Unsafe heavy-metal levels have been detected in the Kok, Mekong and Sai rivers in the northernmost provinces of Chiang Mai and Chiang Rai.
The pollution is believed to have come from outside Thailand.
Health deputy pemanent secretary Weerawut Imsamran said on Wednesday that enironmental officias checked the quality of the water in the three rivers on May 1 and 2. They found unsafe levels of heavy metals, especially arsenic and lead, at many locations in both northern provinces.
“This can affect the health of people who regularly eat fish caught in these areas. Children, pregnant women and elderly people can suffer nerve disorder, hand and foot numbness, thickening and darkened skin and irregular heartbeat,” Dr Weerawut said.
Officials from the Department of Disease Control and the Department of Health would look for arsenic in food chains over the next four months, he said.
Dr Weerawut advised consumers to wash vegetables and fruits carefully to minimise arsenic contamination.
Aweera Pamat, director of the Environmental and Pollution Control Office 1, said his staff collected surface water at 12 locations along 157 kilometres of the Kok River, which flows from Myanmar, in Chiang Mai and Chiang Rai from April 21-24.
The office found unsafe levels of arsenic at nine of the 12 locations. Three locations were in Mae Ai district of Chiang Mai and six in Muang district of Chiang Rai.
Officials checked the sources of wastewater along the Kok River within Thailand, including factories, mines, wastewater treatment facilities and large farms, and did not find any evidence of any heavy metals big discharged, Mr Aweera said.
He said tap water in Chiang Mai and Chiang Rai remained safe for consumers.
Heavy-metals are typically released from mining waste.