Dr Shanna Swan, a professor of environmental medicine and public health at Mount Sinai school of medicine in New York City, talks to Rachel Humphreys about declining fertility in men. Over the past 40 years, average sperm counts among western men have more than halved. She tells Rachel how certain chemicals can interfere with reproductive health. Phthalates, used to make plastic soft and flexible, are of particular concern. They lower testosterone, which can affect sperm count, though they are bad for women, too, and can increase the risk of miscarriage and premature birth. The reproductive crisis is serious, she says. We need to try and limit our exposure to these chemicals otherwise human survival could be threatened.