An insecticide still used as an ingredient in head lice treatments in some countries causes cancer in humans, the World Health Organization has said after reviews by expert panels.

Already banned in the Unites States and Europe for agricultural use but still found in some products such as head lice shampoos and scabies treatments in the U.S., Canada, China and India, the insecticide lindane has been classified as carcinogenic to humans by WHO's International Agency for Research on Cancer.

Lindane and another long-banned insecticide, DDT, have been specifically linked to several types of cancer, including non-Hodgkin's lymphoma, liver cancer and testicular cancer, say researchers who've conducted studies in the U.S. and Canada.

DDT has been banned for most uses since the 1970s, but it is highly persistent in the environment and in animals, leading to humans still being exposed to it, mostly through diet.

While is can still be legally manufactured in the U.S., DDT can only be sold to foreign countries.

After a review of the most recent scientific literature on insecticides and herbicides, the WHO's cancer research agency issued a new classification for lindane as "carcinogenic to humans," and has classified DDT as "probably carcinogenic to humans" and another insecticide, 2,4-D, as "possibly carcinogenic to humans," publishing its findings in the journal Lancet Oncology.

California banned the pharmaceutical use of lindane in 2002, and the Canadian Pediatric Society has recently warned against treatments containing lindane for infants, young children and pregnant and nursing mothers.

The classification of lindane as carcinogenic was based, for the most part, on studies conducted among agricultural workers who found a consistent increase of cancer risks as high as 50 percent, says Dr. Kurt Straif, head of the International Agency for Research on Cancer.

"This agricultural usage of lindane has been severely restricted starting in the 1970s and current general population exposure is mainly through the diet or when treated for scabies or lice," he notes. "There are currently no epidemiological studies to quantify the lymphoma risk from these exposures."

2,4-D, introduced in 1945, is still widely used for weed control in many agricultural, forestry, urban and residential settings.

Exposure to 2,4-D can occur both during its manufacture and its application for those who handle it directly, and the general public can experience exposure through water, food or during spraying, the WHO agency says.