What were the conclusions of the IARC 1997 and 100C Monograph?

In 1997, the International Agency for Research on Cancer (IARC) changed its evaluation of crystalline silica (i.e. quartz and cristobalite) from probable human carcinogen (IARC Monograph of 1987) to human carcinogen (Group 1). The experts had difficulty reaching a consensus and, as a result, an unusual explanatory note preceded their conclusion: "In making the overall evaluation, the Working Group noted that carcinogenicity in humans was not detected in all industrial circumstances studied. Carcinogenicity may be dependent on inherent characteristics of the crystalline silica or on external factors affecting its biological activity or distribution of its polymorphs. Crystalline silica inhaled in the form of quartz or cristobalite from occupational sources is carcinogenic to humans (Group 1). Amorphous silica is not classifiable as to its carcinogenicity to humans (Group 3)".

This means that at the workplace (not in the general environment) in certain circumstances, exposure to some forms of crystalline silica could lead to cancer in man.

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Paradoxically, the same working group evaluated coal dust, which may contain up to 15% crystalline silica as a Group 3 agent i.e. a substance which cannot be classified as to its carcinogenicity due to inadequate evidence in humans and animals.

The 1997 IARC Monograph on crystalline silica did not resolve the uncertainties surrounding crystalline silica and its potential carcinogenicity.

For its 100th Monograph celebration, IARC reviewed in 2009 in a Volume 100 all human carcinogens previously evaluated by the Agency. During the week 17 to 24 March 2009, Meeting C “Metals, Particles and Fibres” was held and a Working Group reviewed 14 agents including crystalline silica. The conclusions of the meeting were published in Lancet Oncology on 4 May 2009.

On silica, the article mentions "The Working Group reaffirmed the carcinogenicity of crystalline silica dust as Group 1. An increased risk of lung cancer was observed across various industries and processes". This means that IARC has confirmed its 1997 evaluation but has now qualified the agent as "crystalline silica dust".

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