I had intended to ask this question and validate my estimates a long time back but forgot about it.
Based on the table below ( factor in partial pressure of CO at 4ATM for 1hr for recreational divers ) I arrived at a quick guesstimate that anything more than 10ppm would be risky. Consider also that it’s the very first dive of a 10 day vacation that cost you $$$$ and you don’t want to retire to bed with a headache and unsure about diving the next day…
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I request the more experienced and technically minded folks here to validate my assumptions…
(Edit : I think I may have mixed up ppm with percent of gas in the mix, leading to incorrect guesstimates…)