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.