Photo by John Messina
The insecticide gamma-hexachlorocyclohexane (lindane) is carcinogenic, according to experts from the International Agency for Research on Cancer (IARC), the specialized cancer agency of the World Health Organization.
The experts said they found sufficient evidence that lindane, which is banned or restricted in most countries, can cause non-Hodgkin lymphoma (NHL).
The group also discovered that 2 other chemicals might cause NHL.
The evidence suggests the insecticide dichlorodiphenyltrichloroethane (DDT) is probably carcinogenic, and the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) is possibly carcinogenic.
A summary of these findings is available in The Lancet Oncology, and the experts’ detailed assessments will be published as Volume 113 of the IARC Monographs.
The group, which consisted of 26 experts convened by the IARC Monographs Programme, reviewed the latest scientific literature on lindane, DDT, and 2,4-D and used their findings to classify these 3 chemicals according to carcinogenicity.
The classification (Group 1, Group 2A, etc.) indicates the strength of the evidence that a substance causes cancer, not the level of risk associated with exposure. The Monographs Programme identifies cancer hazards even when risks are very low at current exposure levels, because new uses or unforeseen exposures could engender risks that are significantly higher.
Lindane
The experts classified lindane as carcinogenic to humans (Group 1), saying they found sufficient evidence that it can cause NHL. Large epidemiological studies of agricultural exposures in the US and Canada showed a 60% increased risk of NHL in people exposed to lindane.
Lindane has been used extensively for insect control, including in agriculture and for the treatment of human lice and scabies. High exposures have occurred among agricultural workers and pesticide applicators. However, the use of lindane is now banned or restricted in most countries.
DDT
The experts classified DDT as probably carcinogenic to humans (Group 2A), saying they found sufficient evidence that DDT causes cancer in experimental animals and limited evidence of DDT’s carcinogenicity in humans.
Epidemiological studies have shown positive associations between exposure to DDT and NHL, testicular cancer, and liver cancer.
There was also strong experimental evidence that DDT can suppress the immune system and disrupt sex hormones. However, overall, there was no association between breast cancer and DDT levels measured in samples of blood or fat.
DDT was introduced for the control of insect-borne diseases during World War II and was later applied widely to eradicate malaria and in agriculture. Most uses of DDT were banned in the 1970s. However, DDT and its breakdown products are highly persistent and can be found in the environment and in animal and human tissues throughout the world.
Exposure to DDT still occurs, mainly through diet. The remaining and essential use of DDT is for disease vector control, mainly for malaria. This use is strictly restricted under the Stockholm Convention.
2,4-D
The experts classified 2,4-D as possibly carcinogenic to humans (Group 2B), saying they had inadequate evidence in humans and limited evidence in experimental animals.
There is strong evidence that 2,4-D induces oxidative stress and moderate evidence that 2,4-D causes immunosuppression, based on in vivo and in vitro studies. However, epidemiological studies did not show strong or consistent increases in the risk of NHL or other cancers in relation to 2,4-D exposure.
Since its introduction in 1945, 2,4-D has been widely used to control weeds in agriculture, forestry, and urban and residential settings. Occupational exposures to 2,4-D can occur during manufacturing and application, and the general population can be exposed through food, water, dust, or residential application, and during spraying.
