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 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.
Previously, only the crew was required to be supplied with fresh air, the rest of the cabin was required to be "ventilated" but no ventilation rate was specified, so it was frequently turned off to save fuel or increase power. (This is approved and is still done on some older generation aircraft that are still in service, for instance the Boeing 737NG for certain high performance situations, search 737 Packs Off Takeoff) The current requirement is .55 lbs of outside air per minute per passenger minimum, through all phases of flight. Permissible CO2 has been reduced from 3% in 1964 to 0.5%. today. The carbon monoxide requirement is unchanged at 1 part per 20,000. Multiple other requirement changes, but those are the ones that are specific and measurable.
Current requirements are published in 14 CFR 25.831: https://www.ecfr.gov/current/title-14/chapter-I/subchapter-C...
Original requirements are on page 47 of this PDF: https://archives.federalregister.gov/issue_slice/1964/12/24/...
How does that work though? The cabin pressurization requires constant bleed air from the engines because there are intentional leaks. So the air is constantly renewed anyway.
In ye olden days before a DCPCS, not only were the flow requirements not as stringent, but there was culture of "cheating the chart" when manually setting the system. More airflow = more fuel burn, and at one time pilots were compensated for minimizing fuel burn. Anyone can see that this is a perverse incentive from a safety standpoint, just look at what former astronaut Hoot Gibson did to the passengers on his 727 trying to save a buck and playing flight test pilot with paying passengers onboard.
https://en.wikipedia.org/wiki/TWA_Flight_841_(1979)
Also, from a cabin crew perspective, drowsy passengers are conveniently compliant passengers.
But stupid shortsightedness aside; it was crazy, they needed to put the smokers (which was me at the time) at the back of the plane as that's the direction of the airflow inside, but the toilets were at the back so everyone had to walk through the smoking section anyway..