The pesticide widely used to fight Zika-carrying mosquitoes in Florida and across the nation has been linked to deficits in motor functions in Chinese babies, according to a new study.

The study, whose authors say it is the first to examine real-world exposure to naled outside workplace accidents or lab experiments, used cord blood from 237 mothers who gave birth to healthy babies at a hospital in southeast China between 2008 and 2011. At six weeks, the babies displayed no problems. But at nine months, the babies had slight problems with coordination, movement and other motor functions.

The University of Michigan study was published in the journal Environment International.

While the study provided only a close snapshot of a particular group of mothers, the authors say it suggests the need to take a closer look at using naled to fight mosquitoes, particularly since problems surfaced at lower exposure levels than previous studies.

"Just because changes are small, that doesn't mean they should be discounted," said lead author Monica Silver. "We really need to know more about it."

Naled has been used for decades in Florida to control pesky marsh mosquitoes, sprayed by plane mostly over suburban fringes bordering marshes and mangroves before dawn. But last year, it drew more attention, and quick opposition, when Miami-Dade County and other urban areas battling Zika began using it in neighborhoods where active transmission of Zika was occurring. Zika began rapidly spreading through Brazil over the summer of 2015, leading to the births of an estimated 2,300 babies with microcephaly and severe brain damage.

A spokesman for Amvac, the U.S. manufacturer of the pesticide, said the company had no record of selling naled in China and said the U.S. government has approved its use for mosquito control for five decades.

"We do not sell naled into China and have no idea how it may have been used or how much is applied," spokesman Brian Maddox said. "We cannot verify the validity of the China study, knowing nothing about the source of the product or how the population was selected."

Silver said it was impossible to determine how the Chinese mothers ended up with naled in their blood, although she suspected it was used on crops or mosquito spraying. The team used cord blood collected between 2008 and 2011 by co-author Betsy Lozoff for another study that looked at iron deficiency and brain development. They found a number of pesticides but focused on five that occurred in traceable levels.

To measure problems, researchers used a standard motor-skill test that looks at reflexes, body control, movement and hand and eye coordination. As exposure to naled increased, they found deficits also rose.