Also consider that humans "outgas" in many ways and this process increases as the ambient pressure decreases. As we expel the excess gases produced they mix with the available mass of existing gases in the environment. If you operate at a lower pressure your "exhaust" will have a greater impact on the existing mass of gasses. NASA does not advertise this fact but you can bet that it is plenty ripe enough on the ISS at one atmosphere. Removing these contaminants takes time and the processing is more efficient at higher pressures.

Missions on ISS are much longer term that in the past and the cumulative impact of the environment has more opportunity to multiply the effect.

Consider that humans "outgas" in many ways and this process increases as the ambient pressure decreases. As we expel the excess gases produced they mix with the available mass of existing gases in the environment. If you operate at a lower pressure your "exhaust" will have a greater impact on the existing mass of gasses. NASA does not advertise this fact but you can bet that it is plenty ripe enough on the ISS at one atmosphere. Removing these contaminants takes time and the processing is more efficient at higher pressures.

Missions on ISS are much longer term than in the past and the cumulative impact of the environment has more opportunity to multiply the effect.