Phthalates are a group of chemicals used to make plastics more flexible and harder to break. In several studies over the last two decades, they have been shown to disrupt the operation of male hormones like testosterone and have been linked to genital birth defects in male infants.

Dr. Swan, who conducted a 2008 study about phthalate exposure, said that scientists have had the ability to measure exposure to plasticizers only since about 2000, via urine. That has led to a 20-year lag in the process since researchers cannot enroll men to produce sperm until they are in their 20s.

That evidence is the “missing piece of the puzzle,” she said.

Professor Pacey cautions that while the changes in data may be driven by “greater exposure of pregnant women or adult men to more man-made chemicals,” it is too soon draw a conclusion.

No trend studies were performed in the first half of the 20th century, said Niels Skakkebaek, a reproduction researcher at the University of Copenhagen, but in the 1940s, fertility doctors claimed that men should have at least 60 million sperm per milliliter to be considered normal and that many had more that 100 million per milliliter.

“Nowadays, average young men have 40-50 per milliliter,” he said.

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.

“We must find out which ones are to blame for the problems with male reproduction, including male infertility and testicular cancer,” Professor Skakkebaek said.

The website for the Centers for Disease Control and Prevention says the effects from low-level exposure to phthalates are unknown, but it acknowledges that some types of phthalates have affected the reproductive system of laboratory animals and that more research is needed. The agency declined to offer further comment.