We investigate whether some segments of the population face disproportionate health burdens from environmental contaminants. Such disparities may arise from differences in biological susceptibility, baseline health status, exposures, or other factors. Persons may face a higher environmental health burden based on socio-economic position, racial/ethnic minority, urban/rural environment, age, or other conditions. Our work in this area considers environmental justice issues broadly, including which populations have the highest exposure levels as well as which populations have the highest response to a given pollutant. Further, we study how choice of method to estimate exposure may impact risk estimates for various populations, for example due to the nature of existing air pollution monitoring networks. As an example, we found higher ozone-mortality effect estimates in communities with a higher proportion of racial minorities or lower income. Exposure patterns may also play a role, as we observed lower effects for communities with high prevalence of central air conditioning, which alters penetration of ambient air. Our studies of air pollution and birth weight also found differential effects by race, with higher impacts on infants of African-American mothers than White mothers. Currently, we are investigating how socio-economic status affects air pollution and weather-related mortality in Latin America and how air pollution effects differ by gender.