1. Under anesthesia, when a patient exhales carbon dioxide (CO2) and anesthesia gas into their breathing tubes, it travels to a CO2 absorber canister. 2. Then, when a patient inhales, the CO2-containing anesthesia gas passes through the CO2 absorber, which (as its name suggests) absorbs the CO2 and removes it from the gas. 3. This process allows any exhaled anesthetic agents to be rebreathed, which: - minimizes the level of anesthetic medications needed to perform surgery - minimizes pollution from anesthetic gases that are vented out of the hospital and into the atmosphere an - protects patients from the dangers of rebreathing carbon dioxide. Why is the canister purple? CO2 absorption is a chemical reaction where exhaled CO2 (an acid, in the form of carbonic acid) is neutralized by a base (sodium or calcium hydroxide). As more acid gets neutralized and the canister becomes more acidic, an indicator dye (ethyl violet) that will turn puplish when the pH of the absorbent decreases.