Falling sperm counts and changes to sexual development are barrelling us towards a fertility crisis according to Professor Shanna Swan. The environmental and reproductive epidemiologist predicts that most couples may have to use assisted reproduction by 2045. In 2017, Swan and her team of researchers completed a major study which found that over the past four decades, sperm levels among men in Western countries have dropped by more than 50 percent. They came to this conclusion after examining 185 studies involving close to 45,000 healthy men. Her new book Count Down lays out the ways in which chemicals in the modern environment are implicated in changing and endangering-human sexuality and fertility and provides practical guidelines for reducing exposure, both as individuals and societies. Professor Shanna Swan is based at Icahn School of Medicine at Mount Sinai in New York.