We present an analysis of the atmospheric dust content and its effects on astronomical observations at Observatorio del Roque de Los Muchachos (ORM), in La Palma Island (Canaries). We have detected atmospheric particles having size 0.3, 0.5, 1.0 and 5.0 mu m, closer to Telescopio Nazionale Galileo (TNG) dome and using a power law we have derived the content of 10.0 mu m particles. Using a model we have calculated the contribute of aerosols to astronomical extinction in B, V and I from a 5 years series database in two typical conditions: clear and dusty days. A correlation is also analysed comparing the aerosol extinction and total astronomical extinction using the Carlsberg Automatic Meridian Circle (CAMC) extinction database in V. We have estimated that the dust reach an altitude of about 5 km above the sea level during the sand storms. Finally we have concluded that in the visible band extinction is dominated by particles at 1.0, 5.0 and 10.0 mu m.