A tar-sands facility from the air.A tar-sands facility: oil doesn’t get any dirtier than this.At ForestEthics, persuading the world’s largest corporations to treat the Earth ethically is our bread and butter. And it often starts with a letter.

Last week, we mailed letters to more than 100 Fortune 500 companies, warning that their continued consumption of fuels from Canada’s tar sands—the world’s dirtiest oil—puts their brands at risk.

As ForestEthics Executive Director Todd Paglia documented in a vivid slideshow for Grist last year, the tar sands manage to combine multiple local and global environmental hazards into a single industrial project—in fact, the largest industrial project in the world. In the parlance of addiction, the tar sands are proof that we’re getting pretty close to rock bottom. It’s a giant step backward for a world that is ready to break its addiction to oil.

Tar-sands oil production generates three to five times the greenhouse-gas emissions of conventional oil production. Communities downstream of tar-sands projects are facing elevated levels of cancer. Tar-sands production creates toxic lakes so vast they can be seen from outer space. Production of tar-sands oil destroys fresh drinking water, pollutes the air, and razes North America’s Great Boreal Forests. Tar-sands sludge, extracted primarily in the province of Alberta, Canada, cannot be made clean by technological solutions.

And the tar-sands problem is coming to America.  An increasing percentage of U.S. transportation fuels–consumed in massive quantities to ship American products and power American cars–are derived from Canadian tar-sands oil. This means that despite what you may have heard, a lot of America’s favorite products—from cans of soda to bars of soap to books purchased online—have a dirtier carbon footprint than they’ve ever had before.

The tar-sands industry is proposing new pipelines and refineries that would expand U.S. capacity for converting their tarry sludge into fuel. If these plans move forward, America will have moved Alberta’s toxic local impacts to our towns and cities. At the precise moment America has concluded that our economy must be cleaner, tar-sands oil threatens to make it dirtier.

In last week’s letter, we offered a hand in helping companies rely more on cleaner fuels and less on dirty tar-sands fuels, while also notifying them that a public campaign could be launched against any company that does not act ethically in response to the tar sands’ devastating environmental and health impacts. The choice is theirs to make.

Both the sincere offer of help and the legitimate threat of public action are critical, and this “carrot/stick” approach marks a return to the strategies that made ForestEthics’ reputation. As ForestEthics has found over the years, the old adage “the customer is always right” can be a powerful tool for change.

And America’s Fortune 500 companies are some of the most powerful customers in the world.  Many of these companies did not know they were customers of Canada’s tar sands until they received our letter.  Now that they know, they can either burnish their brands by helping to lead us into a clean energy future, or they can ‘tar’-nish their brands by passively accepting Big Oil’s latest plan for keeping us addicted to fossil fuels.

A version of this post was originally published at ForestEthics.org.