County Chairman Jeff Cogen, a coal export opponent, requested the report on health effects, The Oregonian newspaper reported.
Local governments can't stop the export projects, he said, but "the burden should be on the coal companies and the train companies to prove that this is not going to damage the health of our residents."
One in nine Multnomah County residents lives within a third of a mile of potential coal-train routes, the report said.
Three of the five terminals being considered for coal exports could send trains through Portland -- one in Coos Bay and two along the Columbia River in Longview, Wash., and at a Port of St. Helens industrial park near Clatskanie.
The analysis looked at the impact if all three projects succeed, bringing up to 90 million tons of coal through the county on 16 to 19 trains each day. But some of the traffic might be on the Washington side of the river, and two of the terminals haven't applied for permits.
"The bottom line is a lot of the information on coal dust dispersal is proprietary, and it's not well validated," said Gary Oxman, who recently retired as county health officer and oversaw the report. "It doesn't mean there's a terrible risk from train transport, but it needs to be illuminated more."
The report says the federal government should do a regional study of export proposals, a call similar to one made by Oregon Gov. John Kitzhaber.
The dust contains harmful metals, including cadmium. But little is known about how it's dispersed or the size of the particles. Smaller particles are more likely to lodge in the lungs.
BNSF Railway has estimated that up to a ton blows off of a single car. But terminal and rail officials say most of the dust is lost near mines in Montana and Wyoming's Powder River Basin.
Coal shipments have been going through Washington to export ports in British Columbia for decades with no complaints made to regulators there, say advocates such the Alliance for Northwest Jobs and Exports, a trade group that includes railroads and coal companies.
"Coal dust is one issue where people involved in the alliance feel very, very comfortable that it's not a concern," said spokeswoman Lauri Hennessey. "I really feel it's a red herring."
The report concludes the trains a mile long would generate relatively small increases in diesel pollution and noise, but they would go through areas already heavily affected by pollution. The trains could create cumulative delays of up to two hours per day at at-grade rail crossings, the report said.
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