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This guest post was contributed by Emily Ponder, a legal intern in our U.S. office. Emily is a first-year law student at the University of Virginia School of Law.

Everyone knows that oil is a dirty business, but tar sands oil may be the dirtiest. That is why environmentalists, indigenous groups, and small-town Nebraska famers alike are celebrating President Obama’s rejection of the Keystone XL pipeline Jan. 18.

The tar sand oil extraction process and its transport poses serious health and environmental hazards, and the Keystone XL pipeline would have made 2,000 miles of land—and communities—vulnerable to its destructive risks.

The energy-intensive process of extracting tar sands emits three times more carbon dioxide emissions three times higher than the extraction of conventional oil. [Edit: As noted in the comments, this refers only to emissions during extraction, which is a small percentage of total emissions. However, because of these high extraction emissions, the “well-to-wheels” emissions of Canadian tar sands are still the highest of US oil sources, 16.5% higher than the baseline and 22% higher than domestic crude oil.] What’s more, extraction of the tar sands oil in Canada, at the head of the pipeline, poisons water downstream with chemicals such as cyanide and anomia. Indigenous communities downstream from extraction sites in Alberta have seen an increase in rare cancer, renal failure, lupus, and hyperthyroidism.

At the other end, refineries in Texas would have increased air pollution, making communities vulnerable to respiratory problems and lung disease, not to mention high levels of smog and acid rain.

And the miles of pipeline in between Canada and Texas wouldn’t have gone unscathed, either. TransCanada’s Keystone I pipeline spilled a dozen times in less than a year of operation. If the Keystone XL had been built, a similar track record of spills could have contaminated not only drinking water, but also the water which irrigates the US’s vital agricultural heartland.

It’s a sweet victory to be sure, but the fight isn’t over. The Obama administration has said the pipeline wasn’t necessarily rejected on environmental merits, but because of procedural hang-ups that prevented further investigation from taking place before the Congressionally imposed deadline. Congressional Republicans are also meeting to come up with a strategy to continue pushing for the pipeline. Advocates must continue to demand that the government take a stance against tar sands oil and investigate the dangerous risks associated with it.