When do we talk about genetic damage?
How chemicals affect the genetic make-up of cells: mutagenesis

Genetic changes can occur during cell multiplication, when chromosomes are replicated, or during cell division, when chromosome groups are separated. This is when they are most vulnerable to environmental influences and xenobiotics.

Some nano-sized or low-molecular-weight substances can enter the nucleus through nano-sized pores in its wall, facilitating the exchange of ions or mRNA, to stick to parts of genes on the DNA helix, forming adducts. Pore size has been estimated at 39 nm (nanometers).