“It’s the unknowns that are scary,” says Drew Dutcher, a resident of a north Denver neighborhood that in 2016 was deemed the most polluted ZIP code in America.

As Dutcher sits at a picnic table, describing what it’s like to live in a neighborhood surrounded by polluting and smelly industries, a cocktail of pungent odors drift by: earthy manure from passing rail cars stacked with cattle, a faint whiff of weed from local marijuana grow warehouses, the overwhelming stench of diesel from nearby I-70 highway.

Residents of ZIP code 80216, who suffer from some of the highest rates of asthma, cancer, cardiovascular disease, diabetes and obesity in Colorado, have been dealing with these unpleasant—and often harmful—odors for years. A major culprit of the more toxic stink is the nearby Suncor refinery, a massive complex that processes a variety of crude oils, including Canadian tar sands.

Suncor is a serial polluter, despite its greenwashed name. And over the past decade, it has increased its pollution, partly as an effort to process larger volumes and dirtier kinds of crude.

Tired of being saddled with an unfair share of Denver’s industrial pollution, residents like Dutcher are fighting back. Most recently, community members represented by Earthjustice petitioned the EPA, asking it to prevent Colorado state regulators from approving Suncor’s request to modify its existing permit so that it does not have to report its emissions of hydrogen cyanide. A colorless gas that’s a byproduct of oil refining, hydrogen cyanide is an agent of chemical warfare. Exposure to it can cause both short- and long-term effects like chest pain, breathing difficulties, developmental harm to children, and even brain and heart damage.

Thanks to a national refinery rule issued in 2015 because of Earthjustice litigation, Suncor and other refineries are now required to do a one-time test of their hydrogen cyanide emissions. Suncor’s test in 2015 revealed that, for some time, it had been emitting dangerous levels of hydrogen cyanide.

But rather than limit its emissions, the company instead decided to petition the state to give it a free pass to continue emitting as much hydrogen cyanide as it wants into the community—all without telling its neighbors just how much it’s emitting and without ensuring its pollution isn’t harming human health.

Suncor’s request to disregard the law is unfortunately business as usual. The company has repeatedly violated its air pollution permits, releasing hundreds of tons of smog-forming chemicals each year. It also has frequent accidents and malfunctions, sometimes spewing giant plumes of orange, chemical-filled smoke among the clouds. These emissions from Colorado’s only refinery contribute to Denver’s poor air quality, which is so bad that federal regulators have designated it unsafe for breathing since 2008. All too often, nearby residents experience shelter-in-place orders over concerns about odors, smoke and spikes in hazardous pollution.

Due to health and safety threats like these, a group of 13 community, scientist, and environmental groups, represented by Earthjustice, sued the EPA in 2017 to end the delay of the Chemical Disaster Rule, which was designed to strengthen safety requirements at oil refineries like Suncor. The new rule, years in the making, was delayed by the Trump administration until 2019 at the request of the oil and chemical industries.

At the same time, community members are speaking out. Last August, more than 100 people packed a public hearing about Suncor’s request to modify its permit so that it could emit pollution while evading transparency requirements for reporting its hydrogen cyanide emissions. Many of them came from nearby Commerce City and Denver’s Globeville and Elyria-Swansea neighborhoods, which are made up of primarily Latino and low-income residents. Despite calls from a majority of speakers to limit Suncor’s pollution, the state approved the company’s proposed modification at exactly the level Suncor asked, not at a level that would protect the community’s health.

Residents aren’t giving up. In addition to filing the petition over the hydrogen cyanide emissions, people are banding together to fight other local sources of pollution, including the expansion of one of the many highways that crisscross North Denver, a power plant, and several Superfund sites.

“We’re not going away,” says Ean Tafoya, a board member of the Colorado Latino Forum who is working with affected community members. “Every time they don’t hear us out, it’s just another opportunity to come together. It only strengthens our resolve.”