An oxygen self rescuer or self-contained rescue device (SCSR) is a portable piece of equipment that supplies breathable air when the surrounding atmosphere lacks oxygen or is contaminated with toxic gases, such as carbon monoxide. SCSRs are intended for use in mines or other areas where there are risks of sudden fires or explosions, which can change the quality of the surrounding air, making it dangerous to breathe. Where no external rescue is immediately available, workers must turn to self rescue and make their own way to safety or to a pre-equipped underground refuge – and SCSRs ensure they have adequate time to do so.