Thanks Simon - I'd replied to the other version of this question before realising that there were several copies of the same enquiry...
Might not now make sense in the context of the series of posts above, but copied from the other thread for reference before it get deleted:
If you're making a food product, I'd be vary wary of using the "CCP" designation for something that isn't a food safety hazard. If an incubator temperature or gas mix potentially leads to e.g. pathogen growth then I'd call it a microbiological hazard, but if these are things that are process critical rather than safety critical then I'd give them some other sort of name/designation.
In terms of the hazard categorisation, I think most in the food sector would think of a physical hazard as something physical that poses a potential hazard to the consumer, and similarly a chemical hazard would be a chemical at a concentration that poses a potential hazard to consumers. The CO2 level would therefore probably only really be such a hazard if it impacted your process in such a way that an incorrect CO2 level caused some sort of chemical contamination of your final product. Both of the examples you've given are hazards to your cell culture, but it doesn't seem like they're a potential hazard for consumers?
In general, there is no specific HACCP-based reasoning why any given step couldn't be a CCP if it meets the requirements of whichever HACCP-based process you're using to do your assessment (there are differences between standards, regulators/countries on the interpretation around this ), but to me it does not sound like this step is one, unless I'm misunderstanding some of the risks here. It sounds as though the step is operationally critical to you, but I'd separate that out from HACCP and the associated nomenclature, as it'll only cause confusion otherwise IMO