Packed in Metal Tubes: How Smart Exhaust Systems Could Transform Air Quality on Trains

This week, we dive into a fascinating study published in Railway Engineering Science titled Study on Air Quality Improvements in Train Compartments by Coordinating Exhaust of Multiple Air Outlets, to explore a question that fundamentally challenges how we think about ventilation in mass transit: What if the single biggest barrier to protecting passengers on high speed trains isn't our lack of technology—but our complete failure to recognise that static, one size fits all ventilation systems can never deliver both thermal comfort and effective infection control across changing seasons? Key Topics Discussed: The Physics of Human Thermal Plumes: Most modern trains pump fresh conditioned air in from vents in the ceiling. But where does that stale air go? Usually it's exhausted through vents down near the floor. This study used CFD modelling to look at what happens if we rethink that. What if we coordinate exhaust at both the top and the bottom of the carriage? Human bodies create thermal plumes. We are basically 100-watt heaters and warm air naturally rises off us, carrying our exhaled breath upwards. In the summer, when the air conditioning is trying to cool the cabin, if you exhaust the air entirely from the top of the carriage, like near the luggage racks, you are working with that natural thermal plume. The hot, polluted air rises off the passengers and gets sucked right out of the top vents. The study found this significantly improves the cooling efficacy and efficiency. You aren't forcing hot air down past passengers to reach the floor vents. It also clears out pollutants near the passengers' breathing zones much faster. The Winter Problem: In the winter, the scenario flips. You're trying to heat the cabin. If you pull all the air out from the top vents, you are just sucking all that nice buoyant warm air straight out of the roof before it reaches the passengers, leaving their feet freezing. It creates massive temperature stratification and makes people incredibly uncomfortable. The study found that in winter, a 50 50 split using both the top and the bottom exhaust provides the best balance. The bottom exhaust pulls the warm air down to feet level, maintaining thermal comfort, while the top exhaust still helps capture the rising respiratory pollutants. The End of Static Ventilation Design: This proves that static one size fits all vent designs is dead. You cannot have a ventilation system that operates the exact same way in January as it does in July and expect optimal results for both comfort and importantly infection control. We need dynamic systems. If you want to truly manage the risk of airborne transmission in mass transit, while simultaneously hitting our net zero energy efficiency targets, train manufacturers and frankly designers of any large, densely occupied space, need to implement variable exhaust systems that adapt to the season, the thermal load and the occupancy. The Future of Mass Transit Ventilation: It's a brilliant example of how nuanced, targeted and responsive our ventilation strategies need to become to protect public health in the built environment, especially technical ones like trains. You can imagine the exact same thinking being applied to buses, to aeroplanes and so on. This is the frontier of mass transit air quality management. https://link.springer.com/article/10.1007/s40534-025-00398-0 The Air Quality Matters Podcast in Partnership with Particles Plus https://particlesplus.com/ Eurovent (https://www.eurovent.eu/) - Aico (https://www.aico.co.uk/) - Lindab (https://www.lindab.ie/) - S&P UK (https://www.solerpalau.com/en-uk/) The One Take Podcast in Partnership with SafeTraces (https://www.safetraces.com/) - Inbiot (https://www.inbiot.es/?utm_campaign=simon&utm_source=airqualitymatters&utm_medium=podcast) - Farmwood (https://farmwood.co.uk/) - iE Electronics (https://www.eielectronics.ie/) and iAir Group (https://iair-group.com/) Zehnder https://www.zehndergroup.com/en Do check them out in the links and on the Air Quality Matters Website. (https://www.airqualitymatters.net/podcast) If you haven't checked out the YouTube channel its here (https://www.youtube.com/@airqualitymatters-SimonJones). Do subscribe if you can, lots more content is coming soon.