WASHINGTON (Reuters) – Environmental groups said on Thursday they plan to sue the U.S. Environmental Protection Agency for failing to regulate aircraft emissions after a 2016 agency determination that those emissions pose a danger to public health.

The Center for Biological Diversity and Friends of the Earth, represented by Earthjustice, filed a notice of intent to sue the EPA.

Airplanes are the third-largest source of U.S. transportation-related greenhouse gas emissions and the largest not subject to greenhouse gas emissions standards.

The groups said the EPA last year announced it would release a proposed rule by September 2019, but failed to do so. EPA did not immediately comment.

(Reporting by David Shepardson; Editing by Chris Reese)