Filters

Filters are used in conjunction with the gas mask. Without fresh filters, the apparatus could not properly filter the poisonous particles in the air on the surface, or the toxic pockets of air caught in the metro system, the inhalation of which ultimately leads to death.

Overview
All filters are universal and will fit any gas mask found in the metro. Also, they are widely traded throughout the settlements. Anyone regularly travelling between Metro stations or the surface should carry these.

These filters are likely charcoal filters as charcoal is a natural poison adsorbent, and can be found and made just about anywhere. This may also be the reason why they do not last very long, since the technology to make more advanced filters was likely lost after the disaster or it is prohibitively difficult and expensive to mass produce better filters due to the scarcity of the resources and expertise that are required to make them.

Appearances
Filters can be found just about everywhere in the Metro:
 * They are traded in a few settlements (Riga and Market).
 * They can be found on corpses.
 * Given to Artyom by some other NPCs
 * Can be found as individual filters in various locations in the Metro and surface.
 * Can be found in wooden supply crates/boxes throughout the game.
 * Will come attached to recently swapped gas masks.

Uses
Filters must used in compliance with the gas mask, they must be swapped out regularly with a fresh filter every few minutes of continued use. A clean filter lasts almost exactly one minute regardless of difficulty played on or the activities being performed by Artyom (such as constant sprinting).

Artyom keeps a wrist-watch on at all times to keep track of how much time his filters have left before they have become too used to entirely filter the toxic air. There are, however, other signs that the filter may need to be swapped, if for example: Artyom's breathing becomes very heavy, Artyom can be heard wheezing as he struggles for air, his vision starts to tunnel or blur, then the filter may need to be changed. When a filter runs out, Artyom will automatically swap in a fresh one. If the filters are not swapped out when needed, Artyom will slowly suffocate and ultimately die.

The duration of the air filters can exploited by constantly putting on and taking off the gas mask. Since Artyom has a short period of time (up to 30 seconds) that he can last without a mask on, this time elapsed with the gas mask off can be valuable if he is nearly out of time on his filter. As soon as the gas mask is put on, the timer will reset, so there is no need to keep the mask on for any particular length of time.

Trivia

 * When the gas mask is in play, the duration of the air filter is reduced. In real life, a crack in the mask would not affect the filter at all. It would simply escape into toxic air and in many cases render the gas mask unusable. If the filter is damaged, it may have less usage time, but is most likely unusable.
 * Most filters found cannot be picked up because they have been used or the filter is expired.
 * The filters are shown in an odd way. The filter units shown are not the number of filters, but rather, they are shown as minutes of time. Still, when buying a filter, one cannot keep more than 20 of these units at a time.
 * The filters last for a very short period of time (about 5 minutes), even though the air is still relatively clean, as the player character can survive there for a brief-period of time.
 * It's relatively easy for filters to run out, so use them wisely and do not waste them or take too long in areas where a gas mask is needed. In the levels where filters must be used, pass them quickly to conserve them. Also, it's recommended to purchase filters whenever possible (especially in levels like "Market" in Metro 2033), as some parts of the games will require the player character to spend long periods of time on the surface, where filters are essential for survival.
 * In Metro 2033, filters can only be bought in early parts of the game, at Riga Station and Market Station, so buying as much as you can is wise.
 * In Metro 2033 Redux, filters can be bought at most stations before beginning the Library stage.
 * Rangers do not use regular filters on their gas masks since their helmets have built in cheek filters.
 * In Metro: Last Light and Redux, the gas masks used by Rangers have regular filters on it.
 * The gas mask filters the player character uses appear to be very large (bigger than Artyom's hand), but the placed filters appear very small before being picked up. They are likely 60mm filters. In Metro: Last Light and the Redux version, the filters are larger, fixing this developer oversight.
 * The Nazi gas mask and the Ranger gas masks are the same, but the Nazi masks have filters and a helmet over them, while the Rangers' masks are attached to their helmets.
 * In Metro: Last Light, the maximum amount of filters Artyom can carry at one time is 59:59 (59 minutes, 59 seconds). He will, however, refuse to pick up any filters that would put the total number higher than his limit. E.g. When carrying a total of 56:56, Artyom will not pick up any filters with over 3 minutes and 3 seconds left on them.
 * Each filter in the Redux version lasts 5 minutes, like the Metro: Last Light version.
 * In Metro Exodus, when starting a new game using the New Game+ feature having collected the Consumables carrier in the previous playthrough, Artyom can carry double the usual maximum amount of filters (up to 60 minutes), until the cutscene where he is shot and thrown to the pit by Hanza soldiers (halfway through the Moscow level). After this cutscene, the maximum amount resets to 30 minutes. This is the case on "easy" or higher difficulties but on "reader", Artyom can only have a total of 15 minutes worth of filters on him at a time.

Gasmaske und Filter Фильтр