The science15 questions39 studies, all linkedLast reviewed
Heat, polyester
and your sperm. The research, straight.
Laptops, saunas, polyester, microplastics, ice packs, cold plunges. Men are searching all of it, and most answers online are either scarier or simpler than the evidence. Here's what each study actually measured, what it doesn't prove, and what's worth doing about it. Nobody taught us this either.
By the NumbNuts teamFounder: Jaspal RaiInformation, not medical advice
The short version
Six things worth knowing- Your testes work best about 2 to 4°C below core temperature. Heat pushes them out of that zone. How heat works
- A laptop on your lap, a long drive or twice-weekly saunas measurably heat them or lower counts. Where men were followed up, counts mostly came back. The numbers
- New sperm take about ten weeks to make, so a change you make today shows up in a test about three months later. The 74-day rule
- Polyester and microplastics: striking findings, but no proof that everyday pants harm fertility. Pants and plastic
- Ice packs, cold plunges and cold showers have no good evidence behind them for sperm or testosterone. The cooling studies used a gentle 1°C drop. Cold
- Trying for a year? NICE says both of you should be offered tests. Sooner if your partner is 36 or over. Getting tested
01Heat
Does heat affect sperm?
Short answer
Yes. Your testes hang outside the body because sperm production works best about 2 to 4°C below core temperature (Maroto 2025). Studies have measured sitting, laptops and driving pushing scrotal temperature up by around 2°C, and regular saunas lowering counts. Where researchers followed men after the heat stopped, counts mostly came back.
That 2 to 4°C gap is the headroom, and your day spends it. Heat comes from outside (a chair, a laptop, a heated seat, a sauna) and from inside (a fever). Researchers measure it with small sensors taped to the scrotum, and every number we could verify is in the heat sources table below.
NICE, which writes health guidance for England, puts it carefully: raised scrotal temperature is linked with poorer semen quality, but it's uncertain whether looser pants improve fertility (NG257, 1.11.1). That's the honest shape of the evidence. Heat is a real, measurable stress. It's also one factor among several, and the effect is usually reversible.
Why is everyone suddenly asking? Partly because of a 2023 analysis of 223 studies, which estimated that average sperm concentration fell by 51.6% between 1973 and 2018 in men who weren't selected for fertility (Levine 2023). It doesn't pin that on any one cause, heat included. Weather plays a part too: in 33,234 sperm donors, hotter-than-usual spells in the hot season were linked with lower concentration (Xiao 2024). More on that in our guide to summer heat and sperm.
Testes normally sit around 2 to 4°C below body temperature. Heat raises germ-cell death, oxidative stress and DNA damage in the testes.
A rise in temperature isn't a diagnosis. Most heat studies are small and measured temperature or semen, not pregnancies.
DataMeasured, not guessed
Heat sources, measured.
Every everyday heat source we could find a real measurement for, in one place. Your headroom is about 2 to 4°C (Maroto 2025). The bars show temperature rises on a 0 to 3°C scale.
| Heat source | What was measured | Result | Study |
|---|---|---|---|
| Laptop on lap, thighs together, 60 min | Scrotal temperature | +2.6 to 2.8°C | Sheynkin 2005 29 men |
| Sitting with thighs together, no laptop, 60 min | Scrotal temperature | +2.1°C | Sheynkin 2005 29 men |
| Laptop on a lap pad, thighs together, 60 min | Scrotal temperature | +2.1 to 2.2°CThe pad didn't protect | Sheynkin 2011 29 men |
| Laptop on a lap pad, legs apart, 60 min | Scrotal temperature | +1.4 to 1.5°CHalf the rise, slower | Sheynkin 2011 29 men |
| Driving for 2 hours, vs walking | Scrotal temperature | +1.7 to 2.2°C | Bujan 2000 9 men |
| Heated car seat for 90 min, vs unheated | Scrotal temperature | +0.5 to 0.6°COn top of sitting | Jung 2008 30 men |
| Sauna, 2 × 15 min a week at 80 to 90°C, 3 months | Sperm count and motility | Fell sharplyNormal again 6 months after stopping | Garolla 2013 10 men |
| Regular hot tubs or hot baths, then stopping | Total motile sperm count | +491%After stopping, in 5 of 11 men | Shefi 2007 11 men |
| Fever during sperm production | Sperm concentration | −32.6 to 35%Temporary | Carlsen 2003 27 men |
| Polyester sling, day and night, 12 months | Sperm in semen | Zero in 14 of 14Back about 157 days after stopping | Shafik 1992 14 men |
| For comparison: gentle overnight air cooling, 12 weeks | Scrotal temperature | −0.8 to 1°CConcentration rose in men with low counts | Jung 2001 · Jung 2005 Two small studies |
02Laptops
Does a laptop on your lap affect sperm?
Short answer
It heats them. In 29 healthy men, an hour with a working laptop on the lap raised scrotal temperature by up to 2.8°C, and sitting the same way without one raised it by 2.1°C (Sheynkin 2005). A lap pad didn't stop the rise. Sitting with legs apart helped most. Nobody measured sperm.
The follow-up is the useful study. The same team ran three one-hour sessions: legs together, legs together with a lap pad under the laptop, and legs apart at about 70 degrees with the pad. The rises were up to 2.6°C, then about 2.1 to 2.2°C, then 1.4 to 1.5°C. With legs together it took 11 minutes to warm by 1°C. Legs apart, it took 28 (Sheynkin 2011).
Their conclusion: posture does most of the work, and the pad doesn't protect you. So put the laptop on a desk, and if you must use your lap, open your knees and keep sessions short.
While we're here, the phone. In 2,886 young Swiss men, keeping a phone in a trouser pocket wasn't linked with poorer semen. Heavy use, 20 or more times a day, was linked with a lower total count, though that link faded over the study years as phones changed (Rahban 2023).
+2.6 to 2.8°C with a laptop, +2.1°C just sitting with thighs together. Legs apart cut the rise to about 1.5°C.
These studies measured temperature, not sperm or fertility. They lasted an hour, so they can't tell you what years of laptop use does.
03Driving
Do heated car seats and long drives affect sperm?
Short answer
They warm things up. In nine men, two hours of driving left scrotal temperature 1.7 to 2.2°C higher than walking (Bujan 2000). In 30 men, 90 minutes on a heated seat added another 0.5 to 0.6°C on top of sitting (Jung 2008). Neither study measured sperm, so treat it as a heat source, not a verdict.
The heated-seat study has a detail worth knowing. On the heated seat, men passed the highest temperature reached by sitting alone within the first third of the session, so about half an hour in (Jung 2008).
If you drive for a living, the easy wins are dull and free: seat heater off, a proper stop to stand and walk every couple of hours, and trousers with some room in them. None of that has been tested on sperm. It just takes out heat that was measured.
Two hours of driving: +1.7 to 2.2°C versus walking. A heated seat: an extra 0.5 to 0.6°C over 90 minutes.
Small groups, single sessions, temperature only. No sperm counts, no fertility outcomes.
04Saunas
Does the sauna kill sperm?
Short answer
Not permanently, but it can knock your count down. Ten men with normal semen took two 15-minute saunas a week at 80 to 90°C for three months. Their sperm count and motility fell significantly, and the way sperm DNA was packaged suffered too. Six months after they stopped, everything was back to normal (Garolla 2013).
Their hormones didn't change, so this isn't a testosterone story. It's the sperm factory running hot. The sperm's mitochondria, the bits that power swimming, also worked less well at the end of the three months, and that recovered too (Garolla 2013).
If you're trying for a baby, or you have IVF or a semen test in the next three months, pausing the sauna is the simplest swap on this page. If you're not, it's your call: the effect here was reversible. Nobody has tested shorter or cooler sessions, so there's no known safe dose to quote, and we're not going to invent one.
Sperm count and motility dropped significantly after three months of twice-weekly saunas, then fully recovered within six months of stopping.
Ten men, no control group, all with normal counts to start. Nobody tested a smaller dose, so there isn't a safe one to quote.
05Hot tubs
Do hot tubs and hot baths lower sperm count?
Short answer
They can, and stopping can help. Eleven men at a fertility clinic had regularly used hot tubs, jacuzzis or hot baths. After giving them up, five saw their total motile sperm count rise by 491% on average, mostly because motility jumped from 12% to 34% (Shefi 2007). The other six didn't improve.
The men who didn't bounce back had smoked far more: 5.6 pack-years on average, against 0.11 in the men who improved (Shefi 2007). A useful reminder that heat is rarely the only thing going on.
Water is very good at moving heat into you, and in a hot tub there's nowhere for anything to hang out of the way. If you love a long soak, the evidence here says pause it while you're trying. It doesn't say never again.
Five of 11 men improved after stopping wet heat, with total motile sperm count up 491% on average.
Eleven men who already had fertility problems, and no control group. It shows improvement after stopping, not how much one soak does.
06Fever
Can a fever lower your sperm count?
Short answer
Yes, for a while. In 27 healthy men tracked for 16 months, a fever at a sensitive stage of sperm production cut concentration by about a third, and more days of fever meant a bigger drop (Carlsen 2003). It's temporary: one man followed closely after a fever had a normal count again by day 79 (Sergerie 2007).
A fever is heat from the inside, and you can't swap it for a cooler habit. What you can do is write the date down.
If you have a semen test or an IVF cycle within about three months of a fever, tell the clinic. It can explain a poor result, and they may want to retest or plan the timing differently. That's a reason to tell someone, not a reason to worry.
Concentration fell 32.6 to 35% after a fever during key stages of sperm production. In the man followed closely, it was back to normal by day 79.
Twenty-seven men, plus one detailed case. How much a fever affects any one man varied a lot.
07Polyester
Does polyester underwear lower sperm count?
Short answer
Nobody has shown that everyday polyester pants do. The famous finding is a 1992 contraception experiment: 14 men wore a polyester sling round the scrotum day and night, and all 14 reached zero sperm in about 140 days. Counts came back around 157 days after they stopped (Shafik 1992). A sling isn't a pair of boxers.
Shafik thought two things were going on. The testes ran warmer, and friction between polyester and skin built up static charge, which he measured. The five couples who wanted a baby afterwards all conceived (Shafik 1992).
The same researcher later dressed 12 dogs in loose polyester pants for two years. Their sperm counts fell, while dogs in cotton pants were unchanged, and most recovered once the pants came off (Shafik 1993). One researcher, partly in dogs, never repeated in people.
So the honest version is: striking, plausible, unproven for normal underwear. We'd still rather not wear plastic against the warmest part of the body, but that's a preference, not a proven fix.
All 14 men reached zero sperm wearing a polyester sling round the clock. Counts returned to their starting level after they stopped.
A tight sling worn day and night for a year isn't a pair of pants. One researcher, 14 men, 1992, and never repeated.
08Microplastics
Are microplastics in sperm and testicles harmful?
Short answer
They're there. Whether they cause harm isn't proven yet. Researchers found microplastics in all 23 human testes they tested (Hu 2024), and in the semen of all 113 men in a Chinese study, where each extra type of plastic a man carried was linked with a lower sperm count (Zhang 2024). Those are links, not proof.
A few details rarely make the headlines. In the testes, the most common plastic was polyethylene, the stuff of bags and bottles, and levels in human samples were almost three times those found in dogs (Hu 2024). In semen, the strongest signal came from PTFE, the non-stick pan coating, not polyester (Zhang 2024).
It's not only a male question. Microplastics turned up in the fluid around the egg in 14 of 18 women having IVF, with no link found to fertilisation, miscarriage or live birth (Montano 2025).
You can't avoid plastic entirely, and nobody has shown that cutting your exposure improves sperm. Cutting it where it's easy costs you very little.
Microplastics in 23 of 23 human testes and 113 of 113 semen samples. More types of plastic were linked with lower counts.
Links at one point in time, not cause and effect. Nobody knows yet what level, if any, harms fertility.
09Boxers or briefs
Boxers or briefs: which is better for sperm?
Short answer
Boxers, slightly, going by the biggest study. Among 656 men at a US fertility centre, those who mostly wore boxers had 25% higher sperm concentration and 17% higher total count than men who didn't (Mínguez-Alarcón 2018). But NICE says it's uncertain whether looser pants improve fertility, and a 2026 review agreed.
The Harvard team also found 14% lower FSH in the boxer wearers. FSH is the hormone that pushes the testes harder when output drops, which hints that the tight-pants group was compensating (Mínguez-Alarcón 2018). It was a snapshot, though, with self-reported pants and no data on trousers or fabric.
A 2026 review of the whole field concluded that underwear choice alone is unlikely to meaningfully change fertility, while avoiding excessive heat is sensible general advice (Zleczewski 2026). NICE says the same thing in fewer words (NG257, 1.11.1). Pants are one small dial among several. Worth turning, not worth obsessing over.
Men who mostly wore boxers had 25% higher concentration, 17% higher total count and 14% lower FSH.
A snapshot of men at a fertility clinic, not a trial. Nobody switched pants and got retested, and it says nothing about pregnancy.
Full guideDoes tight underwear affect sperm? Boxers vs briefs →
10Icing
Should you ice your balls?
Short answer
No. There's no good evidence that ice packs raise sperm count or testosterone, and ice straight on skin can cause frostbite. The cooling studies people quote used a gentle overnight air stream that lowered scrotal temperature by about 1°C for 12 weeks (Jung 2001, 2005). That's a long way from a bag of frozen peas.
Those Jung studies are genuinely interesting. Men with low counts had their scrotum cooled every night, and after 12 weeks their sperm concentration and total count rose significantly. But the groups were small, the men already had low counts, the device blew air rather than chilling anything, and nobody tracked pregnancies.
A 2013 review of eight scrotal-cooling studies saw a positive trend, but found too little evidence for firm conclusions (Nikolopoulos 2013). Cooling is worth studying. It isn't proven.
Ice is a different thing entirely. For injuries, the NHS suggests an ice pack or a bag of frozen veg wrapped in a tea towel, for up to 20 minutes at a time (NHS). Doctors have reported full-thickness frostbite from ice packs put straight on skin (Rittblat 2025), and frostbite of the penis and scrotum, while rare, is documented (Tople 2023). Keep them cool, not frozen.
Overnight cooling of about 0.8 to 1°C for 12 weeks raised sperm concentration and total count in men with low counts.
Nothing about ice, cold plunges or men with normal counts. Small, not randomised, no pregnancy data.
Full guideShould you ice your testicles? What's real and what's hype →
11Cold plunges
Do cold plunges or cold showers boost sperm count or testosterone?
Short answer
There's no good evidence they do. In 11 male athletes, four days of 10-minute cold showers made no significant difference to testosterone (Traylor 2026), and a 2025 review found cold-water immersion's effects on testosterone inconsistent (Qaoud 2025). We couldn't find a study showing a plunge raises sperm count.
The largest review of cold-water immersion so far pooled 11 trials and 3,177 people. It found lower stress 12 hours after a plunge and better sleep quality, plus a spike in inflammation straight afterwards. It didn't measure sperm or testosterone at all (Cain 2025).
A review of testosterone strategies in athletes went further and suggested cold-water immersion may lower testosterone (Lazarev 2026). And colder isn't automatically better for sperm: in the study of 33,234 sperm donors, unusually cold weather was linked with poorer sperm quality as well as unusually hot weather (Xiao 2024).
If you like plunging, plunge. It may help your head. Just don't do it for your sperm count.
No significant change in testosterone after four days of cold showers. Reviews call cold-water effects on testosterone inconsistent, and sperm wasn't studied.
Absence of evidence, mostly from small studies. A longer, bigger trial could still find something. Nobody has properly tested it.
12Spermmaxxing
What is spermmaxxing, and what actually works?
Short answer
Spermmaxxing is the social media trend of optimising your sperm, from sensible habits to icing and supplements. The parts with evidence are dull but real: stop overheating them, don't smoke, drink within 14 units a week, sleep well, stay a healthy weight, and never take testosterone if you want children. Most supplements don't help.
Here's the evidence behind the dull list, so you can skip the garlic.
- Heat. Everything above. For many men it's the biggest and most reversible dial.
- Smoking. A meta-analysis of 5,865 men linked smoking with lower sperm count, movement and shape, and the effect was bigger in heavier smokers (Sharma 2016).
- Alcohol. NICE says drinking within 14 units a week, spread over several days, is unlikely to affect semen quality. Heavy drinking is the problem (NG257, 1.6.2).
- Cannabis. In 1,215 young Danish men, smoking it more than once a week was linked with 28% lower concentration (Gundersen 2015).
- Weight. Across 21 studies of 13,077 men, obesity was linked with higher odds of a low or zero sperm count (Sermondade 2013).
- Sleep. In 842 men, under six hours a night or poor-quality sleep was linked with lower motility (Chen 2020).
- Testosterone. Taking it from outside tells your testes to stand down. In a WHO trial, 65% of men on weekly testosterone injections reached zero sperm within six months (WHO 1990). NICE asks doctors to check for TRT, anabolic steroids, finasteride and GLP-1 medicines (NG257, 1.13.1). Don't stop a prescribed medicine without talking to whoever prescribed it.
- Supplements. In a trial of 2,370 couples, folic acid and zinc for six months didn't improve live births or most semen measures (Schisterman 2020).
Smoking, heavy drinking, regular cannabis, obesity, poor sleep and outside testosterone are all linked with poorer semen. Folic acid and zinc didn't help.
Most of these are links from observational studies. Changing a habit improves your odds. It can't guarantee a result.
Full guideSpermmaxxing: what actually works, and what is just dangerous →
1374 days
How long does it take for sperm to regenerate?
Short answer
About ten weeks, start to finish. The classic 1963 estimate was around 64 days (Heller & Clermont 1963), and when researchers labelled new sperm in 11 men, it reached semen after 64 days on average, anywhere from 42 to 76 (Misell 2006). Drug trials use 74 days as one cycle. Changes show up about three months later.
That's where the "74-day rule" comes from. It's the figure drug trials use as one full human cycle when they set contraception rules (Banholzer 2012). It's also why NICE says to repeat an abnormal semen test ideally three months after the first, to give a full cycle time to finish (NG257, 1.17.4).
The hopeful part: the sperm you'll have in three months are being made in the conditions you live in now. The chair, the car, the sauna, the duvet. It cuts both ways, too. Heat damage isn't instant, and it isn't instantly undone. Give any change one full cycle before you judge it.
New sperm reached semen after 64 days on average, with real variation between men: 42 to 76 days.
These are averages from small groups. Your cycle may run a little faster or slower, so plan on three months, not exactly 74 days.
Full guideThe 74-day rule: how long sperm takes to regenerate →
14Protect them
How can I protect my sperm from heat?
Short answer
Take out the big heat sources first. Pause saunas, hot tubs and long hot baths while you're trying. Put the laptop on a desk, sit with your legs apart, get up regularly and switch off the seat heater. Wear looser, breathable pants and sleep cooler. Then give it about three months, one full sperm cycle.
In rough order of how much heat was measured:
- Sauna, hot tub, long hot baths. Pause them while you're trying. These are the only heat sources on this page with sperm results attached (Garolla 2013; Shefi 2007).
- Laptop. On a desk, not your lap. If you can't, knees apart and shorter sessions (Sheynkin 2011).
- Desk and driving. Stand up and walk regularly. Seat heater off (Bujan 2000; Jung 2008).
- Nights. In one study's 24-hour recordings, 88% of daily scrotal temperature peaks came during rest or sleep (Jung 2001). A lighter duvet and a cooler room are free.
- Pants. Looser and breathable. A small dial, but an easy one (NG257, 1.11.1).
- Fever. Write the date down and tell your clinic if a test or IVF cycle is within three months.
- Not ice. Frozen things straight on skin can cause frostbite. Keep them cool, not frozen.
In 24-hour recordings, most daily temperature peaks came at rest or asleep. Nightly cooling plus less daytime heat raised counts in men with low counts.
No trial has tested this whole list together, and none shows it changes pregnancy rates. It takes known stresses off sperm production. That's all anyone can honestly promise.
Not sure where your heat comes from?The free Nuts Check has eight questions about your reported heat, fabric and fit habits. Get your heat & comfort score and practical next steps on the page. No email needed for results; marketing updates are optional.
Take The Nuts Check→15Getting tested
When should men get a sperm test?
Short answer
If you've been trying for a year without a pregnancy, NICE says both partners should be offered tests, including a semen analysis for you. Go sooner if your partner is 36 or over, or either of you has a known fertility issue (NG257). And see a GP about any lump, swelling or ache in a testicle (NHS).
A semen analysis is a lab count of how many sperm you have, how they move and how they're shaped. Results are compared with World Health Organization reference values, such as 16 million sperm per millilitre and 30% moving forwards (WHO 2021; NG257, 1.17.1).
Those limits are the 5th centile of results from fertile men, and the researchers who compiled them say they aren't distinct limits between fertile and subfertile men (Campbell 2021). So falling below one isn't a diagnosis. Fertile and infertile men overlap heavily on all three main measures (Guzick 2001), and motility in particular wobbles from one sample to the next (Chen 2020).
If a result is abnormal, NICE says repeat it, ideally three months later. After two abnormal results you should be offered an examination and hormone tests (NG257, 1.17.4 and 1.17.5). NICE now advises against sperm DNA fragmentation testing (1.17.6), so if someone offers you one, ask what it would change.
If this is where you are, it can be a lot, and it's normal to find the whole thing awkward. Clinic staff talk about sperm all day. The HFEA lists places to get free or low-cost emotional support (HFEA).
WHO lower reference limits: 16 million sperm per ml, 39 million in total, 42% moving, 30% moving forwards, 4% typical shape.
Below a limit doesn't mean infertile, and above one isn't a guarantee. One result is a snapshot, which is why a repeat is standard.
WordsThe jargon, decoded
Glossary
- Scrotal temperature
- The skin temperature of the scrotum, measured in studies with small sensors taped on. It normally runs a few degrees below core body temperature.
- Spermatogenesis
- The process of making sperm in the testes. One full run, from starting cell to sperm in semen, takes roughly ten weeks.
- Semen analysis
- A lab test of a semen sample: volume, how many sperm, how many move and how many are a typical shape. Also called a sperm test.
- Motility
- How well sperm move. Progressive motility means moving forwards, which is the kind that counts for reaching an egg.
- Azoospermia
- No sperm found in the semen. It has many possible causes. In the heat and testosterone studies on this page, it was reversible.
- FSH
- Follicle-stimulating hormone. It tells the testes to make sperm, and it tends to rise when output falls.
- Microplastics
- Plastic fragments smaller than 5 mm, many far smaller. They come from bottles, packaging, fabrics and coatings, and have been found in food, water, air and human tissue.
39Studies · 5 guidelines and health pages
Sources
Every study was checked against its published abstract on Europe PMC in September 2026. Each PMID links to the PubMed record, so you can read it yourself.
Guidelines and health pages
- National Institute for Health and Care Excellence. Fertility problems: assessment and treatment. NICE guideline NG257. Published 31 March 2026; replaces CG156. www.nice.org.uk/guidance/ng257
- World Health Organization. WHO laboratory manual for the examination and processing of human semen, 6th edition. Geneva: WHO; 2021. www.who.int/publications/i/item/9789240030787
- NHS. Testicle lumps and swellings. nhs.uk, accessed September 2026. www.nhs.uk/symptoms/testicle-lumps-and-swellings
- NHS. Sprains and strains. nhs.uk, accessed September 2026. www.nhs.uk/conditions/sprains-and-strains
- Human Fertilisation and Embryology Authority. Getting emotional support. hfea.gov.uk, accessed September 2026. www.hfea.gov.uk/treatments/explore-all-treatments/getting-emotional-support
Studies
- Banholzer ML, Buergin H, Wandel C, et al. Clinical trial considerations on male contraception and collection of pregnancy information from female partners. J Transl Med 2012;10:129. PMID 22720695
- Bujan L, Daudin M, Charlet JP, et al. Increase in scrotal temperature in car drivers. Hum Reprod 2000;15(6):1355-1357. PMID 10831568
- Cain T, Brinsley J, Bennett H, et al. Effects of cold-water immersion on health and wellbeing: A systematic review and meta-analysis. PLoS One 2025;20(1):e0317615. PMID 39879231
- Campbell MJ, Lotti F, Baldi E, et al. Distribution of semen examination results 2020 - A follow up of data collated for the WHO semen analysis manual 2010. Andrology 2021;9(3):817-822. PMID 33528873
- Carlsen E, Andersson AM, Petersen JH, et al. History of febrile illness and variation in semen quality. Hum Reprod 2003;18(10):2089-2092. PMID 14507826
- Chen HG, Sun B, Chen YJ, et al. Sleep duration and quality in relation to semen quality in healthy men screened as potential sperm donors. Environ Int 2020;135:105368. PMID 31830732
- Garolla A, Torino M, Sartini B, et al. Seminal and molecular evidence that sauna exposure affects human spermatogenesis. Hum Reprod 2013;28(4):877-885. PMID 23411620
- Gundersen TD, Jørgensen N, Andersson AM, et al. Association Between Use of Marijuana and Male Reproductive Hormones and Semen Quality: A Study Among 1,215 Healthy Young Men. Am J Epidemiol 2015;182(6):473-481. PMID 26283092
- Guzick DS, Overstreet JW, Factor-Litvak P, et al. Sperm morphology, motility, and concentration in fertile and infertile men. N Engl J Med 2001;345(19):1388-1393. PMID 11794171
- HELLER CG, CLERMONT Y. Spermatogenesis in man: an estimate of its duration. Science 1963;140(3563):184-186. PMID 13953583
- Hu CJ, Garcia MA, Nihart A, et al. Microplastic presence in dog and human testis and its potential association with sperm count and weights of testis and epididymis. Toxicol Sci 2024;200(2):235-240. PMID 38745431
- Jung A, Eberl M, Schill WB. Improvement of semen quality by nocturnal scrotal cooling and moderate behavioural change to reduce genital heat stress in men with oligoasthenoteratozoospermia. Reproduction 2001;121(4):595-603. PMID 11277880
- Jung A, Schill WB, Schuppe HC. Improvement of semen quality by nocturnal scrotal cooling in oligozoospermic men with a history of testicular maldescent. Int J Androl 2005;28(2):93-98. PMID 15811070
- Jung A, Strauss P, Lindner HJ, et al. Influence of heating car seats on scrotal temperature. Fertil Steril 2008;90(2):335-339. PMID 17919605
- Lazarev A, Pujalte GGA, Philibert C, et al. Testosterone-Optimizing Strategies in Athletes. Sports Health 2026;18(3):477-488. PMID 41630126
- Levine H, Jørgensen N, Martino-Andrade A, et al. Temporal trends in sperm count: a systematic review and meta-regression analysis of samples collected globally in the 20th and 21st centuries. Hum Reprod Update 2023;29(2):157-176. PMID 36377604
- Maroto M, Torvisco SN, García-Merino C, et al. Mechanisms of Hormonal, Genetic, and Temperature Regulation of Germ Cell Proliferation, Differentiation, and Death During Spermatogenesis. Biomolecules 2025;15(4):500. PMID 40305231
- Mínguez-Alarcón L, Gaskins AJ, Chiu YH, et al. Type of underwear worn and markers of testicular function among men attending a fertility center. Hum Reprod 2018;33(9):1749-1756. PMID 30102388
- Misell LM, Holochwost D, Boban D, et al. A stable isotope-mass spectrometric method for measuring human spermatogenesis kinetics in vivo. J Urol 2006;175(1):242-6; discussion 246. PMID 16406920
- Montano L, Raimondo S, Piscopo M, et al. First evidence of microplastics in human ovarian follicular fluid: An emerging threat to female fertility. Ecotoxicol Environ Saf 2025;291:117868. PMID 39947063
- Nikolopoulos I, Osman W, Haoula Z, et al. Scrotal cooling and its benefits to male fertility: a systematic review. J Obstet Gynaecol 2013;33(4):338-342. PMID 23654310
- Qaoud Y, Morcos M, Macdonald M, et al. A systematic appraisal of emerging alternative therapies in men's health and wellness. Can Urol Assoc J 2025. PMID 41406341
- Rahban R, Senn A, Nef S, et al. Association between self-reported mobile phone use and the semen quality of young men. Fertil Steril 2023;120(6):1181-1192. PMID 37921737
- Rittblat M, Katabi S, Kotovich D, et al. Avoiding harm in pediatric heatstroke: Lessons from a case of ice-related frostbite. JPRAS Open 2025;45:386-389. PMID 40799262
- Schisterman EF, Sjaarda LA, Clemons T, et al. Effect of Folic Acid and Zinc Supplementation in Men on Semen Quality and Live Birth Among Couples Undergoing Infertility Treatment: A Randomized Clinical Trial. JAMA 2020;323(1):35-48. PMID 31910279
- Sergerie M, Mieusset R, Croute F, et al. High risk of temporary alteration of semen parameters after recent acute febrile illness. Fertil Steril 2007;88(4):970.e1-7. PMID 17434502
- Sermondade N, Faure C, Fezeu L, et al. BMI in relation to sperm count: an updated systematic review and collaborative meta-analysis. Hum Reprod Update 2013;19(3):221-231. PMID 23242914
- Shafik A. Contraceptive efficacy of polyester-induced azoospermia in normal men. Contraception 1992;45(5):439-451. PMID 1623716
- Shafik A. Effect of different types of textile fabric on spermatogenesis: an experimental study. Urol Res 1993;21(5):367-370. PMID 8279095
- Sharma R, Harlev A, Agarwal A, et al. Cigarette Smoking and Semen Quality: A New Meta-analysis Examining the Effect of the 2010 World Health Organization Laboratory Methods for the Examination of Human Semen. Eur Urol 2016;70(4):635-645. PMID 27113031
- Shefi S, Tarapore PE, Walsh TJ, et al. Wet heat exposure: a potentially reversible cause of low semen quality in infertile men. Int Braz J Urol 2007;33(1):50-6; discussion 56-7. PMID 17335598
- Sheynkin Y, Jung M, Yoo P, et al. Increase in scrotal temperature in laptop computer users. Hum Reprod 2005;20(2):452-455. PMID 15591087
- Sheynkin Y, Welliver R, Winer A, et al. Protection from scrotal hyperthermia in laptop computer users. Fertil Steril 2011;95(2):647-651. PMID 21055743
- Tople T, Skokan A, Ettinger R, et al. Managing Thermal Injuries of the Penis and Scrotum: A Narrative Review. Eur Burn J 2023;4(2):184-194. PMID 39599925
- Traylor DA, Nunes EA, Lees MJ, et al. A Four-Day Cold Shower Protocol Does Not Increase Salivary Testosterone in a Small Cohort of Male Athletes: A Research Communication. Health Sci Rep 2026;9:e72432. PMID 42064114
- World Health Organization Task Force on Methods for the Regulation of Male Fertility. Contraceptive efficacy of testosterone-induced azoospermia in normal men. Lancet 1990;336(8721):955-959. PMID 1977002
- Xiao L, Wang Q, Ni H, et al. Effects of temperature anomaly on sperm quality: A multi-center study of 33,234 men. Heliyon 2024;10(5):e26765. PMID 38434420
- Zhang C, Zhang G, Sun K, et al. Association of mixed exposure to microplastics with sperm dysfunction: a multi-site study in China. EBioMedicine 2024;108:105369. PMID 39342804
- Zleczewski M, Clay N, Lambert J, et al. Garment fit, scrotal thermoregulation, and male fertility outcomes: a narrative review. Transl Androl Urol 2026;15(5):184. PMID 42293842
EditorialRead this bit too
How we check
How we choose and check studies
We start with national guidance (NICE and the WHO), then the human studies behind each claim. We read every study's published abstract on Europe PMC before quoting a number, say how many people took part, and pair each finding with what it doesn't prove.
We prefer studies in people over animal or lab work, and we say so when we use one. When better evidence lands, we update this page and the date below.
Information, not medical advice
This page explains published research. It doesn't diagnose or treat anything, and it isn't a substitute for your GP, your clinic or a semen analysis.
We make a cooling boxer, so we have an interest in this topic. That's exactly why every claim here links to its source.
When to see a GP
NICE NG257: if you've been trying for a year, both of you should be offered tests. Go now if your partner is 36 or over, or either of you has a known cause or a history of risk factors for infertility. Taking testosterone or anabolic steroids and want children, now or later? Raise it with your GP.
See a GP promptly about any lump, swelling or lasting ache in a testicle, whatever else is going on.
Last reviewed Written and checked by the NumbNuts teamFounder: Jaspal RaiSpotted an error? hello@numbnutsclub.com
Keep cool whenever you overheat.
Keep your core hot.
Keep them cool.
NumbNuts is a comfort product. It wasn't used in any of the studies on this page, and it doesn't diagnose, treat or prevent anything. Worried about your fertility? Your GP is the right first call. This page is the bit you can read alongside it.
