During July and August many of us would traditionally board a plane with our families and take to the sky, bound for a sunnier destination. This year things are a little different. Following the coronavirus pandemic many of us may feel a little cautious about boarding a confined metal tube and sharing the air with over 200 strangers. The cabin environment is a key consideration and cabin air filtration forms part of the risk reduction measures studied by industry experts. We talked to Ian Campbell, Director of Applied Research & Technology for Fire & Safety at Ventura County to explain how cabin filtration works.
Social distancing, masks … there’s something that makes me very nervous about sitting on a plane with people I don’t know, who are not in my social bubble, literally breathing my air. Is it safe to fly?
Ian: That depends on your own personal risk assessment and means looking at all the factors including human behaviour, not just the aircraft cabin environment. Cabin air is some of the cleanest you can breathe, and filtration maintenance is actually governed by legislation. This is how it works. Take a look at the diagram below. The air in a commercial airliner is taken from outside the aircraft, pressurized and conditioned, before being pumped into the cabin at an ambient temperature. The air is distributed through the aircraft from ceiling and personal seat vents and typically exits at foot level, into the cargo compartment.
So it comes from outside, does it then just circulate for the whole flight?
Ian: Once in the cargo bay, approximately 50% of the air is discharged overboard. The remaining air is passed through a High Efficiency Particulate Air (HEPA) filtration system, to remove all particulate contaminants before being mixed with fresh air and the process starting over.
Why is only 50% expelled?
Ian: There are a number of factors that drive this level. Air at cruise altitude has almost zero moisture. Using 100% outside dry air in the cabin would be very uncomfortable. By only using 50% (and retaining 50%), the aircraft can maintain balanced humidity levels for passenger comfort while minimizing aircraft material degradation.
All air that is taken from the engine to pressurize the cabin and give us clean air, results in a lower engine efficiency which drives fuel consumption.
How do we ensure recirculated air is clean?
Ian: One of the main factors regarding the safety of cabin air is the filtration system. The system uses a HEPA filter designed to capture 99.97% of all particles with a diameter of 0.3um, which is the hardest particle size to capture. The diagram below illustrates this.
What does HEPA stand for?
Ian: High Efficiency Particulate Air filters, they are made of boron silicate microfibers formed into a flat sheet in a process similar to paper making. The sheets are then pleated to increase the overall surface area and the pleats are separated by aluminum baffles designed to direct the airflow through the filter.
The effectiveness of HEPA filters is defined by three key mechanisms. The first mechanism is interception, this is when air particles being carried in the airflow around the filter fibres naturally stick to the filter.
Larger particles are often captured by the second mechanism, impaction. Due to their size, these particles cannot adjust to sudden changes in airflow around the filter and essentially run into the filter fibre and become embedded.
The final mechanism is diffusion. This is a little trickier to explain, it occurs because of the way microscopic particles move and interact with surrounding molecules. It’s known as Brownian motion. Basically molecules move in a random, zig-zag pattern because they collide with surrounding molecules. This motion slows down a particle’s path through the HEPA filter and increases the probability that the particle will be captured by either interception or impaction.
Trapping 99.97% of all impurities, that’s quite impressive.
Ian: HEPA filters, combined with air in a commercial airliner being changed 20-30 times per hour, which is equivalent to 15-20 cubic feet of air per person per minute, helps keep the air clean.
Thank you so much for explaining how HEPA filters contribute to maintaining cabin air quality.