Wickson Cheung

While outdoor urban greening is recognised for its benefits, indoor green infrastructure (iGI) in shaping indoor environmental quality (IEQ) - including air quality, thermal comfort, and bioaerosols - remains underexplored. This ten-question paper identifies key challenges, opportunities, and research gaps in the iGI-IEQ nexus, organised under 10 questions across five thematic clusters: (1) biophysical and technical performance; (2) ecological and microbiological dynamics; (3) human health and wellbeing; (4) equity, access, and socio-economic factors; and (5) implementation and systems integration. Findings indicate that iGI can improve air quality, regulate humidity, and enhance thermal comfort. However, its performance depends strongly on plant density, species selection, and ventilation. Most evidence comes from controlled settings. iGI may offer positive psychological and cognitive benefits, and can reduce health inequalities through affordable indoor interventions. However, significant data scarcity exists for long-term field studies, indoor microbial ecosystem effects, and socio-economic accessibility. Widespread adoption of iGI requires quantification of proven benefit conditions, followed by overcoming technical, operational, and regulatory barriers via adaptive design, digital monitoring, and interdisciplinary collaboration. As a culminating synthesis, this study introduces a newly developed comprehensive matrix that classifies twenty-six indoor greening types across twenty IEQ parameters, incorporating an assessment of current data confidence. This matrix lays a foundational framework for informed decision-making and design guidance. This review offers evidence-based insights for researchers, policymakers, and practitioners to effectively leverage iGI where suitable, in creating healthier, climate-resilient residential and commercial buildings, addressing both immediate IEQ challenges and supporting long-term sustainability objectives.

Understanding airborne pathogen transmission in cruise ship environments remains a critical challenge due to the confined nature of indoor spaces, high occupancy, and limited access for real-world experimentation. This study addresses the gap in empirical data on particulate matter and CO₂ dynamics aboard operational cruise ships, providing a high-resolution dataset that can be used for the validation of Computational Fluid Dynamics (CFD) models and informing infection probability risk assessments. An experimental trial was designed for two mechanically ventilated cruise ship rooms (R01, R02), instrumented at ten locations under eight ventilation scenarios: R01 with 100 % (S1a) and 50 % (S1b) design flow rates; R02 with 100 % (S2a), 50 % (S2b) and 10 % (S2c) design flow rates; R01 with high aerosol rate and 50 % flow rate (S3); and R01 with an air purifier at maximum (S4a, 1300 m3 h−1) and minimum (S4b, 422 m3 h−1) clean air delivery rate (CADR). A live UK-EU cruise hosted the experimental trial. Particulate matter and CO₂ concentration, temperature and relative humidity were collected using portable sensors to build a unique dataset to validate subsequent computational modelling of aerosol dispersion, infection probability and transmission prevention, mitigation and management (PMM) approaches in arbitrary passenger ship spaces. As expected, PM and CO₂ were markedly reduced under 100 % design flow ventilation compared with 50 %. Maximum PM2.5 reductions were 84 % during background, 29 % in build-up, and 72 % in decay experimental phases. An air purifier further reduced particulate matter, with peak PM reductions of 57 % (PM10), 48 % (PM2.5), and 45 % (PM1). These findings offer practical guidance for optimising air quality management strategies in cruise ships and other high-occupancy spaces, besides providing a crucial high-resolution dataset for validating numerical modelling. Moreover, this study provides valuable insights into mechanically ventilated shipboard airflow behaviour.

The COVID-19 pandemic demonstrated a profound inability of pre-pandemic passenger ship policies implemented by both ship operators and governmental authorities to detect and address newly emerging diseases. The essentiality of maritime transport puts into focus the risk of approach to address known and new emerging airborne infectious diseases that, due to increasing capacity, are likely to occur on passenger ships. In order to enhance the passenger experience, prepare shipping for pandemics like COVID-19, and improve the resilience and safety of the industry, this review critically synthesises existing literature on (1) monitoring ventilation conditions and aerosol dispersion, linking them to airborne transmission risk using airborne aerosols and ventilation performance as input parameters for computational fluid dynamics (CFD) simulations, and (2) modelling airborne disease transmission risk in controlled passenger ship environments. This review analysed 39 studies on aerosol monitoring, thermal comfort, and infection risk modelling on passenger ships (2000–2023). Additionally, 55 papers on CFD modelling of airborne pathogen dispersion were reviewed: 22 included validation, with most focused on built environments and only four specifically addressing ship environments. Two major challenges relate to the complexity and poorly characterised ventilation boundary conditions on passenger ships, and the other is the lack of suitable validation data. For this reason, ship experimental studies are required for CFD model validation. Only a handful of studies were found that have measured aerosol concentrations on board passenger ships. To the best of our knowledge, there have been no studies conducted on aerosol mass or airborne transmission sampling on board passenger ships or other types of vessels. The results of this review have the potential to create synergistic connections between experimental and modelling studies to inform, characterise and improve the development of numerical models that can accurately estimate infection risk on ships for prevention, mitigation and management of outbreaks.

Large passenger ships are characterised as enclosed and crowded indoor spaces with frequent interactions between travellers, providing conditions that facilitate disease transmission. This study aims to provide an indoor ship CO2 dataset for inferring thermal comfort, ventilation and infectious disease transmission risk evaluation. Indoor air quality (IAQ) monitoring was conducted in nine environments (three cabins, buffet, gym, bar, restaurant, pub and theatre), on board a cruise ship voyaging across the UK and EU, with the study conducted in the framework of the EU HEALTHY SAILING project. CO2 concentrations, temperature and relative humidity (RH) were simultaneously monitored to investigate thermal characteristics and effectiveness of ventilation performance. Results show a slightly higher RH of 68.2 ± 5.3 % aboard compared to ASHRAE and ISO recommended targets, with temperature recorded at 22.3 ± 1.4 °C. Generally, good IAQ (20 L s−1 person−1) were highly over-ventilated. Dining areas including the pub and restaurant recorded high CO2 concentrations (>2000 ppm) potentially due to higher footfall (0.6 person/m−2 and 0.4 person/m−2) and limited ACH (2.3 h−1 and 0.8 h−1), indicating a potential risk of infection; these areas should be prioritised for improvement. The IAQ and probability of infection indicate there is an opportunity for energy saving by lowering hotel load for the theatre and cabins and achieving the minimum acceptable VR (10 L s−1 person−1) for occupants' comfort and disease control. Our study produced a first-time dataset from a sailing cruise ship's ventilated areas and provided evidence that can inform guidelines about the optimisation of ventilation operations in large passenger ships, contributing to respiratory health, infection control and energy efficiency aboard.