The WHO's International Agency for Research on Cancer reviewed the latest scientific literature on the insecticides DDT and lindane along with 2,4-dichlorophenoxyacetic acid (2,4-D), a herbicide included in many lawn-weed killers. The herbicide was introduced in 1945 and is widely used in weed control across forestry, agriculture and urban settings.
IARC's experts classified 2,4-D as "possibly" carcinogenic to humans, a step down from the "probably carcinogenic" classification given to glyphosate earlier this year. The lower carcinogenity rating was based on "inadequate evidence in humans and limited evidence in experimental animals".
The group's monograph did say there is "strong evidence that 2,4-D induces oxidative stress, a mechanism that can operate in humans, and moderate evidence that 2,4-D causes immunosuppression, based on in vivo and in vitro studies.
"However, epidemiological studies did not find strong or consistent increases in risk of NHL or other cancers in relation to 2,4-D exposure."
The reviewers also agreed there was sufficient evidence that the insecticide lindane was carcinogenic (causing non-Hodgkin lymphoma) and DDT was probably carcinogenic. Neither have been used in the UK for decades, however DDT exposure still occurs through diet as it is slow to break down in the environment.
IARC classifications indicate the strength of the evidence that a substance causes cancer, rather than the probability or risk of cancer occurring. Cancer risk can vary drastically between chemicals that have a similar classification due to the different ways a chemical is used, which result in different levels of exposure.