The first cases of locally transmitted Zika virus in the U.S. this month present a clear concern for pregnant women, with the virus’s connection to birth defects now well-established.

Far less is known about the potential long-term risks of mosquito-repellent chemicals, particularly DEET, which doctors would normally advise pregnant women to avoid.

DEET is a pesticide as well as an ingredient in many popular insect repellents. It is considered the best at warding off bug bites. Government recommendations state that if, used according to instructions, DEET repellents are both safe and effective, even for pregnant women.

But very little research has been done into the effects of a repeated low dose of DEET — the popular name for N,N-Diethyl-meta-toluamide or diethyltoluamide — on the skin, with long-term follow-up, experts say.

Most of the evidence about DEET is focused on very high doses and accidental ingestion, both of which typically cause seizures, said Dr. Robert Wright, chairman of preventative medicine at Mount Sinai School of Medicine. Seizure risk from skin application of DEET by contrast is “quite low,” he said.

But “what’s not known is if a pregnant woman were to put it on fairly frequently — whether or not that has any effects on their baby is essentially unknown,” he said. Since there’s so little research on the subject, “it’s hard to form an informed opinion about how to recommend” repellent usage.

Related: Don’t use these products to protect from the Zika virus

One 2001 study, which looked only at repellent use during the middle and later stages of pregnancy, found that the chemical can cross the placenta. Still, the study said results suggested “that the risk of DEET accumulating in the fetus is low and that DEET is safe to use in later pregnancy.”

So far, in the continental U.S., Zika virus is only thought to be locally transmitted in the Wynwood neighborhood in Miami, though the Wall Street Journal is reporting the existence of Zika cases in the area defy that geographic boundary.

Read more: Florida’s Zika investigation reveals significant questions about extent of the virus

Miami-based lawyer Sheila Oretsky told Law.com that she was using a chemical repellent despite her concerns. “I drench myself in chemically ridden, DEET-filled bug spray, which generally while you are pregnant you probably would stay away from those kind of chemicals, but I guess when you weigh the risk of ‘Do I want to expose myself to chemicals?’ versus ‘Do I want to expose myself to the risk of Zika?’ you pick the lesser of two evils,” Oretsky said.

In areas without Zika virus, particularly northern U.S. states that don’t have the types of mosquitoes that transmit Zika, “I’d probably recommend not using [DEET],” said Wright. “In Florida, I think it’s a harder question. I don’t think I’d be telling you the truth if I said I knew the answer.”

For those Zika-affected areas, he recommends staying indoors as much as possible and using repellents with lower concentrations of DEET, which is effective for shorter periods of time.

See: With 10 more Zika virus cases in Florida, governor asks for emergency response team

There are other government-approved insect repellents, including some made with natural ingredients. But DEET is generally considered the most effective, and natural repellents require more frequent application.

Picaridin, oil of lemon eucalyptus and IR3535 repellents are available from brands such as SC Johnson’s Off! and Spectrum Brands Holding Inc.’s SPB, -1.40% Cutter and Repel.

Any repellent registered with the Environmental Protection Agency has undergone EPA review and has had the agency accept data supporting its claims.