Support for the Standing Rock Sioux Tribe’s battle against the Dakota Access Pipeline continues to grow with the Service Employees International Union hopping (SEIU) onboard October 1.

“The two million members of SEIU stand beside the Standing River Sioux Tribe in their fight to protect their sacred lands and burial grounds from being dug up if the construction of the Dakota Access Pipeline is allowed to continue as planned,” the union wrote in an online statement.

The organization of 2 million members pointed out that the historical environmental justice factors at work make this battle especially important. They write:

Historical disregard for low income communities and communities of color, including those where many SEIU members live and work, has subjected them to toxic air pollution and contaminated waterways for decades. In these communities, asthma and other respiratory ailments caused by toxic air and poisonous toxins such as lead in the water supply, affect our children’s health and ability to thrive. As the nation’s largest healthcare union, we stand with the growing movement of environmental organizations, businesses, students, parents and others demanding cleaner air and water and to address the growing threat of climate change for the health and safety of our families and communities.

SEIU has led #FightFor15 and immigrant-rights movements. It joins the Communications Workers of America, the Amalgamated Transit Union, National Nurses United and the American Postal Workers Union.*

Many other unions have chosen to support the pipeline instead, as Grist reported last month (September 28), highlighting the lingering tension between environmentalists and Big Labor. AFL-CIO, the country’s largest union federation, threw its support behind the pipeline in September even though its member unions like the ones listed above oppose it.*

*Post has been updated to clarify that the AFL-CIO does not oppose the pipeline though some of its member organizations do. The AFL-CIO actually supports the pipeline, not the Standing Rock Sioux Tribe.