Environmental epidemiology is a branch of epidemiology concerned with defining how environmental exposures impact human health. This field pursues to understand how various external risk factors may prompt to or protect against disease, illness, injury, evolving defects, or death. Epidemiological investigations can be grouped into four broad categories: Observational epidemiology, experimental epidemiology, natural experiments, and Theoretical epidemiology. Several types study designs and measures of relationship are used in these investigations.