The country’s biggest nurses union is speaking out about the Keystone XL pipeline, warning that it may lead to adverse health effects. National Nurses United has called on the Obama administration to certify that the proposed Keystone pipeline, a plan which would see oil transported through the U.S., would not have a negative health impact on Americans before making a decision on the proposal.

The union made the request in a formal letter sent to Secretary of State John Kerry, who holds the final say on the pipeline. It was announced at a press conference with California Senator Barbara Boxer, who is backing the union's demand.

A project that places the health and safety of Americans at substantial risk cannot possibly be in our national interest,” the nurses union says.

The nurses union has already come out in opposition to the pipeline. In a press statement, the National Nurses Union explained that “harmful health hazards” have emerged from tar sands oil.

“Tar sands mining pollutants, for example, have been linked to cancer, leukemia, genetic damage, and birth defects,” the union notes. “Tar sands pipeline spills in Michigan and Arkansas have beset local residents with cardiovascular, gastrointestinal, neurological, and respiratory impacts, as well as persistent coughs, headaches, nausea, eye and skin problems.”

The proposed plan to transport oil has sparked a movement–so far successful–to halt the building of the final phase of the pipeline.