For many of us, mercury was that cool silver element in science class, but today some say it's causing a "catastrophic epidemic." I-Team reporter Linda Eggert finds many state residents have unsafe levels of mercury in them -- and so do some samples of local fish. That "quicksilver" is no longer cool. It's an increasing menace to health, in the United States and right here in Wisconsin.

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City firefighter Denise Sullivan builds homes in her spare time, but eight years ago mercury poisoning put her flat on her back.

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Mercury Rising, Part I: What You Need To Know

"I could sleep 10, 14 hours a night and still feel really tired," she said. "Extreme weakness muscular weakness, not just fatigue, and I had a lot of intestinal problems. I was really so sick and just progressively getting worse that it was pretty scary."

Despite her constant nausea, severe headaches, vision and balance problems, doctors had no answers, until tests finally revealed a high level of mercury in Sullivan.

She thinks removal of some dental fillings might be to blame, but no one knows for sure. What scientists do know is the most dangerous form of mercury for people is the kind they get from eating fish.

Fish are exposed to highly toxic "methylmercury," when mercury from coal fired power plant emissions and other sources mixes with water, and changes form.

"When that happens the mercury is then more toxic, it's easier for the methylmercury then to enter the brain once it's in our bodies," said Researcher Lynda Knobeloch.

NEWS 3 INVESTIGATION: MERCURY RISING Part I Part II FREE TESTING Wis. Mercury Study Volunteers Wanted -- Statewide Free Mercury Exposure Assessment Call: (866) 236-3461 Wisconsin Mercury Assessment Study RESOURCES DNR Fish Consumption Advice Natural Resources Defense Council - independent look at effects of pollution EPA: Mercury info EPA: Advice On Eating Fish

News 3 finds this highly toxic form of mercury in fish all over Wisconsin. State tests show the highest levels are in predator fish, like pike and walleye.

So much mercury is out there that Department of Natural Resources officials now have fish consumption advisories for every lake and river in the state.

"If everybody could at least read the advice and then put it in the context of how much fish they eat that, we should be ok, in terms of what kinds of health effects are possible," said Candy Schrank of the DNR.

But News 3's investigation finds a new state health study is raising new concerns. State lab analysts using sensitive tests are measuring the mercury in hair samples submitted by Wisconsin residents.

Knobeloch wants to document the mercury in 2,000 -- and their fish eating habits. So far she's got 800 locks of hair with 306 tested and the early numbers are surprising. Nearly one in five -- or 19 percent -- has mercury levels over the EPA's safe limit. Among Dane County residents, it's one in four.

Statewide, nearly twice as many men as women are over the limit, with some way over.

In fact the highest mercury level so far is in a 54-year-old man. He's got four-and-a-half-times the safe limit of mercury.

The high numbers are cause for new concern because of two major studies out of Europe. Those studies linked men with high mercury levels from fish to "an excess risk of heart attack, as well as death from coronary heart disease, and cardiovascular disease."

Mercury, a neurotoxin, is a well documented risk to children, breastfed babies and developing fetuses, but now men have more reason to be careful.

"The take-home message from those studies is that mercury-contaminated fish, the damage done to by mercury to the heart, can outwieght the beneficial effects of the omega 3 fatty acids, so you really need to eat fish but eat fish that are low in mercury."

But how do people know which fish are safe? To test the waters, News 3 bought five types of fish all over Madison and hired an independent lab to run tests.

It found mercury in each and every sample:

The swordfish had the most -- enough to prompt a state "do not eat" warning for kids and most women.

The mercury in the yellowfin tuna could prompt a "one meal a month" advisory.

The more expensive white canned tuna fish could fall under the "one meal a week" rule.

Even the farm-raised salmon had a small amount of mercury, as did the local walleye fish fry -- though the tiny amounts are considered safe.

Still some say when it comes to mercury, you can't be too careful.

"You don't know when you're going to reach that point when you've tipped that balance from being able to maintain your health, to becoming very ill," Sullivan.

You can't cut away mercury contamination in fish, but state officials still want you to eat them just in the proper amount. You can get a state healthy fish eating guide at any DNR office.

And to get your mercury tested as part of that state study, call (866) 236-3461.