The continuing degradation of the environment increases the risks to the health and well-being of populations everywhere. Pollution reaches drinking water sources, foods and air in both developed and developing countries. In the former, mainly rural areas are affected; in the latter, cities, towns and villages are exposed, in addition to the rural areas. As well as human wastes, livestock wastes add to the production of contaminants. Convection by water is augmented by soil-borne, air-borne and directly contacted bacteria, fungi, parasites and viruses. Food may be contaminated as well. Inadequate sanitary and hydraulic engineering remains the greatest threat to preserving large numbers of the world's population from communicable diseases.
According to a 1998 study of population trends, climate change, increasing pollution and emerging diseases, an estimated 40 percent of deaths around the world can now be attributed to various environmental factors, especially organic and chemical pollutants. More broadly, at least 500 million people each year are affected by unhealthy environments as a result of pollution; many of these are infants and children.
In three regions of the Asia-Pacific zone there is remarkably high incidence of amyotrophic lateral sclerosis, parkinsonism, and progressive pre-senile dementia. Studies have singled out heavy use of the neurotoxic seed of cycad plants for food and/or medicine as a likely key etiological factor.