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Freshly-fractured crystalline silica, such as that emitted during mining and processing, is significantly more toxic than "weathered" silica (from, e.g., agricultural sources, http://ehp.niehs.nih.gov/members/2000/suppl-4/705-712kirkhorn/kirkhorn-full.html), apparently through reactive oxygen species generated mechanically (http://www.cdc.gov/niosh/nas/RDRP/appendices/chapter3/a3-64.pdf).  This same source suggests a half-life of freshly-fractured silica of about 30 hours.

There is also evidence that freshly-fractured silica causes more lung reactivity and inflammation than weather ("aged") silica: http://www.cdc.gov/niosh/nas/rdrp/appendices/chapter3/a3-70.pdf, http://www.sertox.com.ar/img/silicosis_article.pdf

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