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Outbreak of Unexplained Illness in a Middle School -- Washington, April 1994

Mass sociogenic illness (MSI) is the occurrence of a group of nonspecific physical symptoms for which no organic cause can be determined and that is transmitted among members of a group by "line of sight." On April 22, 1994, the Snohomish (Washington) Health District (SHD) was notified of an outbreak of unexplained illness characterized by abrupt onset of nausea and headache among students at a middle school. This report summarizes the investigation of this outbreak by SHD, which determined that MSI was the most likely cause of the outbreak.

The outbreak occurred on April 19 and April 20 and prompted school officials to close the building and relocate children to another facility before notifying SHD. For approximately 10 years, staff and parents of students had complained about the indoor air quality and attributed chronic allergies and nonspecific illnesses to building occupancy. Initial reports about the outbreak suggested that most of the affected students were members of a class where onset of illness on both days occurred immediately after the students returned to class from lunch; the students were preparing for examinations to be held the next week. According to school officials, the outbreak also coincided with a week-long period of "stink bomb" (hydrogen sulfide capsule) discharges set off in the school by students.

A self-administered questionnaire assessing symptoms and perception of indoor-air quality on the dates of the outbreak was administered to 1) all students in the implicated classroom; 2) all students noted by the school nurse to have made a visit for headache and nausea during the week of the outbreak; 3) a systematic sample, stratified by homeroom, of the remainder of students; and 4) all school staff. Additional information gathered through the questionnaire included sex, grade, age, location in the school when abnormal air quality or symptoms were noticed, and perception of air quality and building-attributed symptoms since January 1, 1994.

Survey response rates were higher among students than among staff (187 {89%} of 211 students versus 51 {70%} of 73 staff; p less than 0.01). A case of unexplained illness was defined as a report of both headache and nausea from a student or staff member on either April 19 or April 20. Illness in 71 (30%) persons met the case definition for April 19 and 43 (18%), for April 20. Of the 43 ill persons on April 20, a total of 34 (79%) also had been ill on April 19. Fifty (63%) of the total ill persons were female, and 24 (71%) of the 34 persons with recurrent cases on April 20 were female. In addition to headache and nausea, other symptoms reported by persons affected on both days included dizziness (61% and 74%, respectively), fatigue (59% and 65%, respectively), weakness (55% and 60%, respectively), and itchy/watery eyes (55% and 53%, respectively). Four persons sought care from a physician. The physician of the one person who permitted release of medical information to SHD reported that the office visit was for routine asthma care, and no specific diagnosis was made.

On April 19 and April 20, a total of 33 (51%) of 65 case-patients and 20 (46%) of 45 case-patients, respectively, reported onset during 11 a.m.-1 p.m., the time during which students in the implicated classroom were noted to become abruptly ill. The median duration of illness was reported as 4-6 hours on both days (range: less than 15 minutes-16 days). A total of 49 (69%) of 71 affected persons on April 19 and 29 (69%) on April 20 observed onset of illness in another person before becoming ill themselves. Risk factors for illness included being a student, occupying the implicated room immediately after lunch, perceiving the building as "too hot" or "smelling like rotten eggs," or having a previous history of headaches or nausea while at school (Table_1). Risk for acute illness was not associated with being aware of a history of previous building-associated illness in other persons or a history of having eaten lunch in the cafeteria on April 19 or April 20.

The environmental assessment of the school included an inspection of the heating, ventilation, and air-conditioning (HVAC) system; extensive sampling of indoor and outdoor air for carbon dioxide, volatile organic compounds, particulates, bacteria, and fungi; soil sampling for organic compounds; sampling of the water supply for bacteria; and sampling of dust from the HVAC ducts for heavy metals. Inspection of the HVAC system did not detect damage, condensation, or malfunction. Carbon dioxide, volatile organic chemicals, particulates, and biologic agents were not detected at levels known to be associated with adverse effects.

After the investigation, SHD, the Washington Department of Health, the school district, and MedTox Northwest presented the findings in a public forum to staff, parents, and students, with an explanation that MSI was the most likely explanation for the event. Nonetheless, the school remained closed through the end of the school year while the HVAC system was upgraded to increase the percentage of fresh air circulated and the carpeting was replaced with linoleum. Other measures to reduce building occupants' potential for exposure to indoor-air contaminants also were implemented (e.g., local control of ventilation in classrooms, elimination of volatile organic cleaning compounds, and storage of art supplies away from student work areas). No further events of unexplained illness have occurred since the school was reopened in September 1994.

Reported by: C Spitters, MD, Snohomish Health District, Everett; J Darcy, PhD, MedTox Northwest, Kent; T Hardin, Office of Toxic Substances, R Ellis, JD, Office of Community Environmental Health, Washington Dept of Health. Health Studies Br, Div of Environmental Hazards and Health Effects, National Center for Environmental Health, CDC.

Editorial Note

Editorial Note: MSI is frequently reported as the cause of acute outbreaks of unexplained illness in school settings (1-4). Characteristics of such outbreaks include 1) lack of illness in others sharing the same environment; 2) symptoms including headache, nausea, weakness, dizziness, hyperventilation, and fainting; 3) a preponderance of cases among females; 4) "line-of-sight" transmission, and 5) relapse of illness. The outbreak in Washington, although generally consistent with MSI, was uncharacteristic of MSI in that it extended throughout a multibuilding facility and the investigation did not detect evidence of hyperventilation or fainting.

The findings in this report are subject to at least three limitations. First, inspection of the facility and sampling was performed 24 hours after onset of the outbreak and, therefore, may have resulted in failure to identify a causative agent that was ventilated out of the facility before sampling began. Second, the retrospective, self-administered survey was conducted after extensive media coverage of the event and probably resulted in an overestimate of the actual number of cases. Third, this investigation did not include examination by a physician to ascertain the presence of physical findings among those reporting illness.

Although school closure, extensive environmental sampling, and epidemiologic investigation may not be routinely indicated after events such as this, responses to such outbreaks should be individualized and should take into account the perceptions of building occupants and perceived health and safety concerns. Whenever possible, the diagnosis of MSI should be communicated to building occupants promptly and openly to prevent recurrence and facilitate reoccupancy.


  1. CDC. Mass sociogenic illness in a day-care center -- Florida. MMWR 1990;39:301-4.

  2. Small GW, Borus JF. Outbreak of illness in a school chorus. N Engl J Med 1983;308:632-5.

  3. Goh KT. Epidemiologic enquiries into a school outbreak of an unusual illness. Int J Epidemiol 1987;16:265-70.

  4. Philen RM, Kilbourne EM, McKinley TW, Parrish RG. Mass sociogenic illness by proxy: parentally reported epidemic in an elementary school. Lancet 1989;1:1372-6.

Note: To print large tables and graphs users may have to change their printer settings to landscape and use a small font size.

TABLE 1. Risk factors for unexplained illness in a middle school, by day
of onset -- Washington, April 19 and 20, 1994
                               April 19                April 20
                          ------------------      -----------------
Risk factor                OR *    (95% CI +)       OR     (95% CI)
Student                   5.3     (2.1 15.7)       4.5   (1.5 17.8)
Female                    1.5     (0.8  2.8)       1.6   (0.8  3.4)
Building "too hot"        3.1     (1.6  5.8)       2.6   (1.2  5.5)
Smelled odor of
 "rotten eggs"            2.3     (1.2  4.3)       2.3   (1.0  5.1)
Aware of others having
 previous building-
 attributed illness       1.3     (0.7  2.4)       0.9   (0.4  1.8)
Ate lunch in cafeteria    1.4     (0.7  2.8)       0.9   (0.4  2.0)
Previous history of
 headaches while at
 school                   3.7     (1.7  8.4)       7.2   (2.0 37.8)
Previous history of
 nausea while at school   5.0     (2.5  9.9)       4.2   (2.0  9.2)
Exposed to tobacco smoke
 at home                  1.4     (0.7  2.6)       0.9   (0.4  1.9)
Being in implicated room
 following lunch          3.2     (1.1  9.4)       4.0   (1.3 12.3)
* Odds ratio.
+ Confidence interval.

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