Florida has four new cases of the Zika virus spread by local mosquitoes, bumping the total number of cases to 21 in Palm Beach, Broward and Miami Dade Counties.

As the number of infections rises across the U.S., health officials are urging people to use mosquito repellents, including those that contain DEET. But some pregnant women have fears over whether chemicals -- even those registered with the EPA -- are safe to use, reports CBS News chief medical correspondent Dr. Jon LaPook.

Thirty-one-year-old Ali Simon is 37 weeks pregnant and lives in New Jersey. She knows Zika has reached mosquitoes in the U.S., but like many pregnant women we've spoken to, she is hesitant about wearing insect repellent during her pregnancy.

"To be honest, I don't know that I feel comfortable wearing DEET," Simon said. "I would be really hesitant to wear it and I feel like I would only wear it if I absolutely, positively had to."

Dana Vogel heads the division of the Environmental Protection Agency that examines the health effects of pesticides. Vogel said she understands the fear pregnant women may have about using chemicals to try to prevent he mosquito bites that spread Zika, but she says the repellents are "safe for use by all populations, including pregnant women."

"Being a mom - having been pregnant at one point in time - I would use a repellent. I know that they're definitely safe for use, as I participated in the reviews of them," Vogel said.

There have only been a few published studies addressing the effects of DEET on expectant mothers and their children - there's mostly animal research on the topic. However, one study in pregnant women found insect repellent with 20 percent DEET, used daily in the second or third trimester, was safe for both mother and baby.

"No products are put on the market unless a safety evaluation is done and it's found to be safe," Vogel said.

DEET-based products come in concentrations ranging from five to 100 percent. A higher number doesn't mean a product will work better; it means it will last longer.

According to an industry trade group, five percent DEET lasts about an hour, while 30 percent lasts up to eight hours.

"And to me, that's the most important thing - to follow the label and use the repellents as they're labeled for use," Vogel said.

It's also important to remember that whatever repellent you use, you carefully read the label and follow the directions.