BuildsJets here with your aviation trivia. Cabin Air Quality requirements generally have become a lot more stringent since they were first enacted back in December 1964.
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/...