Improving indoor air quality for transport users

In recent years, there has been increasing concern about the health effects of indoor air quality. Changes in infrastructure design, conceived to improve energy efficiency, have meant that modern infrastructures are more airtight than older structures. Furthermore, advances in building technology have caused a much greater use of synthetic building materials. While these improvements have led to more comfortable infrastructures with lower running costs, they also provide indoor environments in which pollutants are easily produced and may build up in higher concentrations than found outdoors.

With this in mind, in the last year, transport-related infrastructures (such as transport stations and bus stops) have been working to improve ventilation levels in order to reduce COVID-19 virus transmission and to decrease the risk of exposure, but without losing sight of the improvement of comfort and energy efficiency following the green policies approved by governments.

What can be done to improve indoor air quality?

The main objectives of the ventilation of tunnels and stations are different depending on whether it is fire operation or normal operation. In the event of fire, the aim of ventilation is to drive the fumes out of evacuation paths. For normal operation, the intention is to renew the air of stations and platforms in order to control temperature and humidity.

Focusing on underground stations, the problem of high indoor temperatures and low air quality has not been a priority issue to be addressed, but it is a health concern, and it occurs in underground transport systems all over the world, regardless of weather conditions. The problem is linked to the presence of high internal thermal loads, mainly due to the operation of the trains – especially the braking system, which accounts for about half of the total internal thermal load – followed by the facilities and, to a lesser extent, the occupants. In order to ensure proper ventilation, forced or mechanical ventilation systems are used, especially on lines that run deep and where it is particularly difficult for a natural ventilation system to be effective.

Currently, the measures that are being adopted in stations are hybrid ventilation systems (natural and forced) or natural ventilation systems (ventilation shafts). In addition, architecture has much to contribute by introducing natural conditioning strategies and qualitative factors that prioritise the user. The plans that contemplate forced ventilation systems in stations feature misting, conventional and tunnel fans.

Safety screen doors are the other most effective solution, completely enclosing the platform and opening only when the train has completely stopped at the station. It would protect passengers from air pollution and also prevent people from jumping or falling onto the tracks.

According to data collected in transport infrastructures, the pollutants are PMs, NOx, CO, SO₂, CO₂ and other pollutants. The most critical pollutant is PMs, concentrations of which change due to: train movement; by switching the fans on or off; or by the fresh air intake through passengers’ accesses or ventilation shafts. CO, NOx and SO₂ concentrations on the platform are very low and probably controlled by the ingress of traffic‑contaminated street‑level air. CO₂ averages vary, changing during the accumulation and exchange of passengers with each passing train.

The aspects to consider, which involve the air quality in a station or on a platform, are the following: design, ventilation system, location of the station, possible safety screens and the type of train driving, as well as outdoor air quality and the movement of people.

Looking to the future, ventilation systems should seek to improve comfort, air quality and energy efficiency, whilst trying to achieve this with ‘zero’ consumption infrastructures and sustainable systems. But, above all, always preserving the indoor air quality, especially in public spaces.

Smart ventilation systems

As we have seen, ventilation systems in transport infrastructures are not an easy facility to manage manually. This issue is solved by the installation of management ventilation systems, programmed with schedules and static logics, but the infrastructure conditions are changing each minute, making the actual management system ineffective.

In order to achieve future goals, current ventilation systems may be combined with: an air filtering system; adequate insulating materials; good real-time monitoring of the equipment (operation levels, energy consumptions, etc.) and environmental (temperature, humidity, pollutants, etc.) conditions; and the implementation of artificial intelligence (AI) in control logics.

This whole improved system will be known as a ‘smart ventilation system’, since it will be able to make decisions and actions on the equipment on its own, achieving a more optimal and precise operation according to the required needs in each moment.

These smart ventilation and air conditioning systems consist of the possibility of continuous and real-time monitoring – through Internet of Things (IoT) devices – of the parameters involved in comfort, air quality and energy efficiency. This is a solution that could be implemented by public transport organisations.

The artificial intelligence platform can read the temperature, humidity, air quality and electricity consumption in stations in real time. An algorithm combines this data with information such as the weather forecast and expected occupancy levels to predict environmental conditions at the stations, and then accordingly programmes the operation of each fan to regulate temperature and energy consumption. Intelligent control of the ventilation will also bring the maximum amount of fresh air inside, which will increase hygiene and provide better air quality, reducing pollutant levels (PMs, CO, NOx, etc.) and reducing the risk of spread of COVID-19 and other micro-organisms.

The challenges ahead

There are still some challenges to address in this sector, though, such as: making consumers aware of the importance of indoor air quality and the advantages of good air conditioning; transmitting confidence to people who are still insecure when returning to their work centres or using public transport; and establishing operational protocols for action and safety so that the user feels as safe as at home.