History 284 Environmental aspects 142 Law and legislation 137 Christianity 94 Pollution 86 Religious aspects 83 History and criticism 74 Criticism and interpretation 72 Indians of North America 64 Indigenous peoples 61 Prevention 60 Research 59 Study and teaching 59 Air 54 Law 54 Legal status, laws, etc 53 Statistics 52 Legal status 47 Periodicals 47 Politics and government 47 Geology 46 Management 46 Indoor air pollution 44 Nuclear power plants 43 Groundwater 41 Water 41 Campaigns 40 Military campaigns 40 Standards 40 Economic conditions 39