In September, American Airlines introduced new flight attendant uniforms for the first time in nearly 30 years. But just days after the release, over 400 flight attendants alleged that the new clothing was making them sick—causing both hives and headaches. Now, that number has reportedly grown to more than 1,300.

After the initial complaints, American blamed the issue on a wool allergy. But the Association of Professional Flight Attendants (APFA) conducted their own test and found that new uniforms, created by the clothes maker Twin Hill, have detectable levels of chemicals commonly found in pesticides, fungicides, and fertilizers.

“We will continue to invest the resources necessary to get to the bottom of this and we fully expect the Company and Twin Hill to do the same,” the APFA wrote to its members in the notice.

 

Overall, the number of complaints only represents about 1% of the 70,000 employees who are wearing the new uniforms, LaKesha Brown, an American Airlines aal spokeswoman, told Fortune.

“We’re still working on what could be causing it [the issue],” she said, explaining that the airline tested the uniforms twice before mass distributing them to employees, and is now waiting for the results from a third test.

In the meantime, according to Brown, employees can choose to wear their old uniform if they are having problems with the new one. The airline has also offered employees to exchange their wool uniforms for ones made of a different material, like cotton.

“We want our employees to be happy with their new uniforms,” she said.

This article has been updated to reflect comment from American Airlines.

Fortune has reached out to Twin Hill and will update the story if we receive response.