“I’ve already described how airliners are designed. My daughter works maintenance on airliners. If you want to read about aviation regs, you’ll have to do it yourself.”
As long as filters aren’t required, then it’s meaningless, some planes will have them, others won’t - and even the ones that do have them - nothing prevents cheap and ineffective filters. And just this week the FAA stated that health concerns are not in their charter - hopefully that changes though.
As to recirculated air, I’ve heard 3 stories here on FR, one says that most air is recirculated, another says that no air is recirculated, and now half of the air is recirculated.
All may well be true, since it’s up to the airlines and may also vary with the type of plane. Again, that’s why regulations are needed. Really no different than other safety regulations, which seem to work quite well!
“As long as filters aren’t required, then it’s meaningless, some planes will have them, others won’t - and even the ones that do have them - nothing prevents cheap and ineffective filters.”
If you deny what my daughter and what articles by pilots say, the burden of proof is on you.
“Many Delta aircraft, including all of our international widebodies, plus many narrowbodies including 737s, 757s, A220s, A319s, A320s and A321s, are equipped with state-of-the-art air circulation systems, blending fresh outside air that is sterilized with a high-temperature compressor and ozone purifier with existing cabin air that has been recirculated through an industrial-grade HEPA filter. HEPA air filters extract more than 99.999% of even the tiniest viruses, as small as 0.01 micrometers. Coronaviruses, which range from 0.08 to 0.16 micrometers in size, are filtered by the HEPA filter. Delta’s 717s use 100% fresh outside air.”
https://news.delta.com/video-air-filtration-systems-help-keep-aircraft-cabins-safe
“HEPA filters are effective at capturing greater than 99 percent of the airborne microbes in the filtered air. Filtered, recirculated air provides higher cabin humidity levels and lower particulate levels than 100% outside air systems. The cabin air system is designed to operate most efficiently by delivering approximately 50 percent outside air and 50 percent filtered, recirculated air. This normally provides between 15 to 20 cubic feet of total air supply per minute per person in economy class. The total air supply is essentially sterile and particle-free.
Cabin air circulation is continuous. Air is always flowing into and out of the cabin. Total airflow to the cabin is supplied at a bulk flow rate equivalent to 20 to 30 air changes per hour. This provides temperature control and minimizes temperature gradients within the cabin.....
Q2: Are all commercial aircraft fitted with HEPA filters?
The majority of modern, large, commercial aircraft, which use a recirculation type of cabin air system, utilize HEPA filters. A small number of older aircraft types have filters with lower efficiencies.
Q3: What is the smallest particle size that the cabin air filter element can
remove?
Air filters can remove very small particles such as bacteria and viruses. Virtually all viruses and bacteria are removed; even the most difficult particles in the range of 0.1 to 0.3 micron are filtered out with an efficiency level of of 99.995%. Contrary to popular belief, very small particles below 0.1 micron are easily filtered out by the mechanism of diffusional interception.
Q4: How often are filters changed?
The interval varies by aircraft. Most airlines replace cabin air filters at regular “hard time” intervals to fit in with routine scheduled maintenance periods, as long as these intervals do not exceed filter manufacturers’recommendations.
Q5: What happens if the cabin air filter elements are not changed out regularly?
Air flow through the filter might be reduced, but the efficiency of the filter at capturing viruses and bacteria is not affected.” - International Air Transport Association
https://www.iata.org/contentassets/f1163430bba94512a583eb6d6b24aa56/cabin-air-quality.pdf
You might also see here:
https://aerospace.pall.com/en/commercial-fixed-wing/how-cabin-air-systems-work.html