Introduction:
The COVID-19 pandemic has demonstrated a noticeable deficiency in the comprehensive understanding of airborne infectious disease transmission risk specific to the human transportation sector, particularly the ship-board environments. Large passenger ships can be characterised as enclosed and crowded indoor spaces with frequent interactions between international travellers, providing the perfect conditions for the transmission of disease^1-3.

Methods:
This paper presents the results of an indoor air quality (IAQ) monitoring study at nine different ship environments with low-cost CO₂ sensors, including mass gathering locations and cabins, onboard a passenger cruise ship voyaging across the UK and EU. CO₂ concentrations, temperature and relative humidity (RH) were simultaneously monitored to investigate the thermal characteristics and effectiveness of ventilation performances aboard. To optimise the comparability of measurements, monitors underwent co-locations with factory-calibrated instruments and subsequent correction factors were applied⁴. ACH and VR was calculated using the decay method, by evaluating the CO₂ concentration when occupants vacated the venue until it reached background level (occupancy estimated based on seating plan). In addition, the risk of airborne infectious disease was estimated by employing a well-mixed Wells-Riley model.

Results:
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 (<1000 ppm) was measured with CO₂ mainly varying between 400-1200 ppm. The estimated air change rates (ACH) and ventilation rates (VR) implied sufficient ventilation was provided in most locations, and the theatre (VR: 86 L s-₁ person-₁) and cabins (VR: >20 L s-1 person-¹) were highly over-ventilated. Dinning areas have recorded high CO₂ concentrations (>2000 ppm) potentially due to its high footfall and limited ACH, indicate a potential risk of infection and should be prioritised for improvement.

Conclusions:
The IAQ and probability of infection indicated that there is an opportunity for energy saving. This study sets the stage for further exploration and provides practical recommendations for the optimisation of ventilation operations in passenger ships.