A scientist studying human fertility has warned Kiwi men's sperm counts are dropping so rapidly, within a couple of decades there might be none left.
In 2017, Shanna Swan - an epidemiologist at Icahn School of Medicine at Mount Sinai, New York - co-authored research looking at falling sperm counts in New Zealand, Australia, Europe and North America.
Looking at 185 studies dating back to 1973, Dr Swan and her team found a 50-60 percent decline between 1973 and 2011.
It was sparked by a paper she saw while on sabbatical.
"I spent six months... looking in detail to see, was this decline due to different method of measuring sperm, or recruiting men, or where they more obese or did they smoke more?" she told Environmental Health News this week, promoting the release of 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.
"Were there other things that could explain the decline? After six months of work, the answer was no. The slope had not changed after all that analysis."
So she started the proper research that would result in the shocking 2017 paper published in journal Human Reproduction Update - which found the decline was indeed real, consistent and showing no signs of slowing down.
Dr Swan says at current trends, it's on track to hit zero between 2040 and 2045.
"What does that mean? These are median sperm counts - which means half the men in Western countries will have no sperm. This is a big problem, if it were to come to that... If things continue in the same way, it looks very grim for our population survival."
Similar drops in sperm quantity weren't seen in men from South America, Asia and Africa - the scientists unsure whether that was significant, or just because they didn't have enough data from those regions.
So what's behind the drop in Western men's virility? The initial study didn't go into that, but she does in the book.
"I think everyone agrees this is not genetics - this is too fast for an evolutionary change, we're talking about 40 or 50 years," she told Environmental Health News, saying that environmental and lifestyle factors are likely to blame - including the effects of pesticides, cosmetics, foam furniture, paper plates, work stress, fatty food, processed food and packaging.
"The chemicals I'm particularly interested in... are those that affect the body's hormones. The reason is that reproduction depends on healthy hormones. Anything that would interfere with the body's natural hormone system... is going to challenge the reproductive system."
"If women want to have a baby, they are often told, 'Clean up your act,' " Dr Swan writes in the book. "But it's probably more important for men to do so."