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A toxic planet has been polluted so badly due to neglecting environmental caring measures by it's former inhabitants, that no life can exist without extreme protection measures. This makes every type of working on this planet a hard and relatively unproductive task. Fortunately, there are some waste disposite and environmental rehabilitation measures that can remove the toxic gases slowly but steady. This of course is only successful, if industry cares for environment not producing more pollution than it can compensate.

A cleaned toxic planet will become a Barren planet.

Toxic planets can't be terraformed.

Toxic planets can't be selected as starting planet's type of a custom species.

Toxic planets can be colonized by races that either have the Terraforming technology or have a Barren starting planet.