Portland residents will decide whether to fluoridate their drinking water supply in the May 21 special election.

Multnomah County has set a new record for the thickness of its voters' guide. A total of sixty-three arguments are included -- for and against.

Michael Clapp / OPB

It's enough to make anyone's head spin.

When Portland's fluoride debate first flared in September, Kristian Foden-Vencil looked at two scientific studies that were often cited in the fluoride debate. Here's an update.

In the middle of the 20th century, researchers learned high levels of naturally occurring fluoride in towns like Colorado Springs were causing children's teeth to grow in brown and mottled.

They also noticed their teeth were surprisingly resistant to decay.

So in 1945, Grand Rapids, Michigan, agreed to add small amounts of fluoride to its water. A study found the rate of cavities among children there dropped 60 percent -- and the U.S. Surgeon General has supported fluoridation since the 1950s. ?So, the debate has been running for decades.

One of the latest studies to raise hackles came out of Harvard University, last year. Researchers dug through dozens of previous studies and concluded that there is "the possibility of an adverse effect of high fluoride exposure on children's neurodevelopment."

Repeated calls and emails to the study's U.S. author, Anna Choi of the Harvard School of Public Health, were not returned last fall. But on Thursday, after rebroadcasting the story, OPB received an email from the study's authors, who said they were traveling at that time.

The researchers say their study summarized the findings of 27 studies on intelligence tests in fluoride-exposed children. Twenty-five of the studies were carried out in China.

The researchers say that on average, children with higher fluoride exposure showed poorer performance on IQ tests.

Fluoride released into the ground water in China in some cases greatly exceeded levels that are typical in the U.S.

The researchers say that complete information was not available on these 27 studies, and some limitations were identified. All but one of the 27 studies, they say, documented an IQ deficit associated with increased fluoride exposure.

The average loss in IQ was reported as a standardized weighted mean difference of 0.45, which would be approximately equivalent to seven IQ points for commonly used IQ scores with a standard deviation of 15.

(According to the Harvard School of Public Health's website, this sentence, that refers to IQ loss, was updated on September 5th, the same day that OPB's original story on fluoride aired. OPB was not contacted about this change).

However, the researchers concluded that these results did not allow them to make any judgment regarding possible levels of risk at levels of exposure typical for water fluoridation in the U.S.

On the other hand, they say, neither can it be concluded that no risk is present. They recommended further research to clarify what role fluoride exposure levels play in brain development.

Kim Kaminiski is fighting to stop fluoridation with Clean Water Portland.

She said, "We're seeing negative health effects at very low levels of fluoride."

She added, "I mean we can talk all day about parts per million, but the bottom line is, when we start putting it in our drinking water, that's the major exposure that most people have."

But perhaps the most controversial fluoride study came out in 2006. It found that, "For males less than 20 years old, fluoride levels in drinking water during growth, is associated with an increased risk of osteosarcoma."

Osteosarcoma is a bone cancer.

Kaminiski of Clean Water Portland says the 2006 study prompted her to start digging into the whole fluoride issue.

Kim Kaminski: "This was peer-reviewed and published. It was a very solid study. And at the time, being a mom it was very concerning to me."

Dr. Catherine Hayes of Health Resources in Action was an advisor for the 2006 study. She said that while it was peer reviewed, it was exploratory.

Catherine Hayes: "And it was done using a data set that was not as ... involved in terms of the questions and they didn't have any bone samples from cases or controls."

Still, the study raised enough eyebrows that a follow-up was done.

Hayes was a co-author on that follow-up. She's now a professor at Tufts University's School of Dental Medicine.

She says that instead of just gathering information about previous cases of osteosarcoma, researchers looked at actual samples of bone from people who had the cancer.

Catherine Hayes: "In that study, the bone was carefully examined amongst individuals who had the osteosarcoma and those that did not. And there was no difference in the amount of fluoride in the bone. And that's really significant, because now we're not estimating fluoride intake, we're really measuring it."

So as far as Hayes is concerned, it means there's no link between osteosarcoma and fluoride.

But fluoride opponents like Kaminiski aren't buying it. They say the study's other co-author, Chester Douglass, received payments from the toothpaste company Colgate-Palmolive.

Hayes said the allegations against Douglass don't hold water.

Hayes told OPB, "He was thoroughly investigated by Harvard University, a very extensive investigation and found completely innocent of any wrong doing. His involvement with Colgate is as someone who provides educational information to them. And there is absolutely no relationship between his consulting work with Colgate and his research."

Suffice it to say, whether you're talking to scientists, activists or health policy experts, it's easy to get bogged down in the details.

The list of organizations that endorse water fluoridation is long and authoritative -- the American Medical Association, the National Institutes of Health. ?But Portland's anti-fluoride campaign has attracted some big names too -- the local chapter of the Sierra Club, a scientist from the National Academy of Sciences Fluoride Committee, and consumer advocate Ralph Nader.

Even the local chapter of the NAACP came out against it.

Committee chair Clifford Walker says for him, it comes down to who you can trust.

Walker said, "You hear the arguments on both sides, but you become suspect of governments telling you it's okay."

For example Walker says infants fed formula mixed with fluoridated water risk being over exposed. He cites the American Dental Association and the Centers for Disease Control.

The CDC's website does say mixing infant formula with fluoridated water on a regular basis may increase the chance of a child developing mild enamel fluorosis -- that is faint white markings on teeth.

But the American Dental Association says mild fluorosis doesn't affect the health of a child or its teeth.

And besides the agencies say, parents can use purified bottled water to lessen the possibility.

But Walker says that's a problem.

He said, "A lot of times the income challenged people are not using the distilled or purified water, but they're mixing it from tap water."

The American Academy of Pediatrics website says that once a child's adult teeth come in -- around 8-years-of-age -- the risk of developing fluorosis is over.

This story originally appeared on Oregon Public Broadcasting.