In respiratory protective equipment, hopcalite is used to facilitate the rapid oxidation of the toxic carbon monoxide to harmless carbon dioxide with the oxygen from the air, which is then chemically bound to a sodium hydroxide layer, thus eliminating CO from the air stream, (which otherwise is not removed by activated charcoal air filters). Water vapor poisons the hopcalite catalyst, so in order to protect against water vapor, an additional filter based on Silica gel is introduced. In addition to that, the hopcalite layer is protected by a mechanical filter and a layer of activated carbon, purify the air of other contaminants. The operation of carbon monoxide (CO) detectors, on the other hand, is based on recording the heat released during the catalytic oxidation of carbon monoxide (CO) to carbon dioxide (CO2). Although primarily used to catalyze the conversion of CO to CO2, hopcalite catalysts are also used to remove ethylene oxide and other VOCs as well as ozone from gas streams. In addition, hopcalites catalyze the oxidation of various organic compounds at elevated temperatures (200–500 °C). See also Gas mask breathing apparatus Self-contained self-rescue device Carbon monoxide detector Catalysis heterogeneous catalysis Carbon monoxide poisoning References Manganese compounds Copper compounds Catalysts Diving support equipment