The top U.S. aviation regulator said the government won’t mandate that airlines inform staff when crew members they’ve worked with have contracted Covid-19, but is encouraging them to take voluntary steps to lower the risks.

Federal Aviation Administration chief Steve Dickson, responding to appeals from the largest pilots’ union in North America, said in a letter released on Friday that the agency takes seriously allegations that airlines aren’t adhering to health guidance from the U.S. Centers for Disease Control and Prevention.

“While the FAA remains steadfast in its focus on safety of flight, we are not a public health agency,” Dickson said in the letter to the Air Line Pilots Association, which was dated April 14. “We must look to other U.S. government agencies for guidance on public and occupational health.”

The FAA has worked with the CDC to create guidance on health monitoring of airline crews, screening protocols, and aircraft cleaning, and is continuing to improve the recommendations, Dickson said in the letter.

ALPA had called on the aviation agency to mandate CDC’s guidance for cleaning and notifying crew members when other employees they’ve worked alongside become ill with the virus, alleging that some carriers haven’t been following them.