It is not possible to comply with the fresh (outside) air requirement of 10CFM/per person of FAR 25.831(a), Amdt 25-87 when ECON or LO flow is selected in conjunction with a full passenger load. This is one of the reasons that the FCOM only allows selection of ECON or LO flow with reduced passenger loads. The ECS flow rates are sized in relation to the airworthiness requirements and also to ensure good temperature control and ensure that bio-effluent levels do not cause the cabin to appear stuffy. The main indicator for bioeffluent is carbon dioxide which itself is the subject of airworthiness regulation FAR §25.831 (b2), Amdt. 25-89: “Carbon dioxide concentration during flight must be shown not to exceed 0.5% by volume (sea level equivalent) in compartments normally occupied by passengers or crew members.” and JAR §25.831 (b2): “Carbon dioxide in excess of 3% by
volume (sea level equivalent) is considered hazardous…The level of carbon Dioxide in the cabin is dictated only by outside airflow, cabin volume and the
generation of carbon dioxide by the passengers, i.e. recirculation flow rate does not help in this case. Therefore, reducing fresh air flows below the existing levels by selecting ECON or LO flow outside of the FCOM limits can cause an increase in carbon dioxide, which can cause the cabin to feel stuffy and in the worst scenario break the carbon dioxide airworthiness regulations.
In summary, Airbus does not recommend selection of low pack flow outside of the FCOM limits, due to the possibility of non-compliance with the airworthiness regulations, a possible increase in bioeffluent levels causing a degradation in cabin comfort and the possibility of reduced temperature control efficiency.