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Investigation of cabin ventilation strategies impact on aircraft cabin air quality and passengers’ comfort and wellbeing through subject study in realistic aircraft environment

Periodic Reporting for period 2 - ComAir (Investigation of cabin ventilation strategies impact on aircraft cabin air quality and passengers’ comfort and wellbeing through subject study in realistic aircraft environment)

Reporting period: 2020-05-01 to 2021-03-31

The quality of the air we breathe is crucial for our health, well-being, comfort, and cognitive performance. Many contaminants can contribute to the degradation of the quality of air, among others, volatile organic compounds (VOC) emitted from different sources like humans. Humans also exhale carbon dioxide used as a marker of air quality but recently suspected to modify the quality of air as well. Aircraft cabins present indoor environments with distinctive features, where passengers are exposed to a mixture of outside and recirculated air. They are characterized by high occupant density, inability to leave the environment, low relative humidity, need for pressurization, and pollutants whose origin are predominantly passengers and their activities.
The ComAir study aimed to investigate the impact of cabin air quality on passengers’ comfort and well-being when reducing outdoor air intake and changing passenger density; this main objective was additionally extended by studying the effects of exposure to elevated CO2 and pollutants emitted in the cabin. The primary reason for this work was to provide an optimal cabin environment for passengers in terms of comfort and well-being with as low as possible environmental impacts of cabin systems in terms of fuel burn and pollutant emissions.
To meet the project objectives, various proportions of outdoor air (representing bleed air on actual aircraft) in the total air supplied to the cabin and realistic profiles of cabin pressure, noise, temperature, and relative humidity were applied. All tests were performed during simulated flights carried out in the Flight Test Facility low-pressure vessel, which contains the front part of an A310 airplane cabin. Two different experimental designs were used. In the first one, a 2 (occupancy) X 4 (air ventilation regime) factorial design was used. Occupancy denotes the number of people in the aircraft (half vs. fully occupied cabin) and examines the psychological important well-being factor of proxemics. The four ventilation regime levels (with target CO2 concentrations between 1,200 and 4,200 ppm) were: baseline with typical aircraft airflow regimes per person, ASHRAE 161 requirement (standard), ASHRAE 161 half (half of the recommended flow), and a recirculation regime with a target CO2 concentration close to regulatory limit. In the second design, a 2 (VOC) X 2 (CO2) test matrix was used to investigate the disentangled effects of these air quality parameters on well-being (CO2 is used in regulations as a proxy for air quality without knowing the separate impact of VOCs and CO2).
To reach the study objectives, the project used two main approaches: an extensive literature review and human subject experiments. The review of typical flight environments and parameters served as input for the experimental designs and, in turn, was extended to form an air quality metric substantiated by the subjects’ reaction to different air regimes. Part of the literature review was published in the Indoor Air Journal (Chen et al., 2021).
For human subject experiments, a reliable and valid test battery was developed to measure passengers’ comfort and well-being including physiological and environmental parameters. The battery was aligned with a typical sequence of a 4.5 hour flight. All procedures were approved by the responsible ethics committee.
Overall, 686 people representative of flight passengers in age and sex participated in the randomized controlled studies containing sixteen simulated flights. Analyses showed comparable environmental data during traditional ventilation regime (baseline) as in real aircraft cabin measurements, found in the literature review. Across exposures, a blind panel of subjects trained to assess air quality rated odor as perceptible and slightly unpleasant during all tests.
The full analysis of the 2 X 4 study design showed a strong impact of the occupancy factor on well-being and comfort: participants in the fully occupied cabin reported less comfort, worse psychological wellbeing, lower air quality and showed more physiological stress than participants in the half-occupied cabin. For several outcomes an interaction between occupancy and ventilation regime was found – there was no or sometimes even positive effect on responses (improvement) when increasing recirculation air rate in the half-occupied cabin and negative trends (worsening of responses) with increasing recirculation air rate in the fully booked cabin. Sensitivity analyses for several control variables confirmed the robustness of effects. Regarding gender the analyses showed consistently more favorable ratings from women than men for all outcomes with two exceptions: Women reported lower comfort regarding temperature and showed more signs of stress than men. In general, levels of comfort and wellbeing gave no reason for concern, even with the highest recirculation regime. For the 2 X 2 experiment no systematic effects were found.
Results of the human subjects’ studies and the literature review were used to form an air quality metric as the last main result of the study. The developed metric included 1) compliance with current regulations, 2) inclusion of pollutants documented to be present in aircraft cabins, 3) representation of different sources of pollution occurring in aircraft, 4) protection against different risks likely to occur during exposure to air in aircraft cabin such as discomfort and compromised health conditions, 5) robustness against other factors that can potentially modify the risks, 6) responsiveness to actions and processes aiming to improve air quality, and 7) pollutants easy to use in the control loops for managing aircraft cabin air quality.
Experimental procedure and baseline data were published as conference paper of the International Conference of Environmental Systems (Herbig et al., 2020). Environmental parameters due to increased cabin recirculation airflow fraction were published in Aerospace (Norrefeldt et al., 2021) and as conference paper in the IOP Conference Series: Materials Science and Engineering (Norrefeldt et al., 2021). Other publications are in preparation.
The integrated results of ComAir can be used to help developing adaptive Environmental Control Systems (aECS) and give recommendations for the future cabin air quality standards that would ensure optimal cabin environment for passengers with as low as possible environmental impact of cabin systems in terms of fuel burn and pollutant emissions
ComAir is one of the very few existing large-scale experimental studies that investigated aircraft cabin air quality effects on passengers under real-world conditions, and probably the only one that used the Gold standard of medical and psychological study design by randomly assigning a large number of people to the different conditions instead of using cross-over designs, repeatedly exposing the same, smaller number of people to different environments. This design allows statements on causation and generalization of study findings as well as application in practice.
The test setup can be used to develop and conduct further research activities, both, within the aviation sector as well as for other indoor air contexts. Results can be used for the development of sensors and algorithms for automatic control of ventilation systems in aircraft cabins, other means of transport and in buildings, especially in those with spaces densely occupied by people.
View into the empty cabin of the Flight Test Facility
Participant filling in questionnaire (reenacted scene)