I instead started thinking about the CO2 / O2 exchange in the alveoli in our lungs.
If I drink a beer, the initial “beer smell” is probably from my mouth. Later a more generic “alcohol smell” comes from alcohol/air exchange in the lungs, and this is what a breathaliser picks up.
I don’t fully understand the CO2 / O2 exchange mechanism in the lungs, but I have to assume evolution got it right - for CO2
Why then does that mechanism also release alcohol as well as CO2 into the exhaled breath?
As blood passes through the lungs, some alcohol evaporates and moves into the lungs.
So it doesn’t matter how the alcohol gets into your body, once it’s in your blood it will get into your lungs and exhaled. Anything in your blood that evaporates will show up in your breath.
The oxygen carbon dioxide transport mechanism isn’t responsible for alcohol in your breath. Haemoglobin is exquisitely targeted to purpose and moves a lot of oxygen and CO2 about. Alcohol just evaporates. The characteristic smell on your breath is more from breakdown products (such as Acetaldehyde) of alcohol as your body metabolises the alcohol, and aromatic flavours (hops for instance) rather than alcohol itself. Ethanol is pretty much odourless. The breathalyser tests look for actual alcohol, not what smells.
The respiratory membrane is formed where the capillary walls and walls of the alveoli (the air sacs) touch (as a simplification anyway) and is very thin - about half a micron thick. Gasses can diffuse across this membrane. So long as the molecule of the gas isn’t too big.
Just saying diffusion might sound like ducking the answer. In essence small molecules can dissolve into the membrane structure and on average, whichever side has the greater concentration of that molecule, molecules will move from that side to the side where the concentration is lower. Breathing perfuses the lungs with air, sweeping away CO2, so CO2 will preferentially flow into the lungs from the blood, and bringing new O2, so O2 will preferentially flow into the blood. The exact makeup of the cell walls helps this process.
Evaporation is just a way of saying that a molecule has enough energy to break free of the surface. Given you are warm, they do.
Just to add a few points from the enforcement point of view. They really don’t want us to know or talk about the science behind it because a defense attorney could lead us down a rabbit hole we couldn’t get out of. Cops don’t need to know the science, they need to know the proper operation of the machine. The science was proven in court in the past.
BAC is Blood Alcohol Concentration. It’s right there in the name. In the few cases I’ve seen where a breathalyzer was used and then blood was taken at a hospital shortly after, the two readings match. The blood test can take it a couple more decimals but that’s not important.
When using the breathalyzer there has to be a 20 minute waiting period where the subject doesn’t have anything in their mouth, doesn’t burp or vomit. It is necessary to ensure the breathalyzer does not measure mouth alcohol. What about roadside breathalyzers you ask? In those states that allow them they are used as one piece of probable cause. The breathalyzer back at the station that gets used after the waiting period is what can be used to get a pro se conviction.
I’ll gently dispute this, having worked in clinical and research labs where ethanol was present. It does have a distinct, slightly sweet odor that can be smelled in blood collection tubes at a high enough concentration.
One memorable day on the clinical urinalysis bench, I opened a specimen cup of urine collected from an ER patient and it reeked of alcohol. I called the ER doc and asked if they were aware that the patient had been drinking, and they said no. What may not be detected on the breath by smell alone can make itself very well known in urine and sometimes blood.
I don’t really understand why one would think the results would be any different than drinking the alcohol. You’re still using your gastrointestinal tract to get the alcohol into your bloodstream. The lungs behave the same way in either case.
That is possible. But more likely a ticket for a refusal and a ticket for DUI. The reading is needed to get a pro se conviction. The arrest happens because there is already probable cause showing intoxication and operation.
The reality is it is much more difficult to get a conviction for DUI without a reading.
Back in my college days the word was to never, ever agree to take a “field sobriety test”. The reason being it is subjective that can result in a conviction regardless of the results of the subsequent BAC test.
The penalties for DUI being rather severe, and BAC limit steadily reduced, having even driven past a liquor store within a day or two prior leaves one under suspicion with overzealous cops. Keep in mind this is their job, they do it every day, day after day. They know what to look for, they know how to spot drunks. Driving without headlights at night around bar closing is a dependable method. Or the windows down in winter.
Anyhoo, one can decline a “field sobriety test”, what one cannot refuse is a breathalyzer or blood alcohol test. This is definitive. If you haven’t been drinking this might be important to know. If you have been drinking, then you shouldn’t be driving.
IANALEO, but I think he mean Per Se conviction, which is based solely on the BAC. If your BAC is over the set amount, you will be convicted without needing to prove impairment. Impairment convictions are based on your actions and responses, and need not have a BAC reading to back it up. Obviously, having both makes convictions easier.
It did occur to me, much later, that both nicotine and THC cross the air/blood barrier when inhaled, so I was completely wrong in my thinking that only CO2 and O2 would cross the barrier.
I was a bit focused on my (wrong) idea that the barrier had some sort of chemical or mechanical system. But it appears to be simple osmosis.
Probably should have posted in a different forum, but such is life. Thanks all for the responses.
Right. Typo. A BAC of .08% or over (in the US) is by law impairment. You don’t have to legally prove anything else to prove impairment.
In trial you would have to present observational evidence that there was probable cause to suspect impairment to make it a legal arrest. That’s not the same as proving beyond a reasonable doubt that there was impairment.
Two avenues of the defense to take is to go after the probable cause for arrest or to try and get the reading tossed. If the probable cause is found faulty everything after gets tossed. If the reading gets tossed then the case will hinge on observation only. Nowadays that’s often backed up with video but even with that it’s much more difficult to get a conviction.
Yes, I remember being pulled over by the police outside a bar. They were so excited that they’d caught someone who forgot to turn on the headlights the one cop jogged a block to the station for their portable breathalyzer. I blew zero. Thing was, I didn’t regularly drive, it wasn;t my car, so I forgot about the lights. I was at the bar with friends (drinking Coke) so became the designated driver. I had to go to the other bar down the street, pick up one friend, then come back for the rest of them.
I was nice and polite to the officers and they didn’t give me a ticket for the “no lights”.
There’s the Gordon Lightfoot Defence which the noted (late) songwriter got away with from a judge who was a fan. “Just because I blew over .08 in the station doesn’t mean my body had absorbed that much from my stomach by the time I was pulled over.”
Needless to say, that was overeturned on appeal by the crown.