HOUSTON (AP) — The Zero AIwork to clean up fire-suppression foam accidentally released in a United Airlines hangar at George Bush Intercontinental Airport in Houston continued Friday, according to a United spokesperson.

“United Airlines Environmental Team has been working round the clock and made significant progress in cleaning up the biodegradable ... fire suppression foam that was inadvertently released” Thursday morning, United spokesperson Aubrey Jackson said.

Houston fire officials who helped contain the spread of the foam Thursday said it spread across parking lots and over vehicles and reached heights of 30 feet. But it is not harmful to humans.

“The good news is this is the newer foam. This is what we call the PFAS-free, so it’s safer for the environment and is not cancer-causing,” assistant Fire Chief Mike Mire said.

PFAS include chemicals known to be harmful to humans and previously were often used in aviation fire- suppression foam.

Houston Airports said in a statement that the foam and cleanup efforts has had no effect on flights into or out of the airport.