The final stage of air treatment is filtration of residual moisture, oil and hydrocarbons, and where necessary catalytic conversion of carbon monoxide. All of these depend on sufficient time in contact with the filter media, known as "dwell time", so either the filter must have a long air path or the air must flow slowly. Slow air flow is easily achieved by high compression, so filtration works best at or near the working output pressure of the compressor, and this is achieved by the back-pressure valve, which only allows air to flow above the set pressure.