Data for five continuous years (1984 to 1989) at the Teide Observatory (Izaña, Tenerife), obtained with a full-disc solar multi-channel photometer devoted to the detection of solar intensity oscillations have now been used for a detailed study of the extinction coefficient behaviour at several wavelengths (450-870 nm) under different atmospheric conditions: days with a clear sky and days with dust coming from the Sahara Desert. From this study we conclude that an extinction coefficient of around 0.075 mag airmass-1 at 680 nm constitutes the boundary between clear days and the effect produced by the Saharan dust on dusty days. In the former the extinction can vary between 0.04 and 0.07 mag airmass-1 (at 680 nm) and in the latter between 0.075 and 0.8 mag airmass-1 (at 680 nm). Moreover, the dust produces a grey effect in the wavelength range analysed here.