A method of operating a controlled atmosphere system to regulate the atmosphere in a cargo storage space, the CA system comprising a gas exchange module operable to vary the level of a component gas in the cargo storage space, a control module to control operation of the gas exchange module, and at least one of an oxygen sensor and a carbon dioxide sensor, each being operable to measure a parameter indicative of a level of oxygen or carbon dioxide respectively in the cargo storage space, the method comprising: the control module determining a respiration parameter value indicative of the rate of change of oxygen level and/or the rate of change of carbon dioxide level in the cargo storage space due to respiration of goods in the cargo storage space; and characterised in that: the respiration parameter value is determined from a plurality of predetermined respiration parameter values, each respiration parameter value corresponding to a respective range of rate of change of oxygen level and/or a respective range of rate of change of carbon dioxide level; wherein the control module determines a rate of change of oxygen level and/or a rate of change of carbon dioxide level in the cargo storage space due to respiration of the goods in the cargo storage space; wherein the control module determines the respective respiration parameter value, from the plurality of predetermined respiration parameter values, associated with the rate of change of oxygen level and/or or the rate of change of carbon dioxide level; and wherein the control module controls operation of the gas exchange module based on the determined respiration parameter value to target an oxygen level setpoint and/or a carbon dioxide level setpoint. the control module determining a respiration parameter value indicative of the rate of change of oxygen level and/or the rate of change of carbon dioxide level in the cargo storage space due to respiration of goods in the cargo storage space; and characterised in that: the respiration parameter value is determined from a plurality of predetermined respiration parameter values, each respiration parameter value corresponding to a respective range of rate of change of oxygen level and/or a respective range of rate of change of carbon dioxide level; wherein the control module determines a rate of change of oxygen level and/or a rate of change of carbon dioxide level in the cargo storage space due to respiration of the goods in the cargo storage space; wherein the control module determines the respective respiration parameter value, from the plurality of predetermined respiration parameter values, associated with the rate of change of oxygen level and/or or the rate of change of carbon dioxide level; and wherein the control module controls operation of the gas exchange module based on the determined respiration parameter value to target an oxygen level setpoint and/or a carbon dioxide level setpoint. the respiration parameter value is determined from a plurality of predetermined respiration parameter values, each respiration parameter value corresponding to a respective range of rate of change of oxygen level and/or a respective range of rate of change of carbon dioxide level; wherein the control module determines a rate of change of oxygen level and/or a rate of change of carbon dioxide level in the cargo storage space due to respiration of the goods in the cargo storage space; wherein the control module determines the respective respiration parameter value, from the plurality of predetermined respiration parameter values, associated with the rate of change of oxygen level and/or or the rate of change of carbon dioxide level; and wherein the control module controls operation of the gas exchange module based on the determined respiration parameter value to target an oxygen level setpoint and/or a carbon dioxide level setpoint.