One night in February, high school principal Matthew Smith got a frightening wake-up call.

The local fire department alerted him that the home of a student at Agua Fria High School was contaminated with liquid mercury that apparently had been taken from a science classroom. The next day, emergency crews descended on the school in haz-mat suits, discovering a toxic trail of mercury vapors in classrooms, locker rooms, and buses.

The high school, in Avondale, Ariz., was shut down for a week so it could be decontaminated. The homes of six students were tainted with mercury, two so severely that the families had to be relocated for 11 days, according to the Environmental Protection Agency. The total cleanup is expected to reach hundreds of thousands of dollars.

The mercury mess in Arizona was only the latest in thousands of incidents where children are exposed to elemental mercury, a poison that can damage the brain, trigger respiratory failure and cause other serious health problems.

Power plants are typically cast as the usual suspects of mercury contamination, since they emit mercury into the air, where it spreads globally. But many children are exposed to toxic levels of mercury much closer to home. Mercury spills inside schools and houses, often unreported, can release vapors into the air for weeks, even years.

Elemental mercury, or quicksilver, is a shiny, silvery liquid metal found in thermometers, thermostats, light bulbs, barometers and LCD screens. Though mercury inside these items poses little risk, once broken, they release mercury that vaporizes as an invisible, odorless gas.

Children are most frequently exposed to mercury when it is mishandled or improperly cleaned up after a spill. Broken thermometers, filled with tiny blobs of mercury, are the most common culprits.

From 2002 to 2006, more than 37,000 calls were made to U.S. poison control centers about children exposed to mercury. Of those, 30,891 concerned broken thermometers and 6,396 were caused by other sources, such as old science laboratories and religious or cultural ceremonies, according to a new report by the Agency for Toxic Substances and Disease Registry, part of the U.S. Centers for Disease Control and Prevention.

The report, released in February, was prompted by a number of high-profile incidents of children exposed to mercury, in particular, a 2004 case involving as many as 100 children in a thermometer-factory-turned-daycare center in New Jersey. U.S. Rep. Frank J. LoBiondo (R-New Jersey) commissioned the report.

"Basically there was concern that other types of events such as the one in New Jersey might be occurring throughout the U.S., so Congress directed ATSDR to do a review to look for events of elemental mercury exposures," said Robin Lee, an epidemiologist with ATSDR's Division of Health Studies and co-chair of the study.

Many science labs at schools, especially older ones, unknowingly house long-forgotten toxic chemicals such as mercury, chlorine gas and formaldehyde.

These chemical stockpiles are just accidents waiting to happen, which administrators at Agua Fria High found out the hard way on February 12. According to a police investigation, the source of the spill was five pounds of mercury stored in a medical bottle that a student took from an unlocked shelf in a science classroom.

"It was our understanding that there was no mercury available on campus to students at all," said principal Smith. "It was a learning experience for us and now we are really dotting our 'i's and crossing our 't's to make sure that there are no unsafe chemicals on this campus."

Other schools also have been forced to evacuate and shut down for days, sometimes even weeks, while emergency crews conduct expensive cleanups to remove dangerous levels of mercury.

At Ballou High School in Washington, D.C., in 2003, a student took liquid mercury from a science laboratory and sold some to other students. Mercury was found in the classrooms, gymnasium and cafeteria, and the school had to be shut down for 35 days to clean it up.

In addition, the students unknowingly carried mercury on shoes and clothing through the streets, onto buses, and into their homes. Eleven homes were contaminated and about 16 families were displaced for a month.

The total cleanup cost: $1.5 million, according to the EPA.

Earlier that same year, a student at Pau-Wa-Lu Middle School in Gardnerville, Nevada, brought a vial of mercury to school that he had found in his grandfather's garage. The student shared the mercury with children on the bus and in the locker room. Liquid mercury was visible on the gym floor and in several classrooms, exposing 61 students. The school was shut down for four days and the decontamination cost more than $100,000.

Then, in 2004, a 17-year-old in Las Vegas had to be rushed to the hospital and spent a week in intensive care after playing with mercury over a period of several months.

The house was so badly contaminated that it had to be stripped to bare concrete, according to the EPA. The excavated materials were all removed and treated like hazardous waste. Even the family dog had to be decontaminated. The cleanup cost nearly $132,000, and that didn't include renovating the house.

Few people become immediately ill when liquid mercury is spilled. But health experts say it is a potent neurotoxin that can have lasting health effects. It is particularly dangerous to children's developing brains because it can cause learning problems. Health problems can be more immediate and extreme than exposure to the other common form of mercury, methylmercury, which comes mostly from eating fish.

Symptoms of mercury poisoning--often called the mad hatter syndrome because of compounds used on felt in the 18th and 19th centuries--can start within a few hours and include tremors, chills, vomiting, abdominal pain, headache and anxiety.

Even a miniscule amount of liquid mercury, equivalent to the couple grams found in a thermometer, can be dangerous.

"Children have higher respiratory rates and metabolic rates than adults, so that would provide some evidence for erring on the conservative side when determining acceptable levels of mercury exposure," explained John Risher, an environmental health scientist with ATSDR's Division of Toxicology and Environmental Medicine and a member of the report's work group.

Exposures to high levels of metallic mercury can lead to a rare condition in children called acrodynia, characterized by neurological symptoms including tremors, irritability and light sensitivity, in addition to joint pain, rashes and painful, swollen hands and feet, according to Maida Galvez, an assistant professor at the Mount Sinai School of Medicine.

Anthony Carpi, an environmental toxicologist at the John Jay College of Criminal Justice in New York City, said mercury spills are difficult to clean up because of the element's "stickiness," which allows it to latch on and embed itself into porous surfaces like carpet.

He recalls an incident a few years back involving a house with significant levels of mercury in a bathroom that came from a mercury thermometer broken there two decades earlier.

"We were able to see traces of that mercury spill 20 years after it happened," said Carpi. "It's sticky so it's hard to clean up and what's left behind can persist for very long periods of time."

Mercury thermometer use in schools is declining, in part because many national retailers have stopped selling them and manufacturers have stopped producing them. At least nine states have laws limiting the use of mercury-containing devices in schools, according to the EPA.

"It just seemed foolish to endanger children and foolish to waste that kind of money trying to clean up what was sometimes very small amounts of mercury," said Steven Kratzer, a mercury reduction and policy specialist at the Department of Environmental Quality in Michigan, which has banned mercury devices in schools.

But many schools still harbor the element inside rusty chemical cabinets or sink drains.

As a senior health and environmental investigator in King County, Wash., Dave Waddell has uncovered many forgotten mercury stashes during school inspections. Between 1998 and 2002, the county's Rehab the Lab project, which helps K-12 schools manage their hazardous chemicals, removed 664 pounds of mercury from schools in King County alone.

Small spills can quickly escalate into full-blown hazards when people are unaware of proper clean-up procedures.

"We've had schools that have had a mercury spill try to clean it up with a vacuum and then blew the mercury vapors out the back of the vacuum," he said. "I find broken mercury thermometers in almost every school I've been in. Teachers have collected the pieces and done their best to clean things up, so there's still residual micro droplets left around that are vaporizing."

Once airborne, mercury can easily spread, contaminating everything in its path.

"The average beginning cost for a mercury cleanup is around $14,000," said Waddell. "You think you're saving money by keeping your mercury thermometers, but think of how many nice digital thermometers you can buy for $14,000."

Traces of long-forgotten or unknown mercury spills are found so often in schools that the Minnesota Pollution Control Agency decided in 2001 to bring in the big dogs, literally.

Clancy, a 70-pound chocolate and black Labrador retriever mix, is the only mercury-detecting dog in the U.S. that's been professionally trained to find spilled or "hidden" mercury in schools.

"Kids will break lab thermometers and try to wash it down the drain, which doesn't work because mercury is heavier than water," said Carol Hubbard, a mercury specialist and Clancy's handler. "When he was younger, Clancy would put his head in the sink to tell us that it was contaminated."

Of the 300 schools that Carol and Clancy have inspected in the last eight years, about 40 percent of them had mercury spills, usually in the plumbing. Though mercury-detecting instruments can do the same job as Clancy, he's faster, completing an assignment in a quarter of the time that the instrument takes.

"He can find a broken fever thermometer in a 5,000 sq. ft. warehouse in less than 10 minutes, but usually under a minute," said Hubbard. "It's amazing to watch him work."

Between 2002 and 2006, 75 percent of mercury-related events that potentially exposed children occurred in private households, according to the ATSDR report.

Some Caribbean religions and folk healers use mercury because they believe its supernatural powers bring good luck and drive away evil spirits. Practitioners apply mercury to the skin, add it to candles or sprinkle it around the home.

About 38 percent of 900 people mostly with Latino or Caribbean backgrounds reported that they used or knew someone who used mercury for religious, spiritual, or health purposes, according to a survey by John Snow, Inc., a Boston health consulting company. The ATSDR report warns that "such use may lead to chronic mercury exposure among those who use it in this manner and for subsequent occupants of the contaminated homes."

"Imagine if you suspected that your apartment might have had a prior occupant that sprinkled mercury on the carpet a decade ago," said Arnold Wendroff, founder of the Mercury Poisoning Project, a website dedicated to the issue. "That's not something you want to live with."

Wendroff has tracked religious mercury use since 1989 after a young boy in a class he was teaching told him his mother sprinkled mercury on the floor of their home to keep away witches. These liquid good luck charms, which can be purchased at medicine shops called botanicas, are often found in 10-gram bottles. Mercury fever thermometers, in comparison, contain only a few grams (.5 to 3.0 g) of mercury.

The report cites a handful of religious mercury use studies in Chicago and New York, including a 2008 study that found mercury levels were higher in residential common areas in communities likely to use mercury for cultural practices. However, Wendroff said that further studies are needed to determine mercury levels in the practitioners' homes.

"We have mercury sold, we know how it's being used and we have indoor air elevated levels of mercury," Wendroff said.

Nevertheless, he said, "since there could be a lot of money involved to clean this up, the government's stance is to let sleeping dogs lie."

The EPA's Office of Inspector General disagrees, maintaining in a 2006 report that the environmental agency is properly addressing the risks of ritual mercury use by sponsoring research and environmental monitoring, among other things.

To reduce mercury exposures, the ATSDR recommends increasing education on mercury's health effects and proper clean-up methods, especially when sales of mercury-containing compact fluorescent bulbs (CFLs) are lighting up.

"Given the potential cumulative hazard from breaking a large number of CFLs, or the disposal of large numbers of CFLs in landfills, the public must learn about the need for proper disposal and have easy access to appropriate disposal facilities," the report says.

Though there is no data on exposure incidents from light bulbs, the EPA estimates that more than 670 million mercury-containing bulbs are discarded each year. Currently, there is no national infrastructure for recycling them.

The authors also recommended decreasing the availability of mercury items altogether. Several states have implemented bans on mercury thermometers. In addition, the Mercury Ban Export Act of 2008 aims to prohibit U.S. exports of elemental mercury by 2013.

"The long term goal is to get mercury out of all consumer products," said Ned Groth, a science consultant at the Mercury Policy Project, an advocacy organization. "Mercury is an equal opportunity poison. It's toxic to everybody."

Tina Toy, whose two daughters attended the contaminated day care center in New Jersey, hopes that the new federal report will help keep awareness of the issue alive.

"I worry about my children's health every day, but at least something good is going to come out of this," she said.

The ATSDR report [PDF] is available here.

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.