AbstractIndoor carbon dioxide (CO2) concentration can be used to estimate the degree of air recirculation and outdoor air supply rate. Three types of CO2detector tubes were evaluated by using Fourier Transform Infra‐Red (FTIR) Spectroscopy as a reference method. Two types of detector tubes (Draeger CH 30801 and Kitagawa 126 B) showed a good correlation with the reference method (r = 0.98), the 95% confidence interval of the slope being 0.89‐1.06 and 0.80‐0.95, respectively in linear regression analysis. The third type (Gastec 2LL.) showed lower correlation (r = 0.91) and a wider 95% confidence interval (0.52‐0.80) of the slope. A t CO2concentrations in the range 800‐1000 5 l/l(ppm), control values suggested for the indoor environment, the Draeger and the Gastec tubes underestimated the CO2concentration, while the Kitagawa tube showed a correct value. The difference in reading between observers was similar for all three brands of detector tubes (5‐7%), expressed as relative standard error No significant influence of the air humidity or temperature on the readings could be demonstrated. It is concluded that some brands of CO2detector tubes can be used to measure indoor carbon dioxide concentration with sufficient precision and accuracy. Since the relative error is relatively large at lower CO2concentrations, the use of such tubes for the determination of air recirculation in ventilation systems should be avoided. As a crude estimate of the outdoor air supply rate, however, CO2detector tubes may be used. In order to minimize the error in reading, the type of detector tube and the need for recalibration should be considered. When using CO2measurements as an estimate of outdoor air supply rate, the influence of age and work‐load on the individual's emission of CO2and the time needed to reach equilibrium, should also be taken into