There is provided a method of determining at least one parameter value relating to nitric oxide, NO, production in the lower respiratory tract of a subject, the method comprising obtaining a plurality of measurements of NO levels in exhaled air and air flow rate during multiple exhalations of a subject performing tidal breathing analyzing the plurality of measurements to identify exhalations where the NO produced in the lower respiratory tract dominates the NO contribution from other sources determining at least one parameter value relating to NO in the lower respiratory tract of the subject from the measurements of NO levels and/or air flow rate obtained during the identified exhalations.
