Controversy also exists regarding the genotoxicity of NP. Differences in the assayed organisms and in the screening test have led to opposite conclusions regarding its genotoxic activity. Although it affects other toxic parameters, NP did not affect DNA integrity in fishes or in Daphnia, nor did it increase micronuclei in mice or chromosomal aberrations in Allium [7], [10] and [16]. Yet, in Scrobularia plana, DNA damage caused by NP was reported, but only at high doses in vitro, as occurred in CHO-K1 cells, where this chemical induced DNA breaks, sister-chromatid exchanges and chromosomal aberrations [4] and [5]. In our study, the genotoxicity of NP to the insect genome of Chironomus was confirmed, and our results are in agreement with previous data on short exposures in this species [16]. It is worth noting that, even though DNA breakage increased in a dose-dependent manner under short NP treatments (24 h), we found that genetic damage significantly decreased after four days exposure, suggesting the activation of repairing mechanisms under prolonged exposures. In vivo studies exposing mussels (Unio tumidus) to different polyphenols for 24 h and 48 h also detected the greatest effect of DNA damage after 24 h [49]. Another possible explanation is that the toxicity of NP compounds could disturb enzymatic processes in the formation of DNA damage. NP-induced DNA damage mainly occurs due to NP-induced oxidation of DNA, particularly guanine oxidation [50].