Metro 2033 filter demonstration

I know people say the physical filters you pick up in the game matter, and that the filter change matters, but I haven't found that it does. In numerous playthroughs I've found that each zone of the game where you can't breathe has a required filter time, and if you exceed the filter time required for that zone you can't pick up another filter. It doesn't appear constant, with Frontline it appears to be for the whole level, with Outpost it appears to be 2 or 3 zones, the library also appears to have 2 or 3 zones. From D6 on there appears almost no limit on filters. The filter change animation doesn't affect air time or even the duration before having to actually replace the filter. This appears to be a scripted event. It could be argued that Artyom is running around with numerous filters with only a few seconds of air time on each one, but I just don't think that's the case.