Male fertility depends on producing enough healthy sperm and delivering them effectively, and a surprisingly wide range of factors can disrupt that process. Everything from body weight and chemical exposures to scrotal temperature and age plays a role. Understanding these factors matters because male issues contribute to roughly half of all infertility cases, and many of the biggest influences are modifiable.
One important number to keep in mind: a full cycle of sperm production takes about 65 days in humans. That means any factor damaging sperm quality today won’t fully show up in a semen analysis for about two months, and any positive lifestyle change you make will take at least that long to produce results.
Body Weight and Sperm DNA Damage
Obesity has a direct, measurable effect on sperm quality. Obese men are roughly 2.5 times more likely to have elevated sperm DNA damage compared to men at a normal weight. Sperm motility, the ability of sperm to swim effectively, is also lower in obese men by about 7 to 9 percentage points even after adjusting for age and smoking. Interestingly, being overweight but not obese doesn’t appear to cause the same degree of damage. The DNA fragmentation risk jumps significantly only once a man crosses into obesity.
The mechanism behind this is oxidative stress. Higher body fat increases inflammation and the production of reactive oxygen species throughout the body, including in the testes. Studies have found a direct positive correlation between BMI and oxidative stress levels in seminal fluid. This oxidative damage chips away at sperm DNA integrity, which can reduce fertilization rates and increase miscarriage risk even when conception occurs.
Chemical Exposures That Mimic Hormones
Two of the most studied threats to male fertility are chemicals found in everyday products: phthalates and bisphenol A (BPA). Phthalates are common in plastics, vinyl flooring, and personal care products like shampoos and lotions. BPA is found in food can linings, thermal receipt paper, and some plastic containers. Both act as endocrine disruptors, meaning they interfere with the body’s hormone signaling.
Phthalates suppress testosterone production. Men with the highest urinary levels of phthalate byproducts have roughly 12% lower testosterone and significantly reduced sperm motility compared to men with low exposure. Animal studies show even steeper effects, with testosterone dropping by as much as 40%. BPA works differently: it activates estrogen-responsive genes in tissues where estrogen signaling shouldn’t be active, effectively feminizing reproductive cells. Men with high BPA exposure show 10 to 15% lower testosterone and delayed sperm maturation.
Both chemicals also ramp up oxidative stress in semen. Men in the highest exposure groups have 25 to 30% higher levels of reactive oxygen species in their seminal fluid, and their sperm DNA fragmentation rates are 1.4 to 1.6 times higher than controls. Perhaps most concerning, animal research has found that prenatal exposure to these chemicals can alter the sperm epigenetic profiles of offspring, with effects persisting across multiple generations.
Heat and Scrotal Temperature
The testes hang outside the body for a reason. They need to stay at about 35°C (95°F), roughly two degrees cooler than core body temperature. Even small increases above that threshold can inhibit sperm production. Studies comparing fertile and infertile men have found that infertile men tend to have scrotal temperatures just 0.4 to 0.5°C higher than fertile men.
Practical heat sources add up quickly. Sitting with your thighs pressed together for an hour raises scrotal temperature by about 2.1°C. Placing a laptop on your lap adds another 0.5 to 0.6°C on top of that. Regular hot tub or hot bath use (30 minutes or more per week) has been shown to reduce sperm counts, and sauna exposure totaling about two and a half hours every two weeks cut sperm counts by as much as 50% in one study. When researchers heated the scrotum to 43 to 47°C for 30 minutes, sperm counts dropped 5 to 7 weeks later but recovered within about 4 weeks after the heat exposure stopped.
The good news is that heat-related damage is typically reversible once the source is removed. Switching to looser clothing, taking breaks from sitting, and keeping laptops on a desk rather than your lap are simple adjustments.
Varicoceles
A varicocele is an enlargement of the veins inside the scrotum, similar to a varicose vein in the leg. It’s the most common identifiable physical cause of male infertility. While varicoceles are found in only 10 to 15% of the general male population, they show up in more than 40% of men being evaluated for primary infertility. The enlarged veins are thought to raise scrotal temperature and increase oxidative stress, both of which impair sperm production over time. Varicoceles are treatable, typically through a minor surgical procedure, and many men see improvements in sperm parameters afterward.
Cannabis Use
Cannabis has a clear negative effect on sperm production, particularly with heavy use. Men who use marijuana more than once per week have been found to have 28% lower sperm concentration and 29% lower total sperm count compared to men who have never used it. In heavy users (10 or more times per week), sperm counts dropped to roughly 27 million per milliliter compared to 68 million in moderate users.
The hormone picture is more complicated. Cannabis consistently lowers luteinizing hormone (LH), which is one of the signals the brain sends to the testes to produce testosterone. Animal studies have confirmed this works through specific receptors in the pituitary gland. Yet the downstream effect on testosterone itself is inconsistent across studies. Some early research found dramatic testosterone reductions in heavy users, while more recent large studies have found no difference or even slightly higher testosterone in cannabis users. The safest interpretation is that cannabis reliably impairs sperm production regardless of what it does to testosterone levels.
Age-Related Decline
Male fertility doesn’t have the sharp cutoff that female fertility does, but it does decline steadily. Sperm parameters generally remain stable until about age 34. After 35, sperm motility, morphology, and vitality begin a measurable decline. The drop becomes more pronounced after age 45, when semen volume starts to decrease and motility declines further. Across the span of 30 to 50 years old, studies have documented 3 to 37% reductions in motility and 4 to 18% changes in normal sperm shape.
Beyond raw sperm numbers, older fathers face higher rates of sperm DNA mutations, which can affect not only conception rates but also pregnancy outcomes and offspring health. Age is obviously not a modifiable factor, but it’s worth understanding if you’re planning to delay fatherhood.
Nutrition and Antioxidants
Because oxidative stress is a common thread linking many causes of poor sperm quality, antioxidant intake matters. Semen naturally contains protective antioxidant molecules, including vitamin C, vitamin E, zinc, selenium, and coenzyme Q10 (CoQ10). When oxidative stress overwhelms these defenses, sperm suffer.
CoQ10 supplementation has the most robust evidence. In men with poor sperm quality, taking 200 to 400 mg per day for three months significantly increased sperm concentration and motility. Men taking the higher dose saw greater improvements: progressive motility nearly doubled from about 14% to 26%, and total motility jumped from 23% to 35%. Studies using 200 mg daily for six months have also shown meaningful gains. A diet rich in fruits, vegetables, nuts, and fish provides a baseline of antioxidant and anti-inflammatory support, though supplementation may offer additional benefit for men with documented deficiency or high oxidative stress.
What Normal Sperm Parameters Look Like
If you’re getting a semen analysis, the results are typically compared against lower reference thresholds set by the World Health Organization. The most recent edition, published in 2021, lists these lower fifth percentile values from fertile men: a total sperm count of at least 39 million per ejaculate, total motility of at least 42%, and normal morphology of at least 4%. Falling below these numbers doesn’t mean you can’t conceive, but it does suggest fertility may be reduced and further evaluation could be helpful.
These thresholds are intentionally low. They represent the bottom 5% of men who still achieved pregnancy. Most fertile men score well above these cutoffs, so values near the threshold warrant attention even if they’re technically “normal.”