Sperm Killers That May Surprise You

Sperm are subject to a range of threats. Here are 10 to dodge.

When it comes to conceiving a child, there are lots of things that can go wrong—sperm allergies, poor egg quality, and ineffective sperm. Approximately 1 in 10 couples is infertile, and it has been estimated that male factors alone contribute to between 20-30% of cases, according to a 2015 study in Reproductive Biology and Endocrinology.

Though men produce millions of sperm a day (compared to the 300–400 eggs that women release during their lifetime), external factors (like temperature) can affect the health of these little swimmers. And because sperm cells take about 75 days to grow to maturity, harming them may affect your fertility.

Here are more than 10 factors that may affect sperm.


Human testes cannot function properly unless they are able to stay cooler than the rest of the body. Thankfully, the male anatomy is designed to create distance between the testes and the core body temperature.

If the temperature of the testicles is raised to 98 degrees Fahrenheit, sperm production ceases, according to Hal Danzer, MD, a Los Angeles fertility specialist. When production is interrupted, sperm can be negatively impacted for months.

"The overall number (of sperm) can be lower, as well as the motility and morphology," said Paul Shin, MD, a urologist in Washington, DC.

Hot Tubs

There is a grain of truth in the myth about hot tubs preventing pregnancy. A 2013 study in Human Reproduction found that sauna exposure induces a significant but reversible impairment of spermatogenesis, a cellular process that ensures that fertilizing sperm are produced.

According to Dr. Shin, wet heat exposure can impact a man's sperm for a surprisingly long time. Because sperm takes so long to mature, "any interventions (to reduce exposure) will usually take at least three, if not six to nine, months to show any benefit," said Dr. Shin.


"When I know that a man hasn't been in a hot tub, smoking dope, or wearing bicycle pants, and that the collection technique for his semen analysis was good, my first question is, 'Were you sick three months ago?'" said Kurt Wharton, MD, a San Francisco OB-GYN specializing in infertility. Often, said Dr. Wharton, his patients say yes.

A study published in 2022 in the journal Reproductive Sciences noted that increased body temperature due to fever can affect sperm output, motility (movement), morphology (structure), and DNA fragmentation (the breaking of DNA strands that's sometimes linked with low pregnancy rates). The study found that fever from SARS-CoV-2 virus infection led to a reversible negative effect on sperm production.


Can a laptop computer really affect one's ability to reproduce? According to a 2011 study published in Fertility and Sterility researchers found a direct correlation between laptop use and increased scrotum temperature—1 degree Celsius elevation was reached at 11 minutes and continued to increase with time.

Scrotal temperature increase has a well-documented harmful effect on spermatogenesis (the process of male reproductive cell formation), so if you're trying to conceive, take the laptop off your lap.


Why do you think Scots brag about their fertility? "It's their kilts," said Dr. Wharton, though he admits the difference between boxers and briefs is usually not great enough to merit a change in sperm count.

"Boxers are better than briefs, if a man's sperm count is on the low side. But it probably has little effect if the sperm count is normal," said Dr. Danzer.

However, wearing tight bicycle shorts for an extended period of time is a bad idea, said Dr. Wharton. The more constricted the pants are, the less hospitable an environment he creates for sperm production.

Varicocoeles (Enlarged Scrotal Veins)

Approximately 15% of men have varicoceles, or enlarged veins in the scrotum, usually in the left testicle, according to Johns Hopkins Medicine. When someone is experiencing a low sperm count, healthcare providers may recommend varicocele repair, a procedure that corrects enlarged varicose veins in the scrotum. This is done surgically or via percutaneous embolization, a nonsurgical procedure using a catheter.

Though it's not clear, enlarged veins may interfere with sperm production by interrupting blood flow, overheating the scrotum, or causing blood to back up in the veins supplying the testes, according to Johns Hopkins Medicine. Though there is little proof that fertility improves after varicocele embolization, some providers believe the surgery may improve semen quality.


The advice varies when it comes to keeping cell phones in proximity to the male reproductive organs.

A 2008 study published in Fertility and Sterility found that men with the highest cell phone usage (more than four hours per day) had significantly lower sperm counts, motility rates, and morphology (normal shapes). Dr. Shin recommends patients carry their phones in their briefcases rather than pockets in order to limit radiation exposure.

A 2022 study published in Reproduction, Fertility and Development found that mobile phone use had a negative effect on sperm motility, which can have a negative impact on fertility.


"Obesity has been associated with increased production of female hormones (estrogen), decreased sperm counts, sexual dysfunction, and infertility," said Daniel A. Potter, MD, reproductive endocrinologist with the Huntington Reproductive Center in California and fellow of the American College of Obstetricians and Gynecologists.

According to a March 2012 study published in the Archives of Internal Medicine, men who were overweight were 11% more likely to have a low sperm count and 39% more likely to have no sperm in their ejaculate. Men with obesity were 42% more likely to have a low sperm count than their peers of average weight and 81% more likely to produce no sperm.

A Party Lifestyle

"Tobacco, alcohol, and marijuana can impair sexual function," said Dr. Potter, who recommends that his patients limit or avoid all of these when trying to conceive.

A 2022 study in Andrologia found that cigarette smoking and heavy alcohol intake can deteriorate sperm quality. The study also found that alcohol consumption deteriorates sperm maturity and damages DNA integrity at significantly higher rates than cigarette smoking.

In addition to slowing sperm down, cigarette smoking can damage sperm DNA and increase erectile dysfunction, according to a 2007 study in the American Journal of Epidemiology.

Marijuana isn't safe either. Smoking pot has been shown to reduce sperm count and function and overall male fertility. A 2022 study published in the American Journal of Obstetrics and Gynecology noted that the use of cannabis in males has been associated with erectile dysfunction, abnormal formation of sperm, and testicular shrinking.

More Trouble for Sperm

In addition to environmental factors, some physiological situations may negatively affect sperm, according to Dr. Potter.

Blockages: "Whether it's caused by a birth defect, infection, trauma, or vasectomy (a surgical procedure), a blockage prevents the sperm from entering the semen."

Genetic disorders: "Chromosome abnormalities can cause severely diminished or no sperm production." For example, one form of cystic fibrosis can cause the vas deferens (the tube that carries sperm out of the testes, which produces the sperm) not to form.

Other detrimental factors: Anti-sperm antibodies, hormonal imbalance, testicular cancer, undescended testicles, and sexual problems can affect sperm too.

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