Researchers have used many different sources of heat to disrupt male fertility: hot water, saunas, incandescent light bulbs, even ultrasound. The principle is similar to suspensories; subject the testes to heat and sperm counts drop. Because an external heat source can be significantly warmer than body temperature, the duration of exposure required for effective contraception is shorter than for suspensories. Although more men have used external heat as a self-administered contraceptive than suspensories, there is less published clinical research on this method.
How does it work?
Optimal sperm production (spermatogenesis) requires temperatures several degrees Celsius below average body temperature. The testes are located outside the body in the scrotum, which has several mechanisms to keep the testes cooler than the rest of the body. Warming the testes to or above body temperature temporarily disrupts spermatogenesis. Read a more detailed explanation of this process in How heat works.
How effective is it?
Multiple researchers in the 1950s and 60s did proof of concept studies of external heat as a male contraceptive (Robinson 1968, Steinberger 1959, Watanabe 1959). These studies universally showed decreased sperm counts of their human and animal participants, but none tested the methods’ effectiveness as a contraceptive.
The main evidence for effective use of external heat as a male contraceptive comes from the unpublished works of a Swiss doctor working in India between 1930 and 1950. Dr. Martha Vogeli advised 100s of men on the use of hot sitz baths, and claims that her patients attained 100% effective contraception using this method (1956).
How is it delivered? How long does it take?
Dr. Vogeli tested baths in a range of temperatures with 9 volunteers to determine the most effective regimen. She determined that 21 consecutive days of 46.5 ºC (116 ºF) sitz baths lasting 45 minutes typically provided 6 months of contraception. However, there was a range of duration of the contraceptive effect from 4 to 8 months just among these 9 men. Dr. Vogeli ’s volunteers did not continuously monitor the temperature of the bath, but presumably it decreased over the course of the 45 minute treatment (Vogeli 1956).
In the accounts of her work, Dr. Vogeli did not mention a delay in the start of effectiveness. However, heating as a contraceptive method would likely require at least 2 months to become effective. The onset of effectiveness depends on the individual.
As a high-tech and perhaps more convenient alternative to hot sitz baths, several inventors have created battery-powered pouches designed specifically to heat the testes. A patent of one such device describes an electrically heated, timer regulated fabric pouch. A control unit worn on a waist belt allows the wearer to set the temperature and timer. The inventor suggests that wearing the device for 30 minutes to 2 hours daily will result in effective contraception within 7 weeks (Walston 1991).
What side effects are expected?
Because this method of contraception has had few clinical trials, safety issues are relatively unexplored. Trial participants have reported no effects on libido, mood or physical health. Although no data have been collected for humans, rats in external heat trials experienced a one-third reduction in the volume of their testes 6 weeks after treatment (Steinberger 1959).
Because heat disrupts the assembly of DNA in forming sperm, and fetal exposure to heat is correlated with increased birth defects, the largest safety concern with external heat treatment is its effect on the genetic integrity of future sperm. If a man recovering from using heat as a contraceptive conceived with his partner, would they have a higher risk of birth defects or miscarriage? The few animal model tests of external heat did not show such increased risks, and Dr. Vogeli reported that all children born after her patients stopped treatment were normal (1954). However, there is simply not enough data to make a definitive statement.
How long does it take to reverse?
The contraceptive effect of external heat treatments can be sustained by periodically returning to the treatment regimen. Otherwise full fertility returns within 2-6 months. After stopping heat treatments, many men actually experience a several month period of super-fertility, with sperm counts up to 250% of pre-treatment levels (Robinson 1968).
Could external heat treatments result in permanent sterility?
Up to 11 ºC hotter than normal testes temperature is the effective therapeutic range for external heat. Exceeding the therapeutic range could theoretically cause irreversible infertility. However, Dr. Vogeli asserts that “no degree of heat within the range of physical tolerance is high enough to sterilize a man permanently” (1956). There are no reports of men causing permanent sterility by using external heat as a contraceptive (Robinson 1968).
In addition to the degree of heat used, the duration of treatment could be a risk factor for irreversible infertility. However, clinical studies of subfertile men exposed to heat as part of their daily work provide contrary evidence. Men in these studies had often been exposed to occupational heat sources such as baker’s ovens or blast furnaces for over a decade. The studies report that these men experienced a longer than average time before conception, but not permanent sterility (Thonneau 1998).
- Robinson, D, and J Rock (1968) “Control of human spermatogenesis by induced changes of intrascrotal temperature.” Journal of the American Medical Association 204(4): 80-7.
- Steinberger, E, and WJ Dixon (1959) “Some observations on the effect of heat on the testicular germinal epithelium.” Fertility and Sterility 10(6): 578-95.
- Thonneau, P, L Bujan, L Multigner and R Mieusset (1998) “Occupational heat exposure and male fertility: a review.” Human Reproduction 13(8): 2122–5.
- Vogeli, M (1954) “Data on the thermic method for temporary male sterilization.” Unpublished. Smith College Archives.
- Vogeli, M (1956) “Contraception through temporary male sterilization.” Unpublished. Smith College Archives.
- Walston, WT (1991) “Male contraceptive device.” US Patent # 5,063,939.
- Watanabe, A (1959) “The effect of heat on the human spermatogenesis.” American Journal of Medical Science 10: 101-17.