I've worked in this field for years, and the stuff about being able to detect a faulty boiler or pollution is absolute guff.
If you do a test on a smoker, say on 10-15 a day, you'll get a score between say 10-30, if they smoke a great deal it can go even higher (that's parts per million).
If you test non-smokers who cycle on polluted roads, it comes up between 1-2 absolute max, and usually 0 for everyone else.
So- a smoker will test at 24, your non-smoker will test at 0 or 1. What does 1 or even 2 tell you? Nothing, you can't attribute it to living by a busy road at all, because there's a margin of error/picking up tiny amounts for most people, and you are not sitting next to an exhaust pipe inhaling it. Even going in a busy pub in the days of smoking didn't cause it to rise above the non-danger zone.
Inhaling smoke directly into your lungs is not the same as living on a busy road in terms of carbon monoxide inhalation- it is not good for you it may well have some health effects, but it just isn't in the same order of magnitude whatsoever.
As for the faulty boiler gubbins, this is rubbish, because the best place to put a carbon monoxide check is in the rooms near the boiler, there's no point in testing the mother and not, say, other children if their bedroom is near a faulty source. I have never ever found anyone with an alarmingly high reading who genuinely doesn't smoke- it's vanishingly rare and they are actually, as someone else said, offering false reassurance about something for which this monitor is not best designed to test.
This monitor tests whether the individual who is blowing into it has smoked in the past 12/24 hours. That's all, and the rest is misinformation to protect themselves.