Context : It is known that the Canary Islands are normally affected by dominant winds flowing from north-northeast , that in some meteorological conditions , can transport sand from the Sahara desert at high altitude .
The dust may affect the efficiency of the telescopes and decrease the transparency of the sky .

Aims : To maximize the scientific return of the telescopes located at the Observatorio del Roque de Los Muchachos ( ORM ) , we present an analysis of the atmospheric dust content and its effects on astronomical observations .
Than B , V and I dust aerosol astronomical extinction are derived .

Methods : Using a 5-year series database of data taken from a dust monitor located inside the Telescopio Nazionale Galileo ( TNG ) dome , we computed mean hourly and daily values of the dust content as measured with a four-channel dust monitor .

Results : We detected particles of 0.3 , 0.5 , 1.0 , and 5.0 \mu m. Furthermore , using a power law we derived the content of 10.0 \mu m particles .
We found a typical local dust concentration ranging from 3 \cdot 10 ^ { 6 } particles per cubic metre at 0.3 \mu m , to 10 ^ { 3 } at 5.0 \mu m and 10 at 10.0 \mu m , increasing up to 3 orders of magnitudes during the dust storms , with a relatively higher increase of 1.0 , 5.0 , and 10.0 \mu m particles .
The number of local dust storm events is the same in the local winter and summer , but the average background and storm-related increases in the dust concentration in summer are significantly higher than in winter .
In a uniform approximation , during the dust storms , an average height of the dust layer of 2.5 km above the telescope is inferred .

Conclusions : During the sand storms , La Palma Island is affected by an almost uniform layer extending up to 5 km above the sea level .
The visible extinction is dominated by particles at 1.0 , 5.0 and 10.0 \mu m. In agreement with the results from Carlsberg Automatic Meridian Circle ( CAMC ) , we find a typical extinction of about 0.2 mag airmass ^ { -1 } during dust storms .