This paper proposes and experimentally validates a distributed temperature alarm system based on carbon dioxide (CO2)-filled side air-holes fiber (SAHF), interrogated through a conventional (incoherent) optical time-domain reflectometer (OTDR). Customizable alarm threshold temperatures can be designed and set by adjusting the pressure of the CO2 filling the air-hole region, which in turn determines a threshold temperature under which CO2 liquefies.