The paper considers the possibility of monitoring reliability of infrared spectroradiometer operation when detecting the presence of a toxic chemical in a dusty atmosphere. We present a method for estimating cumulative concentration of aerosol particles based on evaluating variations in contrast of a test object against the background in the visible wavelength range during gas and aerosol cloud propagation. In order to determine the critical contrast value that corresponds to the aerosol concentration designating further inability of the spectroradiometer to detect the toxic chemical in the air, we propose to numerically solve the problem of radiation transfer in a medium with known parameters. We report the results of solving this problem for an interfering aerosol consisting of water droplets with a diameter of 5--20 µm. We show that for a signal-to-noise ratio of 100 it is possible to detect a toxic chemical at aerosol concentrations of up to 5 g/m2. Implementing the method proposed involves using a video camera to monitor the test object and a computer to process the images recorded.