Given the environment that Rakghouls' thrive in (toxic waste pits, radiation-leaking reactors), it's pretty safe to assume that any kind of airborne toxin that could kill Rakghouls would be extremely fatal to any other species trying to colonize Taris.
Consider, the Rakghouls are based on humans who are not native to Taris.
Something that is highly toxic to humans may be completely harmles to, say, Duros. And vice versa.
Different biochemistry means vulnerability to different toxins.