Health-Equivalent Energy Efficiency Factor, combined metric of harm and energy use

The inclusion of health-based performance indicators and metrics in ventilation system design and research is a widely discussed topic in recent years. This is due to increased awareness about the health implication of indoor air quality and due to the need for innovative ventilation system control (smart ventilation) to limit building energy use.  
The main target of most smart ventilation systems on the market today is limiting the building energy use while maintaining a comfortable indoor environment. The health aspect is therefore often overlooked although it would lead to a dilemma is ventilation system optimization: to what extend does achieving energy saving justify an increase in the concentration levels of unhealthy pollutants? 
The rationale behind a new metric is introduced that combines and quantifies the combined performance of energy saving and a harm based metric of a smart ventilation system compared to a chosen reference as the ratio between the energy use indicator of the smart ventilation system and the energy use of the reference line at equal harm: The Health-Equivalent energy efficiency factor. The used reference can be either a pre-defined ventilation concept (e.g. continuous ventilation) or based on energy and health targets. 
Eight smart ventilation strategies are modelled, simulated, and analysed and for each, the health-equivalent energy efficiency factor is calculated using two possible references. The first refence-line is defined by the base continuous ventilation system simulated at 10%, 50% and 100% of the nominal flow rate, ηs_ystem. The second reference-line is a straight line connecting two extreme, theoretical scenarios: (1) which energy use is acceptable for a situation with no harm and (2) what is the minimum target of harm for a situation without any energy use, η_linear.  
Based on the results and insights gained while applying the metric, η_linear is preferred as it results in more versatile and more widely applicable metric. The application of the metric shows that only one of the smart ventilation systems under investigation is unable to provide health-equivalent indoor air quality energy efficient based on both ηsystem and η_linear. The performance of the nominal reference system C (mechanical extract ventilation) is unacceptable based on η_linear.