We can, thankfully, remove one threat to the future existence of the human male from our worry list: The male Y chromosome, after dwindling from its original robust size over millions of years, apparently has halted its disappearing act.

But don’t start cheering yet. Contrary to cultural assumptions that boys are stronger and sturdier, basic biological weaknesses are built into the male of our species. These frailties leave them more vulnerable than girls to life’s hazards, including environmental pollutants such as insecticides, lead and plasticizers that target their brains or hormones. Several studies suggest that boys are harmed in some ways by these chemical exposures that girls are not. It’s man’s fate, so to speak.

First of all, human males are disappearing. Mother Nature has always acknowledged and compensated for the fragility and loss of boys by arranging for more of them: 106 male births to 100 female newborns over the course of human history. (Humans are not unique in this setup: Male piglets, as an example, are conceived in greater proportion to compensate for being more likely than female piglets to die before birth.) But in recent decades, from the United States to Japan, from Canada to northern Europe, wherever researchers have looked, the rate of male newborns has declined. Examining U. S. records of births for the years between 1970 and 1990, they found 1.7 fewer boys per 1,000 than in decades and centuries past; Japan’s loss in the same decades was 3.7 boys.

Boys are also more than two-thirds more likely than girls to be born prematurely – before the 37th week of pregnancy. And, despite advances in public health, boys in the 1970s faced a 30 percent higher chance of death by their first birthday than girls; in contrast, back in the 1750s, they were 10 per cent more likely than girls to die so early in their lives.

Once they make it to childhood, boys face other challenges. They are more prone to a range of neurological disorders. Autism is notoriously higher among boys than girls: now nearly five times more likely, as tallied by the U.S. Centers for Disease Control and Prevention. They are more susceptible than girls to damage from very low-level exposure to lead. Yet another problem: Boys also suffer from asthma at higher rates. There’s also a stronger link between air pollution and autism in boys.

What is up here? Why do boys face such a burden of physical challenges?

The answer is that the male’s problems start in the womb: from his more complicated fetal development, to his genetic makeup, to how his hormones work.

The nine-month transformation from a few cells to an infant is a time of great vulnerability. Many chronic illnesses are seeded in the womb. In our species, the female is the default gender, the basic simpler model: Humans start out in the womb with female features (that’s why males have nipples). The complicated transformation in utero from female to male exposes the male to a journey packed with special perils. When the first blast of testosterone from the Y gene comes along at about the eighth week, the unisex brain has to morph into a male brain, killing off some cells in the communication centers and growing more cells in the sex and aggression centers. The simpler female reproductive system has to turn into the more complex male reproductive tract, developing tissues such as the testis and prostate. Further, it takes a greater number of cell divisions to make a male; with each comes the greater risk of an error as well as the greater vulnerability to a hit from pollutants.

On top of that challenge, the human male’s XY chromosome combination is simply more vulnerable. The two XXs in the female version of our species offer some protection: In disorders where one X chromosome has a genetic defect, the female’s healthy backup chromosome can take over. But with his single X chromosome, the male lacks a healthy copy of the gene to fall back on. The X chromosome, which never shrank, is also a larger chromosome “with far more genetic information than the Y chromosome,” finds Irva Hertz-Piciotto, a University of California, Davis autism researcher, “so there may be some inherent loss of key proteins for brain development or repair mechanisms in boys.” This is a clue to the higher autism rate among boys, she asserts.

Females also have a stronger immune system because they are packed with estrogen, a hormone that counteracts the antioxidant process. “Estrogen protects the brain; it’s a no-brainer, pun intended,” explains Theodore Slotkin, professor of neurobiology at Duke University’s School of Medicine. “It repairs and replaces, even after neural injury.” Low estrogen even leaves boys more sensitive to head injuries. The male brain “is simply a more fragile apparatus, more sensitive to almost all brain insults,” lead poisoning expert Herbert Needleman told writer Julia R. Barrett of Environmental Health Perspectives.

It’s the high levels of testosterone in the womb at critical times in gestation, according to British psychopathologist Simon Baron-Cohen, that are responsible for what he calls “the extreme male brain” – the kind exhibited by autistic boys – low in empathy, high in systematizing. And, in fact, recent decades of U.S. research do find unusually low estrogen and high testosterone levels among boys with autism.

If the balance of hormones is out of whack in males, what made that happen? Researchers are coming up with some clues.

In the New York City neighborhoods near Columbia University’s Center for Children’s Environmental Health, families for years routinely sprayed their apartments with a popular insecticide, chlorpyrifos, until it was banned from household use in 2001. The researchers found that prenatal exposure to the chemical seemed to have more of an effect on reducing the IQs of boys than girls. Disruption of their male hormones may be the reason. “One possible explanation for the greater sensitivity of boys to chlorpyrifos is that the insecticide acts as an endocrine disruptor to suppress sex-specific hormones,” said study leader Megan Horton of Columbia.

Similarly, pregnant mothers’ exposure to phthalates – used in making some vinyl products and toys as well as some personal care products – has been linked to bigger changes in the behavior, such as aggression and attention problems, of their sons than their daughters. Phthalates also may feminize male genitalia.

Boys also seem to be more vulnerable to bisphenol A, an estrogenic substance used to make polycarbonate plastics as well as some thermal receipts and the linings of food and beverage cans. Boys, but not girls, exposed to higher BPA levels in the womb or during childhood had more hyperactivity, aggression and anxiety problems, according to a University of California, Berkeley study. In addition, pregnant women exposed to higher levels of the chemical gave birth to baby boys with lower thyroid hormones. No such effect was detected in the baby girls. No one knows what these lower levels may mean for the boys’ health because they remained within normal boundaries, but it could have important effects because thyroid hormones guide brain development.

Some of these chemicals act like fake estrogens, others like fake testosterone, but both types seem to disrupt normal development. Animal tests show that a dose of these chemicals inflict the most damage when it hits a fetus. And, because of their biological vulnerabilities, it’s boys who may experience the most effects.

While not forgoing the push for fairness and equality, it seems wise to accept the scientific reality of male weaknesses. This likely won’t mean the end of men, but their vulnerability to environmental contaminants and diseases could have serious ramifications for the future of the entire human race unless we find ways to protect them from harm.

Alice Shabecoff is the coauthor with her husband, Philip Shabecoff, of Poisoned for Profit: How Toxins Are Making Our Children Chronically Ill, Random House 2008, Chelsea Green, 2010.

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.