A clastogen is a material that can cause breaks in chromosomes, leading to sections of the chromosome being deleted, added, or rearranged. This is a form of mutagenesis, and can lead to carcinogenesis, as cells that are not killed by the clastogenic effect may become cancerous.
Here is an article published in 2009 which is the most recent research I found on this topic until the one printed in november of this year which I will print tomorrow.
Nature Nanotechnology 4, 876 - 883 (2009)
Published online: 5 November 2009 | doi:10.1038/nnano.2009.313