Quoting a reliable source
It is not enough to pressurise an aircraft; it also has to be ventilated to provide comfort to the occupants and to remove contaminants.
There are two main types of ventilation system: one provides one hundred percent fresh air at all times. As the air coming out of the engine is extremely hot, it is passed through an air conditioning unit before it enters the cabin. This air then passes through the outflow valve and is replaced by fresh air coming from outside and the air conditioning unit.
The other type of ventilation system provides a ratio of fresh air and recirculated air. An example of that system is seen on Figure 2.3 (Appendix ‘C’). In this case, the air leaving the air conditioning unit is directed to a mixing unit where it meets air coming from the cabin. Before entering the mixing unit, the air from the cabin passes through a particulate filter. In modern aircraft, these filters are normally of the HEPA (High Efficiency Particulate) type. There are different levels of efficiency within the HEPA filters and there are no current regulations covering this particular aspect of ventilation. These filters only trap particulates; they have no effect on gases. After leaving the mixing unit, this mixed air is passed into the cabin. A proportion of it is exhausted through the outflow valve and the rest passes back to the mixing unit where the cycle continues. In the current commercial fleet, the ratio of fresh air to recirculated air is usually about 50:50.
In general the cabin ventilation is far superior to an office environment and the entire cabin air is exchanged every 3-4 minutes.
While there has been and still are some controversies about recirculated air, it is well accepted by ventilation experts that one hundred per cent fresh air at all times is not necessary. Indeed, nearly all commercial buildings in the last three decades have been supplied with recirculated air. In other words, if the ventilation system is adequate, recirculated air is totally acceptable. In fact, from a comfort standpoint, one clear advantage of air recirculation in aircraft is the somewhat higher degree of relative humidity.