I would agree with the point that a person can be impaired below a concentration of five nanograms per millilitre. They can be impaired at a concentration of one nanogram per millilitre. Directly correlating a concentration to impairment is very difficult. With respect to impairment and THC, there are a number of factors that toxicologists consider. You consider the route of administration and that will tell you something about how quickly it could come on and how long it will last, so the duration of action.
If you have something like smoking, it's very fast to go into the bloodstream and into the brain, and it has its impact and can last up to approximately six hours. It may be less. If a person consumes it, ingests it, then it takes longer for that active component to get into the bloodstream. That active component, THC, is then metabolized into another active compound. Both of those compounds are contributing to the activity, but you don't see a high peak concentration as you would with smoking. The duration then can be longer than six hours with consumption.
On top of the route of administration, we also consider the potency of the products. Obviously, if it's of greater potency or a person is a more effective smoker.... Chronic smokers tend to be very good at getting all of the active ingredient into their bloodstream and therefore into their brain. The main part about whether we can offer an opinion on impairment is the time elapsed since use. If I have a time and a concentration and all kinds of information, I certainly can be more helpful. Unfortunately, I don't work in that world. I work in the world where I have maybe a concentration and maybe some additional information, but that's certainly not ideal for me to offer a fulsome opinion.