Three of the largest manufacturers of medical-grade nitrogen gas in the U.S. have barred their products from being used in executions, according to The Guardian. The move follows Alabama’s recent killing of death row inmate Kenneth Smith using a previously untested method known as nitrogen hypoxia.

The three companies said March 10 they have put in place mechanisms that will prevent their nitrogen cylinders falling into the hands of departments of correction in death penalty states. It’s the first example of corporate action to stop medical nitrogen being used to kill people, and echoes the almost total boycott that is now in place for medical drugs used in lethal injections.

The campaign has been led by Airgas, is owned by the French multinational Air Liquide, which announced publicly in 2019 that supplying nitrogen for the purposes of execution was not consistent with its values.

Two other major nitrogen manufacturers have also confirmed to the Guardian that they are restricting sales of their gas. Air Products said that it had established “prohibited end uses for our products, which includes the use of any of our industrial gas products for the intentional killing of any person (including nitrogen hypoxia)”.

Matheson Gas said that supplying nitrogen gas for use in executions was “not consistent with our company values,” and that it would not do so.

Other manufacturers of medical nitrogen in the U.S. were more circumspect. Linde, a global multinational founded in Germany and headquartered in the UK, would not say whether it was willing to sell its product for use in U.S. death chambers and declined to comment.