exposed during a chemical release is not cap­ tured directly in HSEES. However, using vic­ tim data and additional information recorded as optional text, one can estimate the number of exposed persons. The contributing causes for the release of mercury are categorized as equipment failure, human error, intentional or illegal release, and unknown cause. The human error category includes breaking of or dropping thermom­ eters or other mercury­ containing devices or equipment. Intentional or illegal releases include events in which children reportedly played with mercury. We included the HSEES events from 2002 through 2006 in this compilation if children were potentially exposed to elemental mer­ cury (ATSDR HSEES, unpublished data). In HSEES, children are defined as persons ≤ 19 years of age. We omitted events in which releases were only threatened. We selected events if they took place at a private residence, at an elementary or secondary school, or at another location for which children were docu­ mented as possibly exposed, injured, or having symptoms associated with mercury exposure. The HSEES database contained 843 mercury events from 41,709 total events in which hazardous substances were reported to be released from January 2002 through December 2006 (Table 2). Mercury was the only toxicant released in 824 of these events; the remaining 19 mercury events included the release of at least one other hazardous sub­ stance. Approximately half of the total mer­ cury events identified (n = 409) were classified as potentially exposing children. All 409 events potentially affecting children were mercury­ only events. These events were reported from 17 states; only 12 states participated during the entire time period from 2002 through 2006. The remaining states participated for either 2 or 4 years. The 409 events potentially affecting chi l­ dren were most frequently classified as non­ volatilization or spill­ only events (88%; 360 of 409). Volatilization of mercury was noted in 6 of the 409 events (2%) as air only and in 40 events (10%) as combined spill and air releases. A fire was noted in one of the 409 events (< 1%). Although liquid mer­ cury has a relatively low vapor pressure and volatilizes slowly at room temperature, some volatilization was likely in some or all of the events described as spill only. Mercury events occurred most frequently in private house­ holds (75%; 307 of 409). The most frequent contributing cause of the event was human error (87%; 357 of 409). Evacuations were ordered in 68 of the 409 events (17%). The median number evacuated per event was 20 people, with a range from 1 to 1,505 people. The total number of people exposed during these 409 events was not cap­ tured in HSEES, although 21 people were injured or had symptoms related to an event. Among the 21 individuals affected, 10 were children. HSEES identified 7 of these children as having specific health effects: gastrointestinal problems (n = 3), eye irrita­ tion (n = 2), respiratory irritation (n = 1), and trauma (n = 1), although these effects may not be mercury related. In addition, 5 children had elevated levels of mercury in blood/urine (data not shown). Mercury biomarkers are not routinely reported to HSEES, and the prevalence of elevated blood/urine levels is probably underreported. Limitations do exist in using HSEES data to identify elemental mercury exposures among children. The HSEES data source is intended to build capacity in state health departments for surveillance of acute releases of hazardous substances and to initiate or improve appro­ priate prevention activities. HSEES was not designed to enumerate and characterize mercury exposure events affecting children. Information on age is captured in HSEES only if the person reports a symptom or requires medical follow­up; for this reason, HSEES data are likely to underestimate the number of children exposed. The magnitude of exposure is difficult to determine because the amount of mercury released or spilled is often reported as a range rather than a specific quantity. Therefore, a reliable calculation of the average amount of mercury released is not possible. Also, the reporting of mercury­related events to HSEES is uneven across the participating states. States with mercury exposure prevention initiatives may report more mercury­related events than do states without mercury initia­ tives (Michigan Department of Environmental Quality 2007; Minnesota Pollution Control Agency 2006). Lastly, HSEES data cover acute releases only; incidents in which mercury expo­ sure continued for an extended period of time are not included. U.S. Coast Guard National Response Center database. Under federal law, the release or spill of 1 lb (33 mL, ~ 2 tablespoons) or more of mercury into the environment is to be reported to the federal government (U.S. EPA 1989). The primary contact for reporting these events is the National Response Center, operated by the U.S. Coast Guard for the National Response Team established under the National Contingency Plan for Oil and Hazardous Substances Releases (U.S. Coast Guard National Response Center 2002; U.S. EPA 1994). The National Response Center receives between 25,000 and 30,000 reports of pol­ lution incidents and response drills each year. To identify events for this report, we down­ loaded data for the years 2002–2007 from the National Response Center Web site and Environmental Health Perspectives • volume 117 | number 6 | June 2009 Children’s exposure to elemental mercury queried them using SAS statistical software (version 9.1; SAS Institute Inc., Cary, NC). We identified mercury­related events by a) a Chemical Abstracts Service registry number recorded as “007439­97­6” (denoting mer ­ cury was released) or b) the word “mercury” reported in the name of the material released, the description of the incident, the description of remedial actions, or the additional infor­ mation provided. A total of 825 events met this definition between 2002 and 2007. Some exposure events may have taken place before the year in which they were reported. To assess the number of events in which children were potentially exposed, we con­ ducted two additional searches on the 825 mercury events. First, we always selected school and child care settings as locations where children were potentially exposed by searching for the terms “school” or “daycare” in the fields for incident description, location of the incident, and additional information. Second, we queried the description of the incident and the additional information fields Table 2. Characteristics of HSEES-reported mercury events, 2002–2006.a No. (%) 843 (100) 409 (49) 5 (1) 8 (2) 0 (0) 56 (14) 73 (18) 129 (32) 5 (1) 4 (1) 6 (2) 19 (5) 7 (2) 32 (8)) 39 (10) 0 (0) 7 (2) 16 (4) 3 (1) 360 (88) 6 (2) 40 (10) 1 (< 1) 2 (< 1) 307 (75) 98 (24) 4 (1) 27 (7) 357 (87) 18 (4) 7 (2) 873 Event Mercury events Total events affecting children State reporting event Reporting all 5 years Colorado Iowa Louisiana Minnesota New Jersey New York North Carolina Oregon Texas Utah Washington Wisconsin Reporting 4 years Missouri Reporting 2 years Alabama Florida Michigan Mississippi Type of release Spill only Volatilization Spill and volatilization Fire Not reported Location of event Private household School Otherb Contributing cause of event Equipment failure Human error Intentional or illegal release Unknown aPercentages may total > 100% because of rounding. bIncludes private property other than a home (3) and a restaurant (1).