We weren't taking about 1000s of PPMs? we were taking about something being in the 200-400ppms region.
Your argument applies to all open flued appliances, not just gas ones. (i.e. blocking vents, and birds nests)
TBH I do not wish to take this thread further off topic than you already have but I will answer your questions.
You brought up a similarity of 1,000's ppm with a coal being in a similar situation and it being perfectly safe. The topic and my argument is solely based upon BBU's. If you wish to open a new thread and discuss, I will be happy to do so there and you can categorise all types of appliances all you wish.
I am indeed taking about levels of 200 - 400ppms and the normal running conditions of a BBU. I am treating a BBU differently to all other open flue gas appliances, such as gas fires, DFE's, etc. Because the other appliances do not have a lint arrester on route to the primary air intake. Thus, they do not suffer as much or certainly as quickly as the other O/F appliances. I did mention this in all my previous posts. If you are used to servicing BBU's, you will be in no doubt aware of this issue.
Hence, my honing in this particular issue as I find they barely remain with low levels of CO outside a year of usage and certainly being that, the lint arrester becomes very much restricted, due to induction of air bringing with it carpet lint, dust, pet hair, etc.
With regard to your comment of 25ppm will also be fatal. How so? 100ppm of exposure would only give slight headaches within 4 hours of continuous exposure. A reasonably heavy smoker's breath would produce about 25ppm, so how would a smoker breathing on me fatal. Perhaps their bad breadth might knock me
Finally, not to be giving gas advice on a public forum, I FGA BBU's via the gas fire. As you know there is a flue way from the gas fire through to the boiler flue. I have an adapted probe that fits nicely in and gives me a reading whilst the BBU is running.
You also mentioned higher levels on HE appliances. This is indeed true, but they are forced draught appliances. Excess air is also riding much closer to stoichiometric combustion and therefore the limits are acceptably higher. Down firing HE in particular may have in excess of 200ppm, however G.A.D. lowered their acceptable ratio to 0.004 instead of 0.008. Some manufacturers such as Worchester Bosch and Vokera lowered their limit to below 0.002 and less than 200ppm, otherwise they require to shut them down.
SE appliances such as the BBU are induced draught and therefore work within an excess air of approx 150 - 200%, thus much lower limits should be obtainable. For example, take a very common Vokera Mynute 16e. This would be in the region of 25 - 35ppm. Both equal in their combustion process, both induced draught burners, but one has a lint arrester and a very low heat exchanger. The BBU encourages a higher CO and therefore higher safety ratio and also has a parameter to encourage flame chilling.
As I said in my initial response to the OP, there is nothing wrong with the safety of a BBU as long as the service engineer understands what to watch for, what to clean and the importance of a CO alarm within the room and the need for an annual service.
What the OP has found is that some major gas companies with "experienced" gas engineers ran a mile from the BBU, not even knowing how to take the fire off the unit and one not even knowing how to use the 3-way gas isolation valve.