The International Agency for Research on Cancer (IARC), a specialized agency of WHO, announced Friday that malathion and another insecticide, diazinon, have been classified as "probably carcinogenic to humans."
The agency issued the same classification for glyphosate, a key ingredient in the weed control product Roundup.
A summary of the agency's findings has been published on the website of the journal The Lancet Oncology.
The City of Winnipeg sprays with malathion when adult mosquito numbers are very high.
In a news release, the IARC says its classification for malathion is based on "limited evidence of carcinogenicity in humans for non-Hodgkin lymphoma and prostate cancer."
That evidence resulted after reviewing mostly agricultural exposures to the insecticide in the United States, Canada and Sweden in studies published since 2001.
"Malathion also caused tumours in rodent studies. Malathion caused DNA and chromosomal damage and also disrupted hormone pathways," the agency's news release states in part.
Malathion use sparks annual debate
The use of malathion to control's Winnipeg mosquito problem has sparked heated debates every summer between people who want to enjoy the outdoors bug-free and those who don't want the chemical sprayed near their homes.
Some experts, including the province's former chief medical officer of health, have maintained that malathion does not pose a risk to human health if it's used properly.
The city plans to use 100 per cent biological mosquito larvicides this summer, meaning no chemical agents will be used.
However, the move only applies to larviciding — that is, interrupting the development of larvae into adult insects — and does not affect adult mosquito control using chemicals such as malathion.
A city spokesperson told CBC News they are working on a response.
Monsanto disputes glyphosate classification
The IARC says diazinon was deemed to be probably carcinogenic based on limited evidence in humans for non-Hodgkin lymphoma and lung cancer.
As for glyphosate, the agency says there is limited evidence of carcinogenicity in humans for non-Hodgkin lymphoma, based on mostly agricultural exposure cases in the United, States and Sweden published in studies dating back to 2001.
It also cited "convincing evidence that glyphosate also can cause cancer in laboratory animals."
Monsanto, the maker of Roundup, issued a news release disagreeing with the IARC classification, saying there was no research or data involved and "relevant scientific data was excluded from review."
"All labeled uses of glyphosate are safe for human health and supported by one of the most extensive worldwide human health databases ever compiled on an agricultural product," the company said.
"In fact, every glyphosate-based herbicide on the market meets the rigorous standards set by regulatory and health authorities to protect human health."