Increasing Bisphenol-A levels in urine, already linked to poor male sexual function and infant brain development problems, also decreases sperm count, a study said Thursday.

BPA is a chemical created in producing polycarbonated plastics and epoxy resins found in baby bottles, plastic containers, dental sealants and food and beverage cans.

For their five-year study of 514 workers in factories in China, researchers found that men with higher urine BPA levels were two to four times more at risk of having poor semen quality, including low sperm concentration, low sperm vitality and motility.

"Compared with men without detectable urine BPA, those with detectable urine BPA had more than three times the risk of lowered sperm concentration and lower sperm vitality, more than four times the risk of a lower sperm count, and more than twice the risk of lower sperm motility," said study lead author De-Kun Li.

Li is a reproductive and perinatal epidemiologist at Kaiser Permanente, a California-based care consortium.

He said urine BPA was not associated with semen volume or abnormal sperm morphology.

The research, published in the journal "Fertility and Sterility," was the first human study to report an adverse association between BPA and semen quality. Previous studies found a negative link between BPA and male reproduction in mice and rats

It was also the third study in a series by Li and his colleagues examining BPA's effect on humans. The first study, published in November 2009, found that exposure to high levels of BPA in the workplace increases men's risk of reduced sexual function.

Increasing BPA levels urine are also associated with worsening male sexual function, according to the second study, published in May 2010.

The latest study, funded by the US National Institute of Occupational Safety and Health, throws further doubt on the safety of BPA.

"The finding of the adverse BPA effect on semen quality illustrates two points: first, exposure to BPA now has been linked to changes in semen quality, an objective physiological measure," Li explained.

"Second, this association shows BPA potential potency: it could lead to pathological changes of the male reproductive system in addition to the changes of sexual function."

The researchers noted that BPA may also affect female reproductive systems and have adverse effects on ailments such as cancer or metabolic diseases.

Canada became the first country to ban the chemical, on October 14. Last year, it was also the first to ban rigid plastic baby bottles, before France, Denmark and other countries followed suit.

In March 2009, the six biggest US producers of baby bottles stopped selling BPA-laced products in the United States.

And in January 2010, the US Food and Drug Administration expressed concern about BPA's potential health risks after having declared it safe in 2008.