to its use against the mosquitoes, lice, and houseflies that carried these diseases. Before World War II, pyrethrum, an extract of a flower from Japan, had been used to control these insects and the diseases they can spread. During World War II, Japan stopped exporting pyrethrum, forcing the search for an alternative. Fearing an epidemic outbreak of typhus, every British and American soldier was issued DDT, who used it to routinely dust beds, tents, and barracks all over the world. DDT was approved for general, non-military use after the war ended. It became used worldwide to increase monoculture crop yields that were threatened by pest infestation, and to reduce the spread of malaria which had a high mortality rate in many parts of the world. Its use for agricultural purposes has since been prohibited by national legislation of most countries, while its use as a control against malaria vectors is permitted, as specifically stated by the Stockholm Convention on Persistent Organic Pollutants. As early as 1946, the harmful effects of DDT on bird, beneficial insects, fish, and marine invertebrates were seen in the environment. The most infamous example of these effects were seen in the eggshells of large predatory birds, which did not develop to be thick enough to support the adult bird sitting on them. Further studies found DDT in high concentrations in carnivores all over the world, the result of biomagnification through the food chain. Twenty years after its widespread use, DDT was found trapped in ice samples taken from Antarctic snow, suggesting wind and water are another means of environmental transport. Recent studies show the historical record of DDT deposition on remote glaciers in the Himalayas. More than sixty years ago when biologists began to study the effects of DDT on laboratory animals, it was discovered that DDT interfered with reproductive development. Recent studies suggest DDT may inhibit the proper development of female reproductive organs that adversely affects reproduction into maturity. Additional studies suggest that a marked decrease in fertility in adult males may be due to DDT exposure. Most recently, it has been suggested that exposure to DDT in utero can increase a child's risk of childhood obesity. DDT is still used as anti-malarial insecticide in Africa and parts of Southeast Asia in limited quantities. Polychlorinated biphenyls Polychlorinated biphenyls (PCBs) are a class of chlorinated compounds used as industrial coolants and lubricants. PCBs are created by