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The objective of the National Institute for Occupational Safety and Health (NIOSH) accuracy criterion is to ensure that measurements from monitoring devices are within ±25% of the true concentration of the analyte with 95% certainty. To determine whether NO 2 and O 3 sensors meet this criterion, three commercially available units (Cairclip O 3 /NO 2 , Aeroqual NO 2 , and Aeroqual O 3 sensors) were co-located three times with validated instruments (NO x chemiluminescence [NO 2mon ] and photometric O 3 analyzers [O 3mon ]) at an outdoor monitoring station. As cofactors of sensor performance such as temperature (T) and relative humidity (RH) potentially influence the response of NO 2 and O 3 sensors, corrections for cofactors were made by using T, RH, and the sensor measurements to predict measurements made by NO 2mon and O 3mon during the first co-location period (training dataset). The developed models were tested in the merged data obtained from the second and third co-location periods (testing dataset). In the training and testing datasets, the mean NO 2 as measured by NO 2mon was 4.6 ppb (range = 0.4-35 ppb) and 9.4 ppb (range = 1-37 ppb), respectively. The mean O 3 in the training and testing datasets as measured by O 3mon was 38.8 ppb (range = 1-65 ppb) and 35.7 ppb (range = 1-61 ppb), respectively. None of the sensor measurements in the training dataset were within the NIOSH accuracy criterion (mean error ≥25%). After correcting for cofactors of sensor performance, the accuracy of the Cairclip O 3 /NO 2 and the Aeroqual O 3 sensors considerably improved when tested with the testing dataset (mean error = -1% and 14%, respectively). However, the Aeroqual NO 2 sensor had an error that was not within ±25%. Raw measurements from the tested sensors may be unsuitable for assessing workers' exposure to NO 2 and O 3 . Corrections for cofactors of Cairclip O 3 /NO 2 and Aeroqual O 3 sensor performance are required for more accurate occupational exposure assessment.