Might surprise you to know some emit sound like your phone makes when battery going flat.
As some will know my brother was very lucky when he suffered long term CO poisoning.
When i use to vist my brother i could not understand WHY i would feel quite unwell.
I thought given my lung issues ITwas because he lived on a hill.
When in fact it was he's kitchen boiler flu and breather had become detached HOW why well anyones guess BUT if he did not smoke with back door open i don't think he would be here.
Shocking part is boiler was serviced few weeks before AND when co alarm went off was not like smoke detector BUT just a low battery bleep.
How do we know well we changed battry but was still making sound.
I guess thats why give post title AS to my brothers long term health YES it as effected him mentally.