4Game

As far as I could find with my google-fu, the community has never found out if air filtering unit is useful or not due to the skill fatigue system (you stop gaining xp on each skill in-raid after a certain amount).

• google query link
• reddit thread 1 where people aren't sure about air filter's usefulness – https://old.reddit.com/r/EscapefromTarkov/comments/ehoauf/air_filtering_unit_and_the_exhaustion_thing/
• reddit thread 2, same – https://old.reddit.com/r/EscapefromTarkov/comments/eo9s4g/can_we_change_the_way_filters_worked_now_that/
  reddit thread 3, also same – https://old.reddit.com/r/EscapefromTarkov/comments/esfhnz/is_the_air_filtering_unit_a_waste_of_money_for_my/
• even the official wiki has no concrete comments on the matter regarding the fatigue/exhaustion and how the air filter affects it.

So I decided to test it myself. Because it's not easy to see when the red arrow is 50% progression speed and when it's getting closer to 0%(fatigue is a gradual process from 129% to 0%, not a simple binary state) & also due to the hideout skills affecting the air filter's bonus value, precise values are meaningless.
But the general values are as follows(Based on strength skill xp gains) – Without the filter, you get around 3.5 points until it basically stops gaining xp. With the filter, I got around 5~6 points until it was meaningless trying to gain more.
Contrary to popular belief, upon observing xp gained it is my understanding that air filtering unit does indeed increase the xp limit per raid.(to be more pedantic per exhaustion cycle which half-resets after some time of inactivity but I digress)

Note: easy way to increase strength and endurance skills which are definitely the most QoL improving skill ingame, is to start the raid ever so slightly overloaded(orange weight) using SAS drives in document cases if you're rich & don't really loot in raids but just do quests/pvp, or using stacks of shotgun shells in backpack which is around 10~30k depending on your gearset.
At the start of the raid, run for about 1 minute. You'll reach plateau in strength after running for just 10 seconds anyway.
Now, offload junk so that you aren't overweight anymore. Endurance isn't gained while overweight, that's why we have to offload. Proceed with the raid normally, and you'll reach plateau in endurance after around 1~2 minutes too.
If you end up getting overweight at the end of the raid due to looting, that's even better since the skill cap penalty would've been partially lifted by now, so you can get a bonus 1~2pt naturally walking to extract.