Our bodies are constantly affected by the substances with which we come into contact in the outside world, and the reproductive system is particularly sensitive to this. We may never manage to catalogue the near-infinite complexity of the ways in which chemicals in the air, in our food, and in objects we touch affect us. But what we do know is enough to understand the importance of environmental factors.
Lead was possibly the first chemical to be linked to lower fertility. In the middle of the 19th century, it became obvious that the wives of men working with lead had lower fertility than would be expected otherwise. Since then the link has been examined in great detail, and lead poisoning has been linked to lower sperm counts and to higher rates of miscarriage.
This is only one of the many medical problems linked to lead, and it is one of the reasons for increasingly tight legislation to control the use of lead. The addition of lead to petrol, for example, has now been made illegal in many countries, and industrial processes using lead are strictly regulated. We can only speculate as to the harm done by lead in centuries past think not just of the 19th-century lead workers, but of the Romans with their elaborate systems of lead pipes.
The story of plants with contraceptive powers is one of the more fascinating cases of evolution in action. Humans have known for millennia that certain plants can prevent pregnancy. In the fourth century BC Hippocrates, a Greek medical expert, wrote that ‘Queen Anne’s Lace’ (also known as wild carrot) had contraceptive powers. Other ancient cultures used plants in the fennel family, and possibly also the pomegranate, for much the same reasons. Modern tests have found that these plants do contain chemicals similar to estrogen. But why?
The answer, as we are only now coming to understand, is that this was a defense mechanism for the plants. Plants have evolved many ways to prevent animals eating them. Some use sharp barbs to hurt animals; others contain poisons to make the herbivores sick. And it now seems that some developed contraceptives.
It works like this. A plant evolves to contain a substance which renders animals eating it infertile. Animals that eat it have no offspring, whereas animals that do not eat the plant are as fertile as they would be otherwise. Over the generations, the animals that are predisposed to avoid the contraceptive plant will prosper, and so the animals will evolve to avoid the plant. The plant, therefore, manages to avoid being eaten.
That explains why plants might contain these estrogen-like substances. What we don’t know is how, and how much, plant estrogens affect human reproductive health. Doubtless more news will come in the future, but for now don’t worry too much about this unless you eat inordinate amounts of one particular plant.
One hundred thousand synthetic chemicals are on the market worldwide, being used in industrial processes, as pesticides and for cleaning, even added to food. Testing facilities can barely keep up: there is a legal requirement to conduct some tests before selling a new chemical, but these required tests only scratch the surface of the impact chemicals can have on human health.
Worse still, chemicals manufactured in countries with lower regulatory standards can traverse the globe, affecting human (and animal) health in other lands regardless of the laws there. In short the rise of the chemical industry over the course of the twentieth century has brought some pretty big risks to human health.
Particularly worrying are those chemicals which mimic the effects of hormones. The first, and still the most famous, case of this is DDT, a chemical created in 1938 and marketed as a pesticide. Though DDT was hailed as a great breakthrough, and its creator won a Nobel Prize, it brought with it some immense problems.
DDT, it turned out, acted like estrogen in the bodies of animals. DDT in the environment has caused reproductive problems in chickens, polar bears, gulls, seals, whales, and many, many other species of animal.