The insecticide lindane causes cancer in humans, says the World Health Organization after conducting a review.
A specialist panel found sufficient evidence to link the chemical, already banned in the EU and the US, to a cancer called non-Hodgkin's lymphoma.
Lindane is still used in some developing countries.
And it is an ingredient in some head lice and scabies treatments used in some countries, including China, India, the US and Canada.
The International Agency for Research on Cancer (IARC) panel also concluded that another insecticide, called DDT, was "probably carcinogenic to humans".
And it classified a herbicide called 2,4-D as possibly carcinogenic to humans.
Most uses of DDT have been banned since the 1970s, but the IARC says exposure to DDT still occurs, mainly through diet.
This is because DDT and its breakdown products are highly persistent and can be found in the environment and in animals.
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, says the IARC.
The Lancet Oncology journal has a summary of the full evaluation.
Dr Kurt Straif, Head of the IARC said the evidence on lindane and cancer was largely based on studies among agricultural workers that showed a consistent, approximately 50% increase in risk, with higher risks in heavily exposed agricultural workers.
"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. There are currently no epidemiological studies to quantify the lymphoma risk from these exposures."