The presence of neurocognitive deficits alone is not usually considered sufficient evidence of neurotoxicity, as many substances may impair neurocognitive performance without resulting in the death of neurons. This may be due to the direct action of the substance, with the impairment and neurocognitive deficits being temporary, and resolving when the substance is eliminated from the body. In some cases the level or exposure-time may be critical, with some substances only becoming neurotoxic in certain doses or time periods.