A matter of

How innovative filtration solutions help reduce aerosol concentrations in buses

How innovative filtration solutions help reduce aerosol concentrations in buses


Public transport is essential for the future of cities and urban regions around the world. The key reasons behind this development are high fuel prices, steadily growing demand for mobility, and social pressure for a switch to low-emission, climate-friendly transport. Metropolitan regions around the world, from London and Vancouver to Bogotá, are promoting sustainable mass transit with ambitious projects and strategic plans. But passenger traffic dropped significantly during the first year of the Corona pandemic. This decline was primarily due to the fear of contracting the disease on crowded public transportation. The cramped conditions in trains and buses definitely pose a risk of infection from not just Corona, but other diseases too. This is a very good reason to start looking for suitable solutions for clean air.

Small droplets increase the risk of infection

The main way in which viruses are transmitted from person to person is through droplets.

These enter the environment when people breathe, sneeze, or cough. While larger, heavier droplets sink and leave the air within seconds, smaller droplets remain suspended in the air for minutes, sometimes even hours. If this air moves, the droplets can spread across a wide area in a short time span. This renders air conditioners and fans in buses and other public transport a risk factor for passengers’ health.

Fresh air and filters to combat pathogens

High-performance air-conditioning and ventilation systems ensure a high air exchange rate with a high proportion of fresh air to recirculated air. However, the recirculation rate must be increased to save energy when heating or cooling the air if the ambient temperature is above 28 °C or below 8 °C. The higher the proportion of recirculated air, the more important it is to have effective air filter elements that reliably separate viruses and bacteria from the air on a permanent basis, even in the event of high passenger volume.

The problem is that conventional filter elements mainly separate dust particles and pollen. They are limited in their ability to effectively filter the smaller aerosols that cause diseases. Without suitable filter elements, air-conditioning systems can even promote the spread of viruses.

“The challenge in developing the filtration solution was to reliably catch aerosols in the range <1 μm. Only with highly efficient solutions do we manage to minimize the amount of these ultra-fine particles, which can become a health risk,”

says Tobias Beisel, Project Engineer Special Applications at Freudenberg Filtration Technologies.

How to reduce the risk of infection on buses

The greater the supply of fresh air, the lower the concentration of airborne aerosols.


High-performance filter elements can be used to filter environmental contaminants, hazardous substances, and harmful aerosols from the air. Viral filtration efficiency increases with filter efficiency.

Filter elements with a disinfectant surface on a functional layer effectively prevent viral re- aerosolization and provide maximum protection.

Using a 3-stage design (comprising a front box filter, a recirculated air filter, and a third filter in the air-conditioning system on the roof of the bus) is ideal for ensuring air quality on a permanent basis.

All good things come in threes

With the micronAir blue, Freudenberg Filtration Technologies has developed a 3-stage cabin air filter that ensures clean air and provides reliable protection against pathogens.

In the main filtration layer, synthetic fibers catch almost 100 percent of all fine dust particles and microorganisms, including fungal spores. The activated carbon layer ensures effective adsorption of contaminant gases and unpleasant odors. The bio-functional layer, which features a nanosilver ion-free impregnation, inactivates allergens and viral aerosols. With these three stages, micronAir blue filters separate approximately 90 percent of viral aerosols from the air and render them almost 100 percent harmless.

Improved filters for an improved recirculation rate

The Technical University of Berlin in Germany has addressed the question of how high the proportion of fresh air in buses must be in order to keep aerosol concentrations below a level of concern. The university’s study reveals that conventional systems operate below critical levels at 37.5 times the air change rate per hour and 80 percent fresh air, even if there are five infected passengers on board who are constantly talking.

In the case of high-performance solutions in which filter elements such as those from the micronAir blue range are installed, it was demonstrated that the proportion of recirculated air can be increased to 50 percent without the risk of infection exceeding the critical threshold. This is advantageous in terms of saving energy at high and low ambient temperatures, which is of particularly relevance for the range of electrically powered buses.

A safe, energy-efficient future

Local transport will play a greater role for mobility in tomorrow’s cities and urban areas. This makes it all the more important for transport companies to protect their customers as best they can from airborne pollutants and p.athogens. Highly efficient, customized filter elements can make a substantial contribution to achieving this level of protection while also reducing the energy consumed by air-conditioning.

Find out more about Freudenberg’s bus filtration solution

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