The Grade E standard is 10 PPM max. I don't believe there is any medical evidence that 10 PPM is hazardous, although there's plenty of speculation that it could be.
In the US, yeah - but I would have to wonder: If the compressor is letting 6 ppm CO into tanks what else are they passing?
10ppm would not be a problem if you didn't dive. If you do submerge, increasing atmosphere, then the problem happens - and it's complicated. Even if you were fine at 130 feet breathing tainted air and loading up your blood with CO, as you ascend - PPO drops but PPCO remains much longer, so the PPO that was keeping you alive stops doing that as well.
Many developed countries limit the max to 5ppm, some 3ppm - for good reason. But fill stations seldom have inline CO monitors nor test.
Because if
@49north names the shop, this thread will be moved to the "Thumbs Down" forum.
No, the reason a person needs to think twice about naming the provider is that law suits happen. Scubaboard once nearly got bankrupted by one. The accused was guilty and the lawsuit was frivolous, but it took a lot of money to establish that. The person posting the accusation may do so with a screen name, but s/he can still be sued.
To be honest, I've never given it much thought as I always get fills from very reputable shops.
Famous last words. Past performance is no indication. Compressors overheat, burn their own lubricating oil, and produce CO at times - maybe in only a few tanks. Ever play Russian Roulette with real bullets?
Not true, not in the least.
Is it risky to not own a CO analyzer?
The agencies don't even require than shops have their air tested. They used to, but it wasn't enforced, and besides - most shops would take samples only at the best times. Even then, 3% failed. 3 poison tanks out of 100?