Should you rely on a home breathalyser?

A pint of beer on a bar top
How long will a pint stay in your system? Unless you check you do not know


I’ve been prompted to write this post following a Periscope broadcast by @SgtTCS about drink driving and the use of home breath test kits.

SgtTCS is a serving police sergeant whom I have followed on Twitter for many years. He is extremely dedicated to his work and to promoting road safety. He has used social media to campaign against all sorts of dangers that motorists encounter on the roads.

This is his tweet with a link to the Periscope broadcast:

Personal breathalysers - #31DaysLive Day 78 suggested by @jred196  https://t.co/ityxGz0cTJ


First, I should say that I do not disagree with the points SgtTCS makes but, having spent many years representing drink drivers, I do feel that he, and many other police officers, underestimate the importance of breath test devices in helping people avoid drink driving.

The argument from many seems to be that people should take personal responsibility by controlling their drinking and “have none for the road”. This overlooks the phenomenon of morning-after drink driving, which SgtTCS does address saying that you should leave sufficient time between drinking and driving for the alcohol to clear your system. It is this portion of the argument that I have a problem with because it ignores the physiological differences that exist between us all and, if followed, leaves people with no option but to guess and hope for the best. It strikes me that this is the very approach that people should be avoiding.

You see the problem is that we do not all eliminate alcohol at the same rate. I was invited onto a BBC London talk show recently in which one of the other guests said that the body eliminates alcohol at one unit per hour; that is a fundamental misunderstanding of how our bodies eliminate alcohol.

A normal, healthy, adult human body will eliminate alcohol from breath at between 3.9ʯg/dL/h and 12.6ʯg/dL/h with an average rate of 8.3 ʯg/dL/h. That is a huge difference and you have no way of knowing how quickly your body eliminates alcohol without a test. Over twelve hours it is the difference between a person at the lower end eliminating 46.86ʯg and somebody at the top end eliminating 151.26ʯg!

Let’s say you have a heavy night and drink sufficient alcohol to reach 120ʯg in breath by time your alcohol level reaches its peak. You then leave a further 12 hours before driving. A person who eliminates alcohol at the top end of the normal range would have an alcohol level of zero while somebody at the bottom end will still be at 73.2ʯg, which is more than double the drink driving limit.

You might say that the person at 73 in breath should know that he is over the limit but people frequently fail to recognise intoxication both in themselves and in others, particularly the morning after. In one case I dealt with, the defendant blew 176ʯg in breath – the police had not really thought he would be over the limit and were so shocked that they did not believe the results of their own Intoximeter, I know that because they wrote it in their witness statements. I should also mention the sheer number of people who tell me they drove because they felt fit to do so – they could all be lying but I doubt it. From personal experience, I have woken up feeling fine after a night of drinking only to test myself and discover I was still over the limit! I seriously doubt I am the only one.

So, how do you know whether you are the person who is at 73 or the person at zero or, as is more likely, somewhere in-between? Unless you have an effective way of testing your breath alcohol level you have no idea and will simply be guessing when you decide whether it’s safe to drive or not.

Home breath tests are no where near as reliable as their police equivalents but they do have a place in helping people to avoid committing a crime and to that extent the police should be encouraging their use.

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