Helium detectors utilizing thermal conductivity are sensitive to the presence of nitrous oxide (N2O) and oxygen (O2). Measurement of the level of these gases within the spirometer may be used to obtain the true helium concentration for use in the calculation of functional residual capacity (FRC). Linear relationships were obtained relating the change (Δ) in reading of the helium meter by nitrous oxide and oxygen. The regression equations are:ΔO2= 0.027 (%O2) – 0.513ΔN2O = –0.276 (%N2O)Failure to correct the helium reading for N2O will overestimate lung volume; elevations of O2above 21% will underestimate the lung volume.