The number of ‘air changes’ refers to the number of times the air is cleared in the machine enclosure per minute.
Reasons for a lower number of air changes (i.e. 1 to 3) could be because the machine cycle is longer than average which means it isn’t necessary to change the air so frequently.
In all cases, the unit should achieve a negative pressure within the machine enclosure which prevents any oil mist egressing from the cabinet.