We hear a lot about how pesticides can undermine children's health. From derailing developing brains to bumping up the risk of childhood cancer, even low levels of exposure can wreak some serious havoc.

It turns out dads should be concerned about how pesticides are affecting them too. Science shows that exposure to these chemicals can pose a serious threat to dad-hood itself.

Sperm counts are down

The news for would-be dads is not good. Worldwide, sperm counts are down and infertility rates are up. The Association of Reproductive Health Professionals points to concerning trends in male reproductive health, including:

• Rising rates of testicular cancer
• Falling sperm counts
• Decline in testosterone levels
• Fewer males being born
• Increases in certain types of birth defects

Science tells us that pesticides and other chemicals are at least partly to blame.

Physicians for Social Responsibility featured this issue in a recent Environmental Health Policy Institute online forum, with various articles exploring links between chemical exposures and rising rates of male infertility — and highlighting the need for prevention.

Messing with biological signals

Endocrine disrupting chemicals are very good at interfering with reproductive health, even when exposure levels are extremely low. Some of these chemicals are structurally similar to human hormones, and can block (or put into overdrive) the body's natural system of biological signals.

A fetus in the womb is especially vulnerable, as hormones are busy regulating the differentiation of cells and development of organs. Infants exposed to a triggering chemical just when the reproductive organs are forming — or the brain developing, or immune system coalescing — can experience harm that can play out over the course of a lifetime.

The Endocrine Disruption Exchange has a compelling online tool showing the vulnerabilities of a fetus at various stages of development. And scientists from the national Endocrine Society explain why the timing of exposure is so important:

...there are critical developmental periods during which there may be increased susceptibility to environmental endocrine disruptors. In those cases in which disruption is directed toward programming of a function, e.g., reproductive health, this may interfere with early life organization, followed by a latent period, after which the function becomes activated and the dysfunction can become obvious.

In other words, exposure to certain chemicals when an infant's reproductive system is developing can completely derail the process. But the effects won’t become evident until years later, when problems arise during puberty or when trying to conceive.

Grownup exposures also matter

Adult exposures to pesticides can undermine male fertility as well. Authors of a 2013 review published in Toxicology examined 17 studies published between 2007 and 2012, all exploring how environmental and occupational exposures to pesticides affect semen quality. Some studies looked at individual chemicals (such as DDT and hexachlorocyclohexane), others looked at classes of pesticides such as organophosphates and pyrethroids.

The majority of these studies (15 of the 17!) found significant links between pesticide exposure and lower semen quality, with decreased sperm concentration the most common finding.

Links between pesticide exposure and damage to men's reproductive ability are hardly new. The classic example is Dow's fumigant Nemogen (DBCP), which sterilized workers both in the California factory where it was produced and in the Nicaraguan banana plantations where it was heavily applied. And exposure to current-use pesticides like atrazine and diazinon has been shown to undermine sperm viability and function too.

Sobering inter-generational harms

Pesticides can damage the male reproductive system in a number of ways. Some chemicals can kill or damage cells; if these are sperm cells, infertility can result. Others alter DNA structure, causing gene mutations that may result in birth defects or an inability to conceive. And still others cause what are known as "epigenetic" effects, meaning they change the way genes are expressed.

It’s been known for some time that pollutants can strip or add chemical tags to DNA, locking the expression of these genes on or off and changing how they function. These changes are called “epigenetic tags,” and have been linked to various health effects including death of sperm-forming cells.

Perhaps most sobering of all, emerging science shows that these genetic changes can be passed from one generation to the next.

Recent studies from Spain and Washington state found that exposure to some chemicals can override the genetic “reset button” designed to protect a developing fetus from such changes. These trans-generational effects give new urgency to efforts to reduce our use of and exposure to harmful pesticides.

A version of this blog appeared on Physicians for Social Responsibility's Environmental Health Policy Institute online forum.