Plummeting sperm counts are threatening the future of human existence, and plastics could be to blame

• Epidemiologist Shanna Swan studied chemical exposure's impact on health for two decades.
• Her book "Count Down" uses research to explain how plastics and other substances mess with fertility.
• If we don't do something about chemical exposure, it could endanger human life, Swan writes.

Shanna Swan is obsessed with studying the length between babies' genitals and their anuses.

That's because Swan is an epidemiologist who's spent two decades researching how our lifestyles and the environments in which we live mess with our hormones and reproductive abilities.

The distance between a person's genitals and their butthole, also called anogenital distance or AGD, is one of the best indicators of reproductive potential and chemical exposure in the womb, Swan explains in her new book "Count Down: How Our Modern World Is Threatening Sperm Counts, Altering Male and Female Reproductive Development, and Imperiling the Future of the Human Race."

As birth rates continue to plummet around the world, a trend that could eventually result in human species endangerment, Swan's book offers insights into why men in Western countries now have half the baby-making potential as their grandparents.

She also explains what we can do now to avoid altering fetus' reproductive potential in the womb, and to protect our health in a world filled with plastics and chemicals.

Solving a worldwide infertility mystery

Swan got her start researching how chemicals, contaminated water, and drugs impacted human health.

So, when she saw a 1992 study that found sperm counts around the world were decreasing at an alarming rate, she pivoted to research whether the paper's author Elisabeth Carlsen was onto something, or if it was bogus.

Swan set out on a six-month research stint with two colleagues, reviewing all of the studies Carlsen used in her analysis, looking for confounding factors and biases that could've skewed the alarming data.

Instead, she found that Carlsen's claims held.

"Not only have I shifted from being dubious to being utterly convinced that a dramatic decline in sperm counts is occurring, I've also discovered that various lifestyle factors and environmental exposures may be acting in tandem or in a cumulative fashion to fuel the decline," Swan wrote in "Count Down."

'Babies are now entering the world already contaminated with chemicals'

Swan found a lifestyle factors like smoking, using antidepressant medication, lack of exercise, and heightened stress could lower both men and women's reproductive abilities.

Even more alarming, Swan found invisible chemicals in plastic water bottles, the dust on shelves, and adhesives most humans come into contact with every day could also mess with reproductive health in grownups, children, and unborn babies.

Phthalates, a type of chemical found in plastic manufacturing parts, are one of the biggest culprits, according to Swan.

That's because they disrupt how the hormone endocrine is produced in the body. In turn, that disruption can contribute to obesity, lower IQs, premature birth. As Swan found, it can also decrease testosterone production, lower sperm counts, decrease fertility, and contribute to smaller penis size.

These endocrine disruptors can affect babies as they grow in the womb, if the person carrying them has been exposed to chemicals, according to Swan and other researchers' work. They can also be passed onto babies in breast milk, said Swan.

"Babies are now entering the world already contaminated with chemicals because of the substances they absorb in the womb," she wrote.