An electronic peak flow meter for measuring a user's respiratory airflow, the peak flow meter comprising: a sensor for measuring the user's respiratory airflow; a transducer for converting the respiratory air flow into an electronic value; a processor for storing the electronic value in a memory (50, 55); a respiratory data section of the memory for storing the converted electronic values; a statistical process control program, stored within the memory for execution by the processor, configured to: provide flags for the converted electronic values which identify measurements taken under different environmental circumstances; form a plurality of data sets, each data set comprising converted electronic values for measurements taken under a different environmental circumstance; calculate, for each of the data sets, at least one pulmonary function parameter and at least one statistical process control variable from the pulmonary function parameter, wherein the pulmonary function parameter for a data set is determined from the converted electronic values for that data set; determine a different pulmonary threshold for each data set from the statistical process control variable for that data set; and perform statistical process control analysis in respect of each of the different environmental circumstances and detect at least one instance when at least one of the pulmonary thresholds is exceeded. a sensor for measuring the user's respiratory airflow; a transducer for converting the respiratory air flow into an electronic value; a processor for storing the electronic value in a memory (50, 55); a respiratory data section of the memory for storing the converted electronic values; a statistical process control program, stored within the memory for execution by the processor, configured to: provide flags for the converted electronic values which identify measurements taken under different environmental circumstances; form a plurality of data sets, each data set comprising converted electronic values for measurements taken under a different environmental circumstance; calculate, for each of the data sets, at least one pulmonary function parameter and at least one statistical process control variable from the pulmonary function parameter, wherein the pulmonary function parameter for a data set is determined from the converted electronic values for that data set; determine a different pulmonary threshold for each data set from the statistical process control variable for that data set; and perform statistical process control analysis in respect of each of the different environmental circumstances and detect at least one instance when at least one of the pulmonary thresholds is exceeded. provide flags for the converted electronic values which identify measurements taken under different environmental circumstances; form a plurality of data sets, each data set comprising converted electronic values for measurements taken under a different environmental circumstance; calculate, for each of the data sets, at least one pulmonary function parameter and at least one statistical process control variable from the pulmonary function parameter, wherein the pulmonary function parameter for a data set is determined from the converted electronic values for that data set; determine a different pulmonary threshold for each data set from the statistical process control variable for that data set; and perform statistical process control analysis in respect of each of the different environmental circumstances and detect at least one instance when at least one of the pulmonary thresholds is exceeded.