Prior to 1940, pesticides consisted of inorganic compounds (copper, arsenic, mercury, and lead) and plant derived products. Most of these were abandoned because they were highly toxic and ineffective. Since World War II pesticides composed of synthetic organic compounds were the most important form of pest control. The growth in these pesticides accelerated in late 1940s after Paul Müller discovered DDT in 1939. The effects of pesticides such as aldrin, dieldrin, endrin, chlordane, parathion, captan and 2,4-D were also found at this time. Those pesticides were widely used due to their effective pest control. Problems with environmental issues of DDT became increasingly apparent, since it is persistent and bioaccumulates in the body and the food chain. In the 1960s, Rachel Carson wrote Silent Spring to illustrate a risk of DDT and how it threatened biodiversity. DDT was banned for agricultural use in 1972 and the others in 2001. Persistent pesticides are no longer used for agriculture, and will not be approved by the authorities. Because the half life in soil is long (for DDT 2–15 years) residues can still be detected in humans at levels 5 to 10 times lower than found in the 1970s.