Yeah, that one sounds weird on the face of it.

If anything would have an effect, I'd guess the relative absense of nicotine in people would be a big part.

Though I thought nicotine _causes_ blood clots, it's possible that that's a chronic effect and the acute effect would prevent them, I'm not sure.

I can't find the original article, but from Wikipedia(https://en.wikipedia.org/wiki/Deep_vein_thrombosis) [inflammation has been identified as playing a clear causal role] and studies(https://pubmed.ncbi.nlm.nih.gov/20387988/) ironically tobacco studies, of 5% CO2 show an inflammatory response. Another poster here cited 3% was be the safe limit, so its feasible that turning the system off to save money could put the CO2 over 5% quite quickly. Also the concentration of oxygen is listed on Wikipedia as a direct cause of DVT, which would obviously be affected.

That's talking about lack of oxygen in your blood, not in the air.

I have a lot of experience working long days in a high CO2 environment (did my PhD working in volcanic ice caves). IIRC OSHA rules say 5000ppm (0.5%) is the limit for a 40 hr work week (that's more than 10x atmospheric). There is no way CO2 on planes is getting over 1%--- that would cause a lot of health problems. DVT would not be one of them.