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Pentane is a highly non-polar, light hydrocarbon solvent, that acutely dissolves THC instead of plant matter.
With a boiling point of 36 degrees celsius, this solvent might be a bit heavier than butane but still functions well for most applications.
It can be found as an impurity in butane, but residual pentane is not a major concern as long as the oil is purged thoroughly, with lots of surface area for the solvent to escape from.
In the face of a growing distaste for solvents and an increasing concern for patients with immune complications though, it should be noted that pentane is actually less toxic than butane.