> a well-understood BAC-like scale for marijuana usage, especially as the drug becomes more and more legal to use.
That'd be tough. BAC is a meaningful metric because alcohol crosses the blood-brain barrier very easily. So, the amount of alcohol in the bloodstream is a very good predictor of the amount of alcohol acting on the brain.
THC, on the other hand, does not cross the blood-brain barrier so readily. So the problem isn't that we can't measure the amount of THC in the blood; it's that it doesn't actually tell us anything meaningful[0].
> Lots of people have a decent approximate sense of what 0.08 BAC is when it comes to alcohol consumption, but everyone who is "high" is just "high" without some sort of well-defined scale.
I actually don't think it's true that most people have an approximate sense of what .08 is. I think that most people believe they do, but most people actually have a very flawed misunderstanding of what .08 means (and doesn't mean).
You can test this out yourself! If you want to be the life of the party, buy a portable breathalyzer for about $100. When you go out, ask your friends to guess what their own BAC is (and what their friends' BAC are, based on how drunk they're acting). I bet you you'll find really large disparities between the numbers. People just aren't very good at gauging how drunk they are (or how drunk their friends are). Or, alternatively, BAC just isn't a very good measure of impairment - you can interpret this either way.
That'd be tough. BAC is a meaningful metric because alcohol crosses the blood-brain barrier very easily. So, the amount of alcohol in the bloodstream is a very good predictor of the amount of alcohol acting on the brain.
THC, on the other hand, does not cross the blood-brain barrier so readily. So the problem isn't that we can't measure the amount of THC in the blood; it's that it doesn't actually tell us anything meaningful[0].
> Lots of people have a decent approximate sense of what 0.08 BAC is when it comes to alcohol consumption, but everyone who is "high" is just "high" without some sort of well-defined scale.
I actually don't think it's true that most people have an approximate sense of what .08 is. I think that most people believe they do, but most people actually have a very flawed misunderstanding of what .08 means (and doesn't mean).
You can test this out yourself! If you want to be the life of the party, buy a portable breathalyzer for about $100. When you go out, ask your friends to guess what their own BAC is (and what their friends' BAC are, based on how drunk they're acting). I bet you you'll find really large disparities between the numbers. People just aren't very good at gauging how drunk they are (or how drunk their friends are). Or, alternatively, BAC just isn't a very good measure of impairment - you can interpret this either way.
[0] http://www.ncbi.nlm.nih.gov/pubmed/12404577