Decontamination gas (often mistakenly called smoke) is a security protocol to stop Class-D personnel from escaping and a gameplay element found throughout the facility.
The gas is deployed in many rooms of the site to stop Class-Ds from escaping and/or to prevent SCPs from progressing in the event of a containment breach.
Decontamination gas will agitate the player's eyes, causing them to blink at faster intervals. The blink icon will be outlined red and the bar will deplete faster. To prevent this, the player can wear a gas mask to protect their eyes. Prolonged exposure to the gas will eventually cause the player to suffocate if they are not wearing a gas mask.
Areas where this gas is commonly found include the Class-D cells, the lockroom, gas catwalk, three-way gas catwalk, a variation of the metal corridors, Storage Area 6, the large testing chamber, SCP-035's containment chamber, the plane, and Gate B's control room. An alternate type of decontamination gas is used in the security gateway, however it does not harm the player.
Due to the effects of the gas on the player's eyes, sight-based SCPs such as SCP-173 pose quite a threat around these areas if the player does not take precautions and equip the gas mask. However, SCP-173 only spawns at the gas catwalks and the lockrooms - with adequate planning, one should have no difficulty in navigating these areas whilst avoiding the SCP.