The sperm count of men in Western countries has been declining precipitously with no signs of leveling off," according to new research, bolstering a school of thought that male health in the modern world is at risk, possibly threatening fertility. An international team of researchers ... looked at semen samples from 42,935 men from 50 countries from 1973 to 2011. They found that sperm concentration - the number of sperm per milliliter of semen - had declined each year, amounting to a 52.4 percent total decline, in men from North America, Europe, Australia and New Zealand. Total sperm count among the same group also tumbled each year for a total decline of 59.3 percent over the nearly 40-year period. Decreasing sperm count was first reported a quarter century ago, but the new analysis shows that "this decline is strong and continuing," said Dr. Shanna H. Swan, one of the study's authors. A 2005 study, Dr. Swan said, showed that prenatal exposure to phthalates, also called plasticizers, affected the development of sons. Phthalates are a group of chemicals used to make plastics more flexible and harder to break. They have been shown to disrupt the operation of male hormones like testosterone and have been linked to genital birth defects in male infants. Professor Skakkebaek, an author of a 1992 study that suggested chemicals play a role in the steady decline in semen quality, has since indicated that a rise in abnormal male reproductive systems may be linked to exposures to endocrine-disrupting chemicals.