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.