Edit, ok got it slightly wrong, heres what wiki says:
The game is played from the perspective of a character named Artyom.  The story takes place in 
post-apocalyptic Moscow,  mostly inside the 
metro system where the player's character was  raised (he was born before the war, in an unharmed city), but  occasionally the player has to go above ground on certain missions and  scavenge for valuables.
 The games' locations reflect the dark atmosphere of real metro  tunnels, albeit in a much sinister and bizarre fashion. Strange  phenomena and noises are frequent, and mostly the player has to rely  only on their flashlight to find their way around in otherwise total  darkness. Even more lethal is the surface, as it is severely irradiated  and a gas mask must be worn at all time due to the toxic air.
[7]  Water can often be contaminated as well, and short contacts can damage  the player, or even kill outright.
 Often, locations have an intricate layout, and the game lacks any  form of map, leaving the player to try and find its objectives only  through a compass - weapons cannot be used while visualizing it. The  game also lacks a health meter, relying on audible heart rate and blood  spatters on the screen to show the player what state they are in and how  much damage was done. There is no on-screen indicator to tell how long  the player has until the gas mask's filters begin to fail, save for a  wristwatch that is divided into three zones, signaling how much the  filter can endure, so players must continue to check it every time they  wish to know how long they have until their oxygen runs out, requiring  the player to replace the filter (found throughout the game). The gas  mask also indicates damage in the form of visible cracks, warning the  player a new mask is needed. The game does feature traditional 
HUD elements, however, such as an  ammunition indicator and a list of how many gas mask filters and  adrenaline shots (med kits) remain.
 
Another important factor is ammunition management. As money lost its  value in the game's setting, cartridges are used as currency. There are  two kinds of bullets that can be found, those of poor quality made by  the metro-dwellers themselves, which are fairly common but less  effective against targets, especially mutants, and those made before the  nuclear war, which are rare and highly powerful, but are also necessary  to purchase gear or items such as filters for the gas mask and med  kits. Thus, the player is forced to manage their resources with care.