09:00The desk
Eight hours. One chair. Zero airflow.
In one study, just sitting raised scrotal temperature by 2.1°C within an hour. Nobody tells you that sitting still quietly turns up the heat.
Sheynkin et al., 2005
It's called NumbNuts.We know.Hear us out.
Your day
runs hot.
Keep them cool.
They're built to sit 2 to 4°C below your core. Then you spend eight hours on a chair with a laptop on your lap. NumbNuts is a 100% merino boxer with a reusable phase-change insert that sits right where the heat builds. Keep cool whenever you overheat.
60-day Keep Your Pants GuaranteeShips Nov 2026No subscription
Thermal scan
Centre sleeveWhere the heat builds
Insert sits here Drag CoolHot
Engineered for
the hot zone.
One boxer. Two inserts. Zero fuss. Slide one in, wear your hot hours, reset it, repeat.
01Natural-rubber waistbandNo synthetic elastic against you.
02100% merino bodyFine, flat jersey that breathes.
03Centre sleeveHolds the insert where heat builds. Never on skin.
04Phase-change insertAbsorbs body heat as it softens. Reusable.
01
Slide it in.
A fully solid insert goes into the merino centre sleeve.
02
Wear your hot hours.
Desk, drive, commute, heatwave. It absorbs heat as it gradually softens.
03
Reset. Repeat.
Two inserts, so one is always ready. Not for sleeping in.
01 · Waistband02 · Merino body03 · Centre sleeve04 · PCM insert ×2No drugs, no hormones
01 / The problem nobody mentions
The desk.The laptop.The heated seat.The sauna.The heatwave.Your nuts havehad enough.
None of it is your fault. All of it is fixable.
A normal day,
measured.
02 / Measured rise in scrotal temperature
+2.1°CJust sitting · 60 min
Sources: Sheynkin et al., Human Reproduction, 2005 · Garolla et al., Human Reproduction, 2013
14:00The laptop
Warm for you. Warmer for them.
With a laptop on the lap it was 2.6 to 2.8°C within the hour. The study measured temperature, not sperm, but the direction is clear.
Sheynkin et al., 2005
19:30The sauna
The sauna you earned.
Twice-weekly saunas cut sperm counts in a small study of 10 men. The good news: they fully recovered six months after the men stopped.
Garolla et al., 2013 · small study
TomorrowThe fix
Same day. Colder.
Merino against you. Room to breathe. A phase-change insert in the centre sleeve that absorbs body heat as it softens. Cut the heat: in the studies, the dip tended to reverse.
Status · Hot
Tight.
Synthetic.
Trapped.
Most underwear is polyester or nylon in a cut that holds everything against you all day.
Status · Cold
Merino.
Room.
Cooling.
A natural fibre that breathes, and a reusable insert that sits exactly where heat builds.
03 / The receipts
We didn't
make this up.
Heat and plastic aren't our theory. Scientists have been measuring both for decades. Here's what they found. And, because we're not that kind of brand, what each study doesn't prove.
01 · Cold12 wks
They cooled men's testes every night. Counts went up.
What they foundTwo studies of men with low sperm counts. A gentle air stream cooled the scrotum by about 1°C overnight. After 12 weeks, sperm concentration and total count rose significantly in both.
Jung et al. · Reproduction, 2001 · Int J Androl, 2005
What it doesn't prove
Small groups of men who already had low counts, overnight air cooling rather than underwear, and no randomised control. NumbNuts wasn't tested.
02 · Polyester14/14
A polyester sling took sperm to zero. In every man.
What they foundFourteen men wore a polyester sling around the scrotum, day and night, as a contraception experiment. All 14 reached zero sperm in about 140 days, and their testes ran warmer. Counts came back around five months after they stopped.
Shafik · Contraception, 1992
What it doesn't prove
A tight sling worn round the clock for a year isn't a pair of boxers. One researcher, 14 men, 1992. Nobody has shown everyday polyester underwear does this.
03 · Testes23/23
Microplastics in every human testis they tested.
What they foundResearchers tested 23 human testes for 12 types of plastic. Every single sample contained microplastics. So did all 47 dog testes they tested alongside.
Hu et al. · Toxicological Sciences, 2024
What it doesn't prove
Finding plastic isn't the same as finding harm. And nobody knows exactly how it got there.
04 · Semen113
More types of plastic. Lower counts.
What they foundMicroplastics were in the semen of all 113 men tested. Each extra type of plastic a man carried was linked with lower sperm count, concentration and movement.
Zhang et al. · eBioMedicine, 2024
What it doesn't prove
A link at one point in time, not cause and effect. The strongest signal was PTFE, the non-stick pan coating, not polyester.
05 · IVF14/18
It's in IVF, too.
What they foundMicroplastics turned up in the follicular fluid, the fluid around the egg, of 14 of 18 women having IVF. At Europe's biggest fertility conference in 2025, another team reported them in 69% of follicular fluid samples and 55% of semen samples.
Montano et al. · Ecotoxicol Environ Saf, 2025 · ESHRE, 2025
What it doesn't prove
The IVF study found no link with fertilisation, miscarriage or live birth. The conference data is early and didn't measure fertility.
0%
Get yours · $69→
Polyester is plastic. Ours isn't.
100% merino body and sleeve, a natural-rubber waistband, and a cooling insert for your hottest hours. You can't control every exposure. You can choose what sits against you, and how hot it gets.
The full research: 15 questions, 39 studies
The fine print, in plain English: NumbNuts is a comfort product. It wasn't used in any of these studies and doesn't diagnose, treat or prevent anything. Trying for a while, or heading into IVF? Your GP or clinic is the right first call. This is the bit you can do alongside it.
Frozen peas
aren't a plan.
| NumbNuts | Polyester boxers | Frozen peas | Do nothing | |
|---|---|---|---|---|
| Natural fibre against you | ● | ○ | – | – |
| Cooling exactly where heat builds | ● | ○ | Messy | ○ |
| Wearable at the desk, in the car, on the train | ● | ● | ○ | ● |
| Never directly on skin | ● | – | Ice-burn risk | – |
| Survives a two-hour meeting | ● | ● | Defrosts by 10:15 | ● |
| Reusable, day after day | ● | ● | Until dinner | – |
| Money back if it's not for you | 60 days | – | – | – |
04 / Why it matters
You can't control much about making a baby. Temperature is one thing you can.
Sperm take roughly ten weeks to develop, so the habits you keep this week are the ones that count in a couple of months. Trying now, planning later, or heading into IVF.
Start todayToday's batch is ready around ten weeks from now
Founder · Jaspal Rai
05 / Why I built it
The only advice men get about heat is "take a break" or "try frozen peas". I wanted something you'd actually wear on a normal day, from a brand that tells you the truth about the science.
Jaspal Rai · Founder, NumbNuts
All the risk
is ours.
Today
Pre-order.
Lock the founder price. One payment. No subscription, ever.
Until dispatch
Change your mind.
Full refund any time before it ships. Just email us.
Delivery
Wear it.
Through your hottest hours. Desk, drive, gym, heatwave.
Day 60
Keep it or don't.
Not for you? Full refund, no worn return. First bundle, one claim per customer.
Founder release 01Product visualisationJoin the NumbNuts Club
The Cool System
$69Founder pre-orderBuy 2 · save $20
- ×1100% merino boxer with centre sleeve
- ×2Reusable phase-change cooling inserts
- 60dKeep Your Pants Guarantee
- UK/USFree delivery
Choose your size
| Size | Waist (in) | Waist (cm) |
|---|---|---|
| S | 28 to 30 | 71 to 76 |
| M | 32 to 34 | 81 to 86 |
| L | 36 to 38 | 91 to 97 |
| XL | 40 to 42 | 102 to 107 |
| XXL | 44 to 46 | 112 to 117 |
Between sizes? Size up. The cut is roomy on purpose.
Ships November 2026 · One payment · No subscription
Keep Your Pants Guarantee. Full refund before it ships, then 60 days from delivery. No worn return.
Not ready? Start with The Cool Protocol, our research-backed 90-day plan, $5.99.
Straight
answers.
What exactly is NumbNuts?
A 100% merino wool boxer with a centre sleeve that holds a reusable phase-change cooling insert. You wear the insert through your hottest hours, like a desk day, a long drive or a heatwave, then reset it. Each kit comes with two inserts.
Is it a fertility product?
No. NumbNuts is a comfort product. It doesn't diagnose, treat or prevent anything, and no underwear is proven to improve fertility. We share the published research on heat so you can make your own call. If you're worried about fertility, speak to your GP.
Does cooling actually do anything for sperm?
In two small studies, men with low sperm counts had their scrotum cooled overnight for 12 weeks and their counts rose significantly (Jung et al., 2001 and 2005). Those studies used an air-cooling device, not underwear, and had no randomised control. NumbNuts hasn't been tested for fertility. What we can say is that the insert absorbs heat right where it builds.
Is polyester underwear bad for sperm?
Nobody has shown that everyday polyester underwear harms sperm. What the research does show: a 1992 experiment where a polyester sling worn day and night took sperm to zero in all 14 men (Shafik, 1992), and microplastics turning up in human testes and semen (Hu et al., 2024; Zhang et al., 2024). We'd rather not wear plastic against the warmest part of the body, so our boxer body and sleeve are 100% merino.
How do the inserts work?
They're made from a phase-change material that absorbs body heat as it gradually softens. Put a fully solid insert in the sleeve, wear it, take it out after your session and reset it using the supplied instructions. Never put it directly on skin, and don't sleep in it.
When does it ship, and what does delivery cost?
The first production run is scheduled to ship in November 2026, with tracking. Pre-order now and you can cancel for a full refund any time before it ships. Delivery to the UK and US is free on orders of US$65 or more, or the local equivalent, which includes a single kit.
What if it's not for me?
The Keep Your Pants Guarantee. Cancel for a full refund any time before your order ships. After delivery, you have 60 days to ask for a refund on your first bundle, with no worn return needed. One claim per customer.
Is it a subscription?
No. One payment, and that's it. You'll never be signed up to anything.
Not ready?
Read first.
All 16 guidesGuide 01
Do Microplastics Affect Sperm? What Studies of Testicles and Semen Actually Found
Microplastics have been found in human testicles, semen and follicular fluid, and a few studies link more p...
Guide 02Merino vs Cotton vs Polyester Underwear: Which Is Best for Heat and Comfort?
For heat, fit matters more than fabric. Roomier underwear is linked with slightly better sperm counts in so...
Guide 03How Can You Improve Sperm Quality Before IVF or ICSI? What Helps in 2, 6 or 12 Weeks
Sperm take roughly two months to develop, so changes made 12 weeks before IVF or ICSI shape the sample coll...
Guide 04Are Heated Car Seats Bad for Sperm? What the Studies Actually Measured
Heated car seats warm the scrotum slightly: in one experiment, 90 minutes on a heated seat left it about ha...
Founder release 01 · ships November 2026
Stop sitting
in it.
Keep your core hot. Keep them cool.
NumbNuts is a comfort product, not a medical device. It is not intended to diagnose, treat, cure or prevent any disease or condition. Figures describe published research, not results from this product. Product and thermal images are visualisations. © 2026 One Third Group · The science · Guides · Heat Check · Pre-order terms · Refunds · Shipping · Privacy · Contact · Instagram · TikTok
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