This chapter examines the current and projected impacts on human health of four key environmental factors – air pollution (focusing on premature deaths from exposure to outdoor airborne particulate matter, or PM, and ground-level ozone as well as indoor air pollution), unsafe water supply and poor sanitation (including in the context of the relevant Millennium Development Goals or MDGs), chemicals (chemical hazards, exposure) and climate change (focusing on the incidence of malaria). For each issue the chapter first describes the current trends, then how the picture could look in 2050 without new policies (the Environmental Outlook’s Baseline scenario), and finally the policy actions required. Air pollution, unsafe water supply, poor sanitation and hazardous chemicals exert significant pressures on human health, particularly for the elderly and the young. While some global trends (e.g. access to improved water sources) are getting better, others – such as urban air pollution and lack of access to basic sanitation – continue to pose a serious risk to human health. In addition, the incremental effects of climate change are contributing to the global burden of disease. Ambitious and flexible abatement policies (e.g. standards, fuel taxes, chemical testing and assessment, green procurement, cap-and-trade emissions trading, transport policies) as well as further investment in water and sanitation services are needed to address these risks. Identified hazards need to be assessed and tackled, and there is also a need to be vigilant and react quickly to new and emerging risks to human health that are not well understood (e.g. endocrine disrupters, manufactured nanomaterials).