The dangerous paicle size causing pneumoconiosis varies from -

Ref:Textbook of pathology (Harsh Mohan)6th edition, page no.487 In general, most of the inhaled dust paicles larger than 5 mm reach the terminal airways where they are ingested byalveolar macrophages. Most of these too are eliminated by expectoration but the remaining accumulate in alveolar tissue. Of paicular interest are the paicles smaller than 1 mm which are deposited in the alveoli most efficiently. Most of the dust-laden macrophages accumulated in the alveoli die leaving the dust, around which fibrous tissue is formed. Some macrophages enter the lymphatics and reach regional lymph nodes.