Do you know your alcohol consumption limits?

Driving home for Christmas, after a bottle of sherry. I take a look at the levels next to me, I am not safe.
14 December 2021

Interview with 

Kai Hensel, University of Cambridge


Driving a car


If you were nipping out to the pub for one drink, would you drive home? While we have guidelines for how much alcohol consumption is deemed safe before getting behind the wheel, ultimately, it's up to us to judge whether we’re over that - and our own - limit. According to research published this week though, we are not at all good at estimating how drunk we really are. Julia Ravey spoke to Kai Hensel from the University of Cambridge about these mis-measurements, and, without too much persuasion, volunteered to do her own self-assessment...

Kai - We had 90 participants that first had a standardized meal to ensure that everyone had the same caloric intake based on sex, age, body, mass index, and so on. Then everyone had the same amount of alcohol over time and we asked participants to come forward when they thought they were now on the edge of not being allowed to drive a car anymore. 40 to 50% based on the study day were actually already exceeding that limit. They then kept drinking for about double of the amount of the echo concentration that you would be allowed to have in your blood to still drive a car. The more the intake was the poorer, the judgment, so the less accurate the self-estimation was.

Julia - Wow. So essentially the more drunk people got the less good they were estimating if they'd reach that limit.

Kai - Exactly.

Julia - I am treating myself to a nice little glass of wine. It's really cold outside and, you know what? I deserve a treat. But I'm gonna see if I can judge when I have reached the legal limit for driving, which in the UK is 0.08%. I've bought myself a little breathalyzer and what I'm gonna do is eat my dinner. I think for me, being five foot five, I could have a medium to large glass of rosé and I think I'd be fine to drive. But I'll take the breathalyzer and see if that is the case. How good am I at estimating my alcohol limits? Cheers to a good week. [drinks alcohol] I've just finished my dinner and my glass of wine. Before I take my breathalyzer, I have to be nil-by-mouth for 15 minutes. Just to clarify, I'm in my house and I'm not leaving for the night, but I feel fine and I think I would be okay to drive a car. While I wait to take my breathalyzer, I thought I'd ask if there is any way that we can improve how perceptive we are about how much alcohol we've had.

Kai - In this study. We asked everyone to come back a week after they participated first in the study, and then we did the same again. We again, looked at how well they could estimate whether or not they were still under the drink-driving limit to drive a car. Interestingly, they became more accurate on the second time. While we didn't tell them the first time how drunk they were, just by participating in the study, they were already improving their self estimates.

Julia - After performing this study, what is the take-home message for people thinking about driving home after having a drink?

Kai - The take-home message is not really surprising. Obviously it's unsafe to drive while you're under the influence of alcohol. The importance is that more people than would you think are actually misjudging how drunk they are already. There's actually quite good estimations you will find apps or websites where you can just put in 'I'm 26. My body weight is six kilograms and I'm female. I had two beers over three hours. What's my estimated blood alcohol level?' You could use those as a guess and they are not too bad on average. This works quite well.

Julia - It's been 15 minutes now and I'm ready to take my breathalyzer. I dunno how much I should trust this because I bought it off the internet and it says it is intended to control overdrinking of alcohol. Well, it's true in a way, isn't it? We don't want to overdrink alcohol. [breathalyzer beep] 0.34? No, that can't be right. That means I'm almost five times over the limit. I'm gonna do that again. [breathalyzer beep] Percentage is 0.06. I think that's more like it. Okay. As a good scientist, let's do one more test. [breathalyzer beep] 0.06. So I am technically under the legal limit right now to drive in the UK. But in other countries like Germany, the limit is 0.05 so with that one glass of wine, I'd actually be over the limit. That first reader really scared me. I can't believe it said 0.34%. I would've drank about three bottles of wine to be at that level. That's the importance of repeating things at least three times. As I'm actually not driving anywhere right now and I'm in my flat for the evening, maybe I should repeat the drink test as well. Cheers.


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