WHO urged to review nitrate drinking water limit

More than 80 international organisations have signed an open letter asking the World Health Organisation (WHO) to review its nitrate drinking water limit.

Signatories include the New Zealand Climate and Health Council OraTaiao and Greenpeace Aotearoa under its international banner.

The maximum acceptable value for nitrate-nitrogen is 11.3 milligrams per litre of drinking water in New Zealand, which is equivalent to the WHO guideline. The letter said the benchmark had not been updated since the 1950s and did not reflect robust evidence linking chronic, low-level nitrate exposure to long-term human health risks including colorectal cancer and pre-term birth.

The signatories asked for nitrate to be fully evaluated as a chemical contaminant in drinking water quality guidelines and for the epidemiological evidence to be reviewed.

Greenpeace agriculture global project lead Amanda Larsson said most governments based their water rules on WHO guidelines.

"The global nitrate guideline hasn't changed since 1958 and hasn't been reviewed in 10 years so we're calling on the World Health Organisation to review nitrate guidelines to take account of this science that has emerged over the last 10 years or so," she said.

Larsson said New Zealand researchers had been at the forefront of studying the health impact of nitrate at levels below the current guideline.

"There is a growing concern here in New Zealand about what that high nitrate in drinking water is going to mean for rural people's health," she said.

Larsson said pressure had been growing from health professionals, scientists and environmentalists to push governments to look at nitrates as a result of growing epidemiological evidence.

Canterbury Regional Council declared a nitrate emergency in 2025 after an annual groundwater survey found nitrate and E coli concentrations were increasing. In September the council voted not to investigate funding for the treatment of private water supplies for nitrate contamination.

The council's 2025 annual groundwater quality survey found nitrate levels had risen in more than half of rural Canterbury water supplies over the past decade. The survey sampled groundwater from 358 wells, finding 38 of them had nitrate-nitrogen concentrations above the drinking water limit.

Larsson said contamination needed to be tackled at its source, such as chemical fertilisers and intensive livestock farming. She called for the expansion of meat and dairy production to be halted and more support for farmers to transition to "nature-friendly" agriculture and clean up contaminated water.

"We think that most people feel that water should be life-giving and not life-threatening. It's a human right," she said.

Federated Farmers president Colin Hurst has previously told RNZ that farmers were working hard to reduce nitrogen loss.

"The dairy farmers are reducing their stocking rates, planting the likes of plantain that has the ability to mop up some of the nitrogen, farmers are undertaking precision farming, so only applying the fertiliser where it's needed," he said.

"Farmers are fencing off their waterways and there are a lot of plants being planted in the waterways to reduce nitrates. It's going to take a little bit of time to sort these things out but we're on the journey."

DairyNZ general manager policy and government relations Roger Lincoln said Canterbury dairy farms had reduced nitrogen loss per hectare by 30 percent over the past decade.

The letter also called for a risk framework that considered source-water protection within the WHO water safety planning and for international support for low and middle income countries to be promoted so chemical safety efforts complemented investments in basic sanitation and microbial safety.