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The environmental and reproductive epidemiologist predicts most couples might need assisted reproduction by 2045.
Swan and and Stacey Colino have just released a book on the subject, Count Down: How Our Modern World Is Threatening Sperm Counts, Altering Male and Female Reproductive Development, and Imperiling the Future of the Human Race.
In 2017, Swan and her team of researchers completed a major study that found over the past four decades sperm levels among men in Western countries had dropped by more than 50 percent. The study involved examining 185 studies involving close to 45,000 healthy men.
"I'm concluding that it may become increasingly difficult for us to reproduce ourselves.
"It's already become more difficult than say 40 years ago, which was the first point of our study.
"Sperm concentration was close to 100 million per millilitre, but now it's dropped to 47," Swan said.
"This is an alarming decline and it's not disputable."
"The majority of men would now be below 40 million per millilitre, which is the range of sub-fertility.
"It becomes increasingly difficult to conceive a pregnancy in the natural way and would lead many more couples to assisted reproduction."
Lifestyle factors affecting sperm health and fertility include exposure to smoking, excessive drinking, lack of exercise, poor diet, and stress. Adopting a healthier lifestyle could improve sperm health, Swan said.
Endocrine-disrupting chemicals, which impact on our bodies' hormones, and other chemicals found in the environment were also taking a toll on sperm health, she said.
"We all know that reproduction depends on hormones like testosterone, estrogen and progesterone... and if those are put out of whack by the chemicals in our daily lives, in our foods, in our personal care products and in our home furnishings, then we are going to experience problems with reproduction."
Studies have linked endocrine disrupting chemicals with girls entering puberty at increasingly younger ages, she said.
Swan has studied phthalates, which make plastics soft and flexible, act as fragrances in personal care products, and are found in foods.
"What they can do is lower testosterone. This has been shown in the laboratory."
The rise of gender fluidity and increasing numbers of transgender people could also be related to chemicals impacting on human hormones.
Swan said when a breeding animal was exposed to phthalates, their male offspring sometimes had "incomplete genitals". The same could occur in human children, with risks of phthalate exposure highest in early pregnancy, she said.
"How those boys are born with those smaller genitals, that is a lifetime effect and it actually is linked to them having lower sperm count."
"Many studies have linked exposure to these chemicals to declines in litter size and endangerment of multiple species."
People could reduce the number of phthalates in their bodies by buying unprocessed foods, which are not wrapped in plastic, she said.
"Definitely do not microwave in plastic.
"I would try to avoid fragrance, because all fragrances have phthalates and phenols in them. For example, wall fresheners, those tags in your car, you want to not be breathing that stuff, you don't want it in your personal care products, in your laundry products, in your cleaning products - try to buy them fragrance free."
Swan said dust posed a risk and recommended leaving shoes at the door to avoid bringing contaminants into the house and having a HEPA filter on the vacuum cleaner.
Some demographers were predicting the world's population would plateau in about 2150 and would then decline and not recover, she said.
While many might think lower human population numbers would benefit the planet, Swan said couples had a right to reproduce.
"They should not have their fertility impaired by chemicals in the environment that they did not ask for.
"This is an assault on our fertility that is immoral.
"Regardless of the number of children in the world, every couple that wants to have a child or wants to have multiple children has the right to do so."
The odds of creating a baby could be improved if men banked their sperm and women had their eggs frozen before their quality began to deteriorate as they aged, Swan said.