Shrugs off

It made Heroditus ashamed the way the generator had been adjusted, its function deliberately violated to transform it from a source of clean, sustaining air to a purveyor of death. Instead of removing the toxins from the air, the generator was now pumping them into it, a vicious cocktail of proticide that would bond with its victims on a molecular level, rendering the protons within the atoms unstable. Death would be almost instant and within an hour, even the evidence of a corpse would be obliterated as the body reduced itself to a sort of protean goo.
...
There were too many orks to simply poison into extermination and, even if they could, there were the Iron Warriors themselves to consider. Against a normal human, the poison was invariably lethal, but the holy Adeptus Astartes and their fallen kin were a different matter. Proticide had proven incapable of defiling the enhanced genetics of a Space Marine, the advanced healing and regenerative properties of their cells nullifying the poison almost instantaneously. The best that exposure could inflict on an Iron Warrior would be a sensation of nausea.

Siege of Castellax pg. 210-211