wilf789 said:
the person who had the fork-lift accident might not have even consumed for a week or two beforehand, and maybe only at a weekend
aren't there ways of testing how immediately prevalent the MJ is in your system - something akin to an MJ breathalyser?
This is exactly the challenge with current MMJ laws and potential legalization. A true urinalysis (liquid chromatography-mass spectrometry) can show nanogram (ng) levels of substances in your system, but are large expensive machines and take time (done them for years in my lab). Not something a police officer can use if you are pulled over and suspected of DWV (driving while vaped). Blood works too, but slightly invasive for traffic stops. You can look at excretion profiles (which is how we usually do it). But you need a baseline measeure of the substance in the body, in our case 11-nor-9-carboxy-Delta(9)-tetrahydrocannabinol (THCCOOH), to determine if there is any change in the profile. This is how WADA (World Anti-Doping Agency, major regulatory body associated with all olympic and most sports drug testing) does it. Atheletes give basline blood levels. If they are tested and the levels do not match the baseline = Red Flag!
It is common knowledge that MMJ is lipid soluble (stored in fat) and so is released in small amounts when you exercise or even sleeping when using fat stores for nutrients (especially if you have a large concentration already stored in you fatty tissues). This is a
small part of the reason why someone "might not have even consumed for a week or two beforehand, and maybe only at a weekend" and it still shows up in your system. However...
Cannabinoids (and most drugs; benzodiazapines, opiates, barbs, etc.) do not work like alcohol. Alcohol works on zero-order kinetics. In other words, it is eliminated at exactly the same rate from your body every hour. The usual rule of thumb is "one drink per hour". And it will almost always work like this. If you have 10 drinks, it will take approximately 10 hours to eliminate all the alcohol. Obviously metabolism, drink size, etc. may alter this but not drastically. If you drink a case of been in 1 hour, you will have alcohol in your system for approximately 24 hours.
Cannabinoids work on first-order kinetics which essentially means they follow the half-life rule. For example, some cannabinoids have a half-life of one hour (for comparison with alcohol kinetics). This means that half of the remaining drug concentration in your body will be eliminated every hour. Thus, after 1 hour, half (1/2) of the drug will be left in your system (at this point, all alcohol would be eliminated, 0.00 BAC). After 2 hours, half of that remaining half will be eliminated, thus giving you 1/4 of the original dose still circulating in your blood. Keep going...the usual rule of thumb is the "5 half-life rule". Enough of the drug is removed from your body after 5 half-lives to essentially be eliminated, but it could still be seen with blood or urine in ng measures. In our example, 5 X 1 hour is 5 hours for elimination compared to 1 hour for alcohol. So, if you consume a large amount in one setting (like the case of beer example above), the amount in your body after even 2 or 3 half-lives might still be realtively large when measured, weeks or months after initial consumption (see below)
Here lies the major problem; some cannabinoids have been shown to have half-lives of 7 - 30 days. Thus, 7 days X 5 is 35 days that the drug can remain in your body. Will this amount come out on a simple 5-panel screen done at most places; doubtful. But that also depends on initial levels, etc.
The other problem with a "breathalyzer" type model is that THC is not water soluble and not measurable in that manner. Alcohol evaporates at room temperature and at even higher rates when you are exhaling (warm body temp heats it up to reduce the vapor pressure). Thus, easily measurable in gas form. With cigarettes, I can measure the amount of CO that you expel on a simple Monoxide Breathalyzer and determine how long it's been since you've smoked a cigarette. MJ smoke does not contain the same levels of CO. In addition, MJ does disrupt some motor control, but not in the same areas of the brain as alcohol. Thus, a field sobriety test is not valid to determine if someone is under the influence of MJ. It might lead the police office to invoke probable-cause and then...well, good luck
Do we go with a zero-tolerance policy? I don't think this is the answer given the complex pharmacokinetics and pharmacodynamics of cannabinoids. With little to no research in the US, and some good stuff coming from Spain, Portugal, Netherlands and Japan, the US may not be the country to set the standard for these impairment measures or even cannabinoid receptor-targeted drugs (the future of MMJ IMO).
Hope this wasn't too remedial or long-winded. But it annoys me when everyone talks about legalization and even MMJ but doesn't consider the legal, research and technological hurdles we still face for basic public safety on the roads and for workplace conditions.
IMO 
