Zirconium compounds (as Zr) TWA 5 mg/m 3 ST 10 mg/m 3 [*Note: The REL applies to all zirconium compounds (as Zr) except Zirconium tetrachloride.] Metal: Soft, malleable, ductile, solid or gray to gold, amorphous powder. Metal: Combustible, but solid form is difficult to ignite; however, powder form may ignite SPONTANEOUSLY and can continue ...