compared with cotton and polyester.
Does polyester underwear cause infertility?
There's no evidence that everyday polyester boxers do. The famous study used a polyester scrotal sling worn day and night.
Is cotton or merino better for sweaty days?
Cotton beat polyester on odour in one study, and we found no comparable merino study. Choose whichever feels better on your skin.
References
- Gavin TP. Clothing and thermoregulation during exercise. Sports Med, 2003. PMID 14606923
- Zleczewski M et al. Garment fit, scrotal thermoregulation, and male fertility outcomes: a narrative review. Transl Androl Urol, 2026. PMID 42293842
- Mínguez-Alarcón L et al. Type of underwear worn and markers of testicular function among men attending a fertility center. Hum Reprod, 2018. PMID 30102388
- Sapra KJ et al. Choice of underwear and male fecundity in a preconception cohort of couples. Andrology, 2016. PMID 26939021
- McKinnon CJ et al. Male personal heat exposures and fecundability. Andrology, 2022. PMID 35924639
- NICE. Fertility problems: assessment and treatment (NG257), 2026. nice.org.uk
- Shafik A. Contraceptive efficacy of polyester-induced azoospermia in normal men. Contraception, 1992. PMID 1623716
- Shafik A. Effect of different types of textile fabric on spermatogenesis: an experimental study. Urol Res, 1993. PMID 8279095
- Callewaert C et al. Microbial odor profile of polyester and cotton clothes after a fitness session. Appl Environ Microbiol, 2014. PMID 25128346
- Napper IE, Thompson RC. Release of synthetic microplastic plastic fibres from domestic washing machines. Mar Pollut Bull, 2016. PMID 27686821
- Chow CM et al. The impact of sleepwear fiber type on sleep quality under warm ambient conditions. Nat Sci Sleep, 2019. PMID 31692485
- Fowler JF et al. Effects of Merino wool on atopic dermatitis. Dermatitis, 2019. PMID 30864970
- Garnsworthy RK et al. Identification of the physical stimulus and the neural basis of fabric-evoked prickle. J Neurophysiol, 1988. PMID 3373270
Last reviewed 24 September 2026.
