game engine was designed so that animal behavior is calculated even if the player is in a different part of the Zone. There are several different variations of anomaly, each one having a unique impact upon those who cross its path. They can be potentially deadly to the player and the NPCs, delivering electric shocks, or pulling them into the air and crushing them. Most anomalies produce visible air or light distortions and their extent can be determined by throwing bolts (of which the player carries an infinite supply) to trigger them. Artifacts are found scattered throughout the Zone, often near clusters of anomalies. As well as being traded for money, a number of artifacts can be worn so that they provide certain benefits and detriments (for example, increasing a stalker's resistance to gunfire while also contaminating him with small amounts of radiation) although certain rarer artifacts provide benefits without any negative effects. The game does not feature controllable vehicles (although vehicles are programmed in the game code, they are not available without the use of a third party modification, and even if added, they are not entirely stable, as a single knife slash can destroy it completely) and thus players are required to go from place to place on foot. A sprint option using a limited stamina bar can be used to temporarily increase the player's rate of movement, though this is reduced by the weight of objects the player is carrying, and weapons cannot be fired while sprinting. It is possible to sprint indefinitely by using artifacts and keeping below a certain weight limit (50 kg); however, it is impossible to sprint with certain weapons (e.g. RPG-7 and SVD). Radioactive contamination caused by the nuclear incidents at Chernobyl occurs in specific invisible patches throughout the Zone. Although most areas are not contaminated, areas near abandoned construction equipment that was used in the post-accident clean-up, certain military vehicular wreckage and a variety of other locations create fields of radiation of varying intensity and size, some of which cannot be passed through without the proper protective equipment and anti-contaminant agents. The equipment is simplified into various sets of armor that have different levels of radiation protection. Additional radiation resistance may be conferred by some artifacts, and radiation sickness may be treated by medication or by consuming vodka. When the player enters a highly irradiated area, they will begin to receive