Residents of Portland, Oregon, are either smiling broadly or gritting their teeth today, after a bid to fluoridate the city's drinking water was rejected by 60 percent of Portland voters in a municipal referendum. It's the fourth time since 1956 that locals have rejected plans to fluoridate city drinking water — and makes Portland the only city among the 30 largest in the US to remain without fluoridated water.

The vote, which was held using mail-in ballots and wrapped up on Tuesday night, highlights the contentious nature of fluoridation — by now a mainstay across the US — in Portland. Since the 1950's, when major metropolitan areas started adding fluoride to their water supplies in an effort to bolster dental health, Portland residents have consistently bunked measures to follow suit.

But in September of last year, Portland's city council quietly, and unanimously, elected to fluoridate the city's water starting in 2014. The move sparked outcry among anti-fluoride advocacy groups, including Clean Water Portland, who gathered 43,000 signatures to force a referendum on the measure. From there, organizations on both sides of the debate raised hundreds of thousands of dollars and launched aggressive campaigns: lawn signs, telephone push polls and boisterous rallies, along with allegations of corruption and sham science, turned the debate into "the city's most contentious of the 21st century."

"At a very fundamental level, people understand that we don't want more chemicals in our water," Kim Kaminsky, leader of Clean Water Portland, told the Portland Tribune. The group and its anti-fluoride cohort claim that fluoridation constitutes the unlawful medication of citizens, threatens to damage water supplies, and might be harmful to human health.

Decades of scientific research, however, doesn't align with those ideas. As Slate points out in a comprehensive report, "almost every credible national, state, and local health and science organization-private and public-gives its blessing to optimal levels of water fluoridation." Some Portlanders, though, now hope to make their resistance to that blessing more permanent: Clean Water Portland wants a ban on fluoride added to the city charter.