“…Maillard [ 19801 and Heppleston [ 19851 reached opposite conclusions, sug-gesting that the reported association between silica exposure, silicosis, and lung cancer could be the result of confounding by smoking and other occupational risk factors [McDonald, 19891. Although mixed coal mine dust contains, on average, more than 4% of free crystalline silica, results among coal miners and subjects with coal workers' pneumoconiosis are generally negative for an association with lung cancer mortality [Jacobsen, 1979;Rooke et al, 1979;Ames, 1983;Miller and Jacobsen, 1985;Atuhaire et al, 1986;Meijers et al, 19881. More recently, further conflicting results have been published on lung cancer risk among workers exposed to dust containing silica, with or without silicosis [Hessel et al, 1990;Hnizdo and Sluis-Cremer, 1991;Carta et al, 1991;Amandus and Costello, 1991;Ng et al, 1990;Chia et al, 1991;Meijers et al, 1990;Amandus et al, 1991;Ahlman et al, 1991;Kusiak et al, 1991;Chen et al, 1991;Mc Laughlin et al, 19921. In most studies no clear dose-response effect for exposure to silica was shown, although an association between silicosis and lung cancer has been observed in many of them.…”