A dramatic decline in sperm counts is occurring around the world — more than 50 percent over five decades. This means a man today likely has only half the number of sperm that his grandfather did. As one of the world’s leading environmental and reproductive epidemiologists, Shanna Swan has been sounding the alarm about the human infertility crisis for more than twenty-five years. An expert in environmental threats to human reproductive health, she researches the causes of low sperm counts and other fertility problems, such as the rate of miscarriages, which has risen 1 percent each year for two decades now.