Men exposed to certain banned but long-lived chemicals at high levels as teenagers are more likely to have defective sperm later in life, according a new study.

Researchers report today that organochlorine chemicals—specifically DDT and polychlorinated biphenyls (PCBs)—may affect how testicles mature and function. It is the first study to examine men’s exposure to the chemicals during the teenage years and abnormal sperm later in life, and suggests that the chemicals—banned in the United States but still lingering in soil, water and people—may contribute to male infertility.

“These chemicals continue to persist in our environment. Levels are going down over the past 30 years, but all of us still have levels in our bodies,” said lead author Melissa Perry, a professor and researcher at George Washington University’s Milken Institute School of Public Health.

Perry and colleagues examined sperm and blood samples from 90 men from the Faroe Islands in the North Atlantic Ocean. They looked at organochlorine chemicals in their blood as adults, and checked their sperm for abnormal amounts of chromosomes. For 33 of the men, they also had blood samples at age 14.

Men with higher levels of DDE—a breakdown product of the pesticide DDT—and polychlorinated biphenyls (PCBs), which were used in transistors and electronics, at 14 years old had higher rates of abnormal sperm. The same link was found for levels of the chemicals in the men as adults, according to the study published today in Environmental Health Perspectives.

"It's another one of those studies that helps us understand why male fertility is in decline in certain areas of the world," said Thomas Zoeller, a professor and researcher at the University of Massachusetts, Amherst who was not involved in the study.

Previous research suggests sperm with an abnormal amount of chromosomes may lead to failed pregnancies and birth defects.

The study doesn’t prove that the chemicals hamper sperm but both DDT and PCBs are known to disrupt the endocrine system. And sperm production is a “very hormone dependent” process, Perry said.

With many endocrine disrupting chemicals, the major risk is exposure as a fetus or young child. However, sperm production is a "continuous process" in men into adulthood, said Zoeller, who is a leading researcher on endocrine disrupting chemicals.

Organochlorine pesticides were used heavily in the U.S. from the 1940s to the 1960s. Concern over their potential to impact health was stoked in the 1960s, in large part due to Rachel Carson’s 1962 book, Silent Spring, which focused on DDT’s harm to birds and the world’s food supply.

DDT and PCBs were banned in the U.S. in 1973 and 1977, respectively, and the International Stockholm Convention on Persistent Organic Pollutants banned their use and production in 2001, with 170 countries signing on. The bans were in response to increasing evidence of the chemicals’ harm: including effects on fertility, immune systems, hormones and brain development.

However, these chemicals don’t just go away. They are extremely persistent in the environment and the human body. Most people are exposed through diet, from eating food such as meat, fish and some dairy.

Also some countries still use DDT to control for disease-spreading insects. A 2012 report found that “global use of DDT has not changed substantially" since the Stockholm Convention went into effect.

“If other countries use organochlorines and we [U.S] import food from them it can be contaminated,” Perry said.

While the researchers in the current study can’t pinpoint exposure, people in the Faroe Islands eat a lot of seafood, such as whale meat and blubber—which act like storage containers for these persistent chemicals. The men in the study had much higher concentrations of DDE and PCBs than the U.S. average.

Human health scientists have for years suspected organochlorines may impact male fertility but studies have had mixed results. However, in a 2002 Netherlands study, researchers reported PCBs in adult mens’ blood was related to decreased sperm counts. And a 2004 study of Greenland men suggested that regional differences in sperm quality may be related to organochlorine exposure.

Zoeller said Perry's study has relevance beyond the banned chemicals and should inform today's chemical regulation.

"With PCBs ... we know they're bad, but it's too late. Forty years after they're banned we're still contaminated and they're still having an impact on us," he said. "That knowledge, seems to me, should make us more cautious when we produce new chemicals, especially those very similar to these [PCBs and DDT]."

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.