Measurement of input respiratory impedance is carried out by superimposing forced oscillations on spontaneous breathing. The latter thus acts as a quasi-steady unidirectional flow component, with effects on the measured impedance that are habitually neglected (linearity assumption). We examined the validity of that assumption in the case of a turbulent steady flow. We tested the validity of a fluid dynamics criterion previously proposed in water channel experiments for gas flow in a tube. This criterion states that oscillatory and continuous turbulent flow may or may not interact if the Stokes boundary layer (ls) is embedded within the viscous sublayer (lv), i.e., if lS+ = lS/lv < or = 10, implying Re7/8 < or = (100 alpha/square root of 2), for a fully developed hydraulically smooth turbulent flow in a tube (where alpha is Womersley parameter and Re is Reynolds number of the steady-flow component). Experiments were performed in long rigid circular and semicircular tubes by superimposing two independent well-defined flows: 1) laminar oscillatory flow obeying the linear transmission line model (frequency = 1.5-250 Hz, i.e., alpha = 6-80) and 2) fully developed turbulent flow characterized by Blasius resistance formula (Re = 3,000-16,000). Confirming the validity of the criterion above, we found that the real and the imaginary parts of the long-tube impedance did not differ from those measured in the absence of a steady-flow component, provided lS+ < or = 10. On the contrary, the real parts measured with and without the continuous component differed greatly as soon as lS+ > 10, both for circular and semicircular tubes and for outward as well as inward steady flows. We concluded that the proposed criterion is pertinent for predicting appropriate oscillation frequency for a given rate of spontaneous flow, such that oscillatory and turbulent flows do not interact. Application of the forced oscillation measurement technique during spontaneous breathing requires use of a range of oscillatory frequencies higher than the frequency range classically used during apnea.