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Elemental mercury was spilled in a chemistry class at Stoughton High School on January 30, 2004. The amount of spilled mercury required intervention and cleanup. Staff and students in the classroom during the spill breathed mercury vapors for a short time at levels not likely to cause harmful health effects. After the cleanup, air sampling did not detect levels of mercury vapors at Stoughton High School and confirmed the cleanup was effective. Follow-up analysis of students' urine found mercury levels within a normal range. No further cleanup is needed at Stoughton High School by the school district, the Wisconsin Department of Health and Family Services (DHFS), or other agencies, regarding this incident. The Stoughton School District is reviewing the use of chemicals in the classroom and storage practices to reduce the potential for future spills of this nature.


On Friday, January 30, 2004, elemental metallic mercury was spilled in the chemistry class of the Stoughton High School, 600 North Lincoln Avenue, Stoughton, Wisconsin. Apparently, during fifth hour (1:15 p.m.) mercury spilled from an open-ended glass manometer in the back of a chemistry classroom. A student reportedly attached one end of a rubber hose to of the manometer and the other end to a pressurized air port. The student then reportedly opened the air port, which forced air into the manometer and blew most of the mercury from the manometer. An estimated 4 tablespoons (or 2 fluid ounces) of mercury spilled onto nearby tables, counters, and the floor. Mercury particles also attached to the ceiling tiles directly above the manometer. Shortly after this incident, school staff discovered the spill and quickly reported to school administration officials, who then implemented the school district's emergency procedure plan.

Responding to a 911 call from school district officials, the Stoughton Fire Department came to the school to evaluate the situation. After consulting with emergency response officials, the school district retained Onyx Special Services (Onyx), a hazardous waste cleanup contractor. Also responding to the incident were the Wisconsin Department of Natural Resources, the Wisconsin Department of Health and Family Services (DHFS), and Dane County Department of Human Services (DCHS). DHFS determined whether any adverse health effects could result from exposure and whether personal items were safe to return to students. DHFS also assisted the school and other agencies with health communications.

Securing the School and Screening Students and Staff

On January 30, immediately after the mercury spill was discovered, the Stoughton High School science wing was cleared of staff and students, then secured. Later in the afternoon, the entire school was "locked down," and to prevent the potential spread of mercury contamination, no staff and students in the school were allowed to leave the building until they were screened for mercury contamination. The 70 chemistry students who were in the chemistry class that day were sent to their locker rooms, where they removed and placed their clothing and shoes in plastic bags, showered to remove any residual mercury, and dressed in Tyvek® suits. These 70 students were then transported to a different school, where they were picked up by their parents and taken home. Their bagged clothes were transported to the Stoughton School District's maintenance building. On Saturday, January 31, students and their parents were allowed to pick up their clothes after the head space in each bag was first screened for mercury. The DHFS Lumex 915+ mercury analyzer was used to screen all shoes and clothing at the community center. Of these bagged clothes and shoes, eight bags had mercury vapor levels at concentrations exceeding the screening value of 10 micrograms per cubic meter (µg/m3). These eight sets of students' clothing and shoes were retained and properly disposed.

Students and staff who did not go in or near the chemistry classroom were required to have their shoes screened for mercury before leaving the building on Friday evening. This screening ensured that mercury would not be tracked or carried around the school or off the school property. DHFS used a Lumex 915+ mercury analyzer (which has a detection limit of 0.002 µg/m3) to screen all shoes and clothing at the high school. Only one person, a science teacher who was in the chemistry classroom when the spill occurred, had elevated mercury vapors coming from her shoes; however, repeated testing verified her clothes were free of mercury vapors. Her shoes were collected and properly disposed. None of the non-chemistry students had shoes or clothing with elevated levels of mercury vapors.

Contractor Investigation and Cleanup

Onyx contractors used mercury field screening meters to identify all indoor areas of the school with elemental mercury vapors. The Onyx VM3000 mercury meter has a mercury vapor detection limit of 0.1 µg/m3. Onyx observed beads of elemental mercury only in the chemistry classroom where the mercury was spilled. Onyx found elevated mercury vapors in this chemistry classroom and in the adjacent hallway and science office. Mercury vapor concentrations at the classroom doorway ranged from 2 to 3 µg/m3 and from 14 to 22 µg/m3 where the spill occurred. Onyx double-screened all areas of the school and confirmed that mercury was elevated only inside and in the immediate vicinity of the chemistry classroom. Onyx then collected and removed all visible mercury from the classroom. Onyx then appropriately removed possible mercury residues from the classroom and the adjacent hallway. The classroom and science wing were then thoroughly ventilated. As a precaution, Onyx also wet-mopped flooring throughout the entire school with a special amalgamating solution to ensure removal of any mercury residues that may have escaped detection. The Onyx staff worked through the night and completed the cleanup by 3 p.m. on Saturday, January 31.

Toxicologic Implications of Elemental Mercury

In humans, elemental mercury affects the nervous system, cardiovascular system, digestive tract, and kidneys, as well as the development of young children, infants, and fetuses. The primary route of entry for elemental, metallic mercury is inhalation of vapors (ingestion and dermal absorption of metallic mercury usually are not significant). Metallic mercury readily vaporizes at room temperature. Laboratory animals that inhaled high levels of elemental mercury vapors (above 1,000 µg/m3) later developed permanent neurologic damage and kidney impairment. Workers exposed for many years to mercury vapors of 14-76 µg/m3 exhibited an increased incidences of mild hand tremors, difficulty with heel-to-toe gait, and impaired performance on neurobehavioral tests.

The Agency for Toxic Substances and Disease Registry's (ATSDR's) chronic inhalation minimal risk level (MRL)(1) for mercury vapor in air is 0.2 µg/m3 (ATSDR 1999). This MRL was derived from a study that found an increased frequency of tremors among works exposed over 15 years to 26 µg/m3 mercury, which was designated as the lowest observed adverse effect level (LOAEL). Because adults in this occupational study were exposed only during working hours, this LOAEL was then adjusted to account for a continuous 24-hour exposure. This LOAEL is comparable to the American Conference of Governmental Industrial Hygienists' (ACGIH's) occupational 8-hour time weighted threshold limit value of 25 µg/m3 (ACGIH 2003). The MRL is then obtained by dividing the adjusted value by an uncertainty factor of 10 to protect sensitive humans and by a factor of 3 because a LOAEL was used rather than a no observed adverse effect level (NOAEL).

In summary, staff and students in the chemistry classroom at the time of the spill breathed mercury vapors for a short time. Taking into consideration the levels they breathed over this time frame, this exposure was not likely to have caused harmful health effects; thus, the exposure posed no apparent public health hazard.(2)

The cleanup goals used by Onyx are found in ATSDR's guidance Suggested Action Levels for Indoor Mercury Vapors in Homes or Businesses with Indoor Gas Regulators (ATSDR, 2000). In this guidance, ATSDR recommends that after a spill, mercury vapor levels in the breathing zone of a home not exceed 1.0 µg/m3 and that measurements at or below this level are acceptable for reoccupancy of any structure. Exceeding the action level of 1.0 µg/m3 prompts the need for cleanup or other remedial actions to reduce exposures. This recommended action level is based on both animal laboratory studies and human epidemiologic studies that examine the long-term health effects from inhaling air containing elevated mercury vapors. ATSDR's guidance- suggested action levels recommend that if mercury vapor levels in a home reach or exceed 10.0 µg/m3, residents be isolated from the exposure and actions taken to remediate the spill. The ATSDR guidance also recommends testing the air inside a plastic bag in which clothing with mercury contamination was placed . When mercury vapors in the bag reach or exceed 10.0 µg/m3, then clothing should be kept from the owner and properly disposed.

Urine Sampling

As a further precaution, DHFS and DCHS offered laboratory screening of urine for mercury. A total of 42 urine samples were dropped at the Stoughton Hospital Laboratory by Tuesday, February 3. These were transported by DCHS staff to the Wisconsin State Laboratory of Hygiene. Laboratory analysis for 41 urine samples did not find mercury above the detection limit of 5 micrograms per liter (µg/L). The single urine sample above the detection limit was at 6 µg/L, which is within the range of an acceptable level (Tsuji, 2003). On Thursday, February 5, DCHS mailed an explanatory letter, with the results, directly to each parent.

Follow-up Air Sampling

In follow-up to the cleanup, air samples were collected from selected locations to provide a final, confirmatory evaluation. A third-party contractor collected air samples in the high school, between 3:45 p.m. and 9:06 p.m. on Saturday, January 31, by drawing air through glass tubes filled with appropriate sampling media. The samples were then hand-delivered by DHFS later that evening to the Wisconsin Occupational Health Laboratory, Wisconsin State Laboratory of Hygiene. On the morning of Sunday, February 1, the air samples underwent preparation and laboratory analysis using the Occupational Safety and Health Administration ID-140 and National Institutes of Occupational Safety and Health 6009 methods.

No detectable amount of mercury vapor was found in any sample. ATSDR recommends 1.0 µg/m3 as the acceptable level for reoccupancy of any structure after an elemental mercury spill. The detection limits of mercury vapors in the seven samples ranged from 0.30-0.39 µg/m3, which are below the ATSDR reoccupancy level. The laboratory results also reported no detectable amount of mercury particulates in six of seven samples. One sample had a detectable amount of mercury particulates at 0.022 µg/m3. This air sample was from a secure storage room/hallway (rooms 146 and 148) in the back of the classroom where the spill occurred (room 336). This level of mercury particulate is below the ATSDR MRL of 0.2 µg/m3. This level apparently is a typical background concentration for indoor air.

DHFS provided these data to the Stoughton School District on the afternoon of Sunday, February 1. On the basis of the results, DHFS concluded the cleanup effectively removed spilled elemental mercury (DHFS 2004). The Stoughton School District then made the final determination that the high school would open for normal use on Monday, February 2.

Risk Communication

Communication by all involved parties was a critical component throughout the incident. DHFS staff used their expertise in risk communication to assist school administrators in developing and communicating effective and appropriate health messages. DHFS Staff assisted the Stoughton School Superintendent and the DCHS Health Officer in drafting written communications and in preparing for a Friday evening press conference to share with parents and the press information about the situation and prudent actions being taken to address the spill. DHFS staff prepared written communication on Sunday for the superintendent that interpreted air sampling results and indicated the school was ready for classes on Monday. DHFS wrote a letter on Tuesday that went to high school parents addressing why mercury spills can be a concern and that no health concern existed with the very low background levels of mercury vapor found in the bags of clothes that were returned to the students. Staff assisted the DCHS Health Officer with a letter to individual parents who submitted their child's urine specimens for evaluating mercury exposure. The DCHS letter interpreted the urine analytical data and briefly summarized results for all students. Staff attended the Stoughton School Board meeting on Monday to present on the public health implications of the incident and to answer questions from board members and the public. Since the school board meeting was broadcast on "local access" television, staff used the opportunity to promote the removal from the homes and businesses and the proper disposal of mercury, such as oral thermometers and thermostats.

Along with the school officials, the responding public health agencies, DHFS, and DCHS collaborated with the Stoughton Hospital Emergency Room and the Wisconsin Poison Center. During the course of the incident, the Poison Center received approximately 40 to 50 calls about the mercury spill from concerned Stoughton parents, with no callers describing health effects or symptoms associated with chronic or acute mercury exposure. School officials and DCHS informed parents that they should seek medical care from their physician or the emergency room if they experienced neurologic or other symptoms related to mercury exposure. The emergency room received a number of inquiries and also reported no cases of known or suspected mercury-related health effects or symptoms.

Follow-up Actions by Stoughton School District

After the incident, the Stoughton school district's environmental management consultant inventoried chemicals in all of the district's classrooms and laboratories to reduce the potential for future spills of this nature. The school district is currently reviewing what chemicals should be kept, and what chemicals are not necessary or inappropriate for a school and should be removed. The school district is also reviewing and updating laboratory and chemical storage safety precautions used in all district schools.


DHFS recognizes that children can be especially sensitive to contaminants. Children are often at greater risk than adults to certain kinds of exposure from hazardous chemicals in the environment. Children engage in activities, such as playing outdoors and hand-to-mouth behaviors, that increase their likelihood of exposure to hazardous substances. Being much smaller than adults and playing on their hands and knees, children breathe air close to the ground that can have more dust, soil particles, and vapors. Children have a lower body weight, but a higher intake rate, which results in a greater dose to hazardous substances per unit body weight. Also, children's bodies are developing and can be permanently damaged if toxic exposures are high enough during critical growth stages. For that reason, DHFS considers children as one of the most sensitive populations evaluated in this health consultation and always takes into account children when evaluating exposures to contaminants.

Stoughton High School students are predominantly between the ages of 13 and 19 years. Children of these ages are less susceptible to the health effects of mercury vapors than infants and very young children. Even if a female student in the chemistry classroom had been pregnant at the time of the incident, exposure to mercury vapors was very short and not likely to pose a health concern to either a student or a fetus.


  1. The mercury spill in a classroom at Stoughton High School on January 30, 2004, was substantial enough to require the intervention and cleanup measures that were taken. Staff and students in the chemistry classroom at the time of the spill breathed mercury vapors for a short time. These levels were not likely cause harmful health effects for this short-term exposure, and posed no apparent health hazard.

  2. The results of confirmatory air sampling did not find detectable levels of mercury vapors at Stoughton High School and supports the conclusion that the cleanup effectively removed spilled mercury.

  3. Follow-up analysis of students' urine found mercury levels within a normal range.


No further cleanup actions are needed by Stoughton High School, DHFS, or other agencies, regarding this incident.


DHFS will continue to collaborate with state and local agencies to promote the reduction and removal of mercury from Wisconsin classrooms.


  1. Agency for Toxic Substances and Disease Registry. Suggested Action Levels for Indoor Mercury Vapors in Homes or Businesses with Indoor Gas Regulators. Atlanta, GA: US Public Health Service, Department of Health and Human Services. 2000.

  2. Agency for Toxic Substances and Disease Registry. Toxicological Profile for Mercury, Update. Report Atlanta, GA: US Public Health Service, Department of Health and Human Services. March 1999.

  3. American Conference of Governmental Industrial Hygienists Worldwide. TLVs and BEIs Based on the Documentation of the Threshold Limit Values for Chemical Substances and Physical Agencies and Biological Exposure Indices. Cincinnati, OH: ACGIH. 2003.

  4. Tsuji JS, Williams PRD, Edwards MR, Allamneni KP, Welsh MA, Paustenbach DJ, and Sheeran PJ. Evaluation of Mercury Urine as an Indicator of Exposure to Low Levels of Mercury Vapor. Env Hlth Persp, 111(4): 623-630, April 2003.

  5. Wisconsin Department of Health and Family Services. Correspondence to M Palomba, Stoughton School District, from H Nehls-Lowe. Madison, WI. February 1, 2004.


Henry Nehls-Lowe, MPH
Bureau of Environmental Health
Division of Public Health
Wisconsin Department of Health & Family Services


This Stoughton High School Mercury Spill Health Consultation was prepared by the Wisconsin Department of Health and Family Services under a cooperative agreement with the Agency for Toxic Substances and Disease Registry (ATSDR). It is in accordance with approved methodology and procedures existing at the time the public health consultation was begun.

Gail D. Godfrey
Technical Project Officer, CAT, SSAB, DHAC

The Division of Health Assessment and Consultation, ATSDR, has reviewed this public health consultation and concurs with the findings.

Roberta Erlwein

1 ATSDR defines an MRL as an "estimate of daily human exposure to a hazardous substance at or below which that substance is unlikely to pose a measurable risk of harmful (adverse), noncancerous effects." There is no evidence that inhalation of elemental mercury vapors causes cancer in humans.
2 ATSDR requires that conclusion categories be used to summarize the findings of health consultations and public health assessments. These categories are: (1) Urgent Public Health Hazard; (2) Public Health Hazard; (3) Indeterminate Public Health Hazard; (4) No Apparent Public Health Hazard; and (5) No Public Health Hazard. A category is selected from incident-specific conditions, such as the presence and duration of human exposure, contaminant concentration, the nature of toxic effects associated with incident-related contaminants, the presence of physical hazards, and community health concerns.

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