Blood THC levels do not correlate to a level of impairment in the way that alcohol does. Numerous factors apply and, while the prosecution may produce a Department of Justice toxicologist that will testify that high THC levels in the blood correlate with impairment, there are numerous studies that cast doubt on such claims, particularly for heavy users.
Knowing what they mean won't necessarily help you. Depending on any priors and circumstances surrounding the arrest, you may be able to get the charge reduced.
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Laws vary from state to state, but in general, operating under the influence of drugs does not require a certain level being present in the blood. It is the fact of their being present at all.
Asking what level of Delta 9 equates to impairment is a very loaded question, because different experts will testify to the fact that different levels may cause impairment in different subjects.
For example, some experts will testify that most people are impaired at a .05% blood alcohol level, while, others will say it's more like .07-.08%, but they all agree that .20% would be "impaired for the purposes of driving."
To be sure, you'd need a blood split to verify the levels and an expert to look over the levels and the case.
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