Carbon dioxide (CO2) is a colorless, odorless, non-flammable gas. Depending on temperature and pressure, carbon dioxide can also exist as a liquid or a solid. Because of the refrigerant qualities of CO2, carbon dioxide gas in high-pressure cylinders may freeze up, causing problems with valves, regulators and other compressed gas equipment. This happens when pressure regulators become clogged with dry ice 'snow' or crystals and the flow of gas through pressure control valves is constricted. Freezing up is contingent on flow rate, inlet, cycle (percentage of time the gas is flowing) and size of the regulator. Heated regulators can relieve or eliminate freeze-up problems. Unlike carbon monoxide (CO), which can be deadly when inhaled at relatively low levels, CO2 is not considered harmful. Humans exhale carbon dioxide at predictable levels, which can be measured to determine ventilation needs. Energy can be wasted on heating, cooling, humidification and dehumidification if occupancy estimates are not accurate. CO2 sensors save energy by adjusting ventilation to actual occupancy.