**4. Conclusion**

There is evidence of etiologic heterogeneity among types of NHL, with different incidence patterns according to age, sex, race and specially geography. The extent to which these differences reflect differences in etiology needs further study.

Epidemiologic studies indicate that environmental factors may play an important role in the etiology of non-Hodgkin's lymphoma. Given the recognition that transmissible agents, especially in the developing world, are a significant cause of some kind of lymphoma, focusing on effective strategies to prevent infection altogether will go a long way to diminish the lymphoma. Additionally, effective strategies for toxic and occupational exposure and changing global lifestyles will yield huge dividends. Future epidemiologic research on NHL will be enhanced by analyses of subtypes of NHL, improved reliability and validity of exposure assessment tools to evaluate occupational, environmental and personal exposures, and evaluation of susceptible subgroups of individuals whose risk of NHL may differ from that of the general population. Finding the relation between environmental factors and genes in lymphomagenesis also important and it needs more investigation.

Lower rate of HL in Asians suggestive of genetic resistance, in addition international and inter-ethnic differences implicate environmental influence**.** Additional insight into the balance of genetic and environment factors on HL risk should be forthcoming. Differences in HL risk reported in several studies indicate that such studies of HL risk factors should be conducted for specific Asian population.

Overall in developing countries the most common subtype of lymphoid malignancies both HL and NHL are those subtype which immunodeficiency have an important role in their pathogenesis.
