A Midland resident and two Michigan-based environmental organizations petitioned the federal Agency for Toxic Substances and Disease Registry (ATSDR) to conduct a public health assessment of dioxin contamination in Midland, Michigan. The present public health consultation addresses soil contamination in the Midland area. The data necessary to determine if dioxin-contaminated soil in the Midland area poses a public health risk are not available; therefore, the site poses an indeterminate public health hazard.
The results of several preliminary soil sampling studies indicate that dioxins have been detected in soil at two locations in the Midland community at concentrations greater than the ATSDR action level. Dioxins detected in most of the soil samples collected from the Midland community fall between the ATSDR screening level and the ATSDR action level. Therefore, the ATSDR guidance indicates that further site-specific evaluation is necessary to determine if dioxin contamination in Midland soil presents a public health hazard.
The Michigan Department of Community Health (MDCH) recommends that soil-sampling plans for the Midland community be developed and implemented, including contingency plans for potential public health actions in residential areas if dioxin concentrations exceed the ATSDR action level. The MDCH further recommends a comprehensive evaluation of site-specific exposure factors for the residents of the Midland area and a comprehensive Exposure Assessment that includes all identifiable sources of dioxin exposure. Priority should be give to people living in areas where dioxin concentrations in soil exceed the ATSDR screening value.
ATSDR and the Michigan Department of Community Health (MDCH) have a cooperative agreement for conducting public health assessments and health consultations for potential health hazards at sites of environmental contamination within the State of Michigan. MDCH staff and a representative from the ATSDR Region 5 office visited the Midland area on July 30, 2001 and toured the Dow Chemical Company (Dow) plant site. The MDCH completed a "Petition Scoping Report" and provided the information obtained from these activities to the ATSDR on August 31, 2001. A copy of the report is provided in Attachment B. The MDCH met with the petitioners on October 3, 2001 to discuss the health assessment process and to provide an opportunity for exchange of additional information.
ATSDR responded in writing (Attachment C) to the petitioners on November 2, 2001, stating that, "After reviewing the public health issues and community concerns about potential dioxin contamination and the Dow Midland facility, ATSDR has found a reasonable basis to prepare public health consultations to address the concerns associated with the Dow facility"(ATSDR 2001). MDCH has agreed to address the petition through a series of health consultations. ATSDR will review MDCH's work and provide technical support as needed.
The present consultation will only address concerns related to dioxin contamination in Midland soil. Additional consultations that address other contaminated media may be developed in the future with the ultimate goal of providing a full multi-media, multi-pathway public health assessment.
The Dow Chemical Company (Dow), founded in 1897, operates a chemical manufacturing plant in the city of Midland, Michigan. The Dow plant encompasses approximately 1,900 acres on the southern perimeter of the city (Figure 1). The Tittabawassee River forms the southern boundary of the plant site and flows east to the Saginaw Bay of Lake Huron.
Chemicals that have been produced at the Dow plant include: styrene, butadiene, picric acid, mustard gas, Saran Wrap, Styrofoam, Agent Orange, napalm, and various pesticides including Chlorpyrifos, Dursban and 2,4,5-trichlorophenol (2,4,5-T). Chlorophenol production began in 1915. Wastes generated from this process were initially disposed of in 600 acres of on-site waste ponds. During high flow periods in the early 1900s, wastes from these ponds would be intentionally released to the Tittabawassee River (Brandt 1997). Dow currently operates its own wastewater treatment plant on-site.
Two rotary kiln incinerators are used for treatment of liquid and solid hazardous and non-hazardous wastes generated from manufacturing activities at the facility (SAF Risk 2001). Ambient air dispersion modeling and monitoring indicate that the northeastern quadrant of the city of Midland has been impacted by emissions from the incinerators. Some site refuse has been and is taken by truck from the Dow plant to local landfills, including the currently operating Salzburg Landfill and the closed Rockwell Road Landfill, via an off-site haul route named Salzburg Road (Figure 1.) The Dow property and releases from the Dow property are subject to the corrective action program administered by the MDEQ under Part 111, Hazardous Waste Management, of the Michigan Natural Resources and Environmental Protection Act (NREPA), 1994 PA 451. The EPA sampled soil in the city of Midland in the 1980s as part of the corrective action plan for the property. Samples collected during these studies were analyzed only for the most toxic dioxin congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD). Elevated concentrations of 2,3,7,8-TCDD were found at several locations and the EPA recommended additional sampling in the future to monitor levels of dioxins in the community (MDEQ 2001).
| Dioxin Toxic Equivalencies Dioxin toxic equivalents (TEQs)are calculated by multiplying the level of a particular dioxin-like compound by its toxicity equivalency factor (see page 7 for additional information). The resulting TEQs are then added together to determine the total dioxin TEQ concentrations in a soil sample. |
Samples collected during the 1996 Midland community study were analyzed for all 17 of the 2,3,7,8-substituted dioxin and furan congeners that are necessary to evaluate the total toxic equivalent (TEQ) concentration of a mixture of dioxins and furans. Samples were collected at 35 locations that included schools, parks, community use and other areas; and at 15 locations on-site at the Dow plant. Sample locations were selected as a follow-up to the studies conducted in Midland during the mid-1980s by Dow and the EPA (MDEQ 1997).
A comparison of results from samples collected in the same or nearby locations generally indicated that levels of dioxins had not changed substantially, although the data suggested a decline in the concentration of 2,3,7,8-TCDD in the Midland community samples (MDEQ 1997). All of the total dioxin TEQ concentrations for the Midland community samples (Table 1) were below the 1,000 parts per trillion (ppt) action level1 used by the ATSDR (De Rosa et al. 1997a). However, many of the samples, especially those located directly adjacent to and downwind to the northeast of the Dow plant exceeded the MDEQ residential soil direct contact criterion of 90 ppt for total dioxins TEQs (MDEQ 2000).
1 A concentration of chemicals at which consideration of action to interdict/prevent exposure occurs (De Rosa et al. 1997a).
| Soil Samples Location | Range of TEQ Detected (ppt) | DEQ Cleanup Criterion(ppt) | ATSDR Screening Level(ppt) | ATSDR Action Level(ppt) |
| Southeast Quadrant | 6-602 | 90 | 50 | 1,000 |
| Southwest Quadrant | 10-95 | 90 | 50 | 1,000 |
| Northeast Quadrant | 77-598 | 90 | 50 | 1,000 |
| Northwest Quadrant | 22-125 | 90 | 50 | 1,000 |
| Dow Plant Site | 16.5-8,840 | 90 | 50 | 1,000 |
The highest detected total dioxin TEQ concentration in the northeast quadrant (598 ppt) was found in a soil sample taken near the intersection of South Saginaw and Bay City roads at the northeast corner of the Dow plant site (Figure 1). Dioxin TEQ concentrations up to 602 ppt (Table 1) were also detected in soil samples taken along the Salzburg Road haul route in the southeastern quadrant (Figure 1). The highest detected total dioxin TEQ concentration on the Dow plant site was 8,840 ppt in a sample taken near the incinerator complex.
Based on the levels of dioxins detected in soil in the 1996 Midland community study, the MDEQ staff recommended that additional phases of study needed to be planned and completed. Pursuant to the corrective action requirements of its federal hazardous waste permit issued in 1988, Dow is responsible for conducting several dioxin/furan monitoring programs (e.g., community and Midland Plant soil, surface water sediment, incinerator stack, ambient air, dust, food chain) under the oversight of the MDEQ and the EPA. Information from these studies was expected to be used to assess the need for corrective action activities (MDEQ 2001).
In 1998, as a follow-up to the 1996 study, Dow collected samples on the Dow Corporate Center property (Dow 1999). The Dow Corporate Center is located in the northeastern quadrant of the city of Midland within the predicted area of impact for the Dow incinerators and adjacent to residential areas (Figure 1). Therefore the MDEQ agreed to accept dioxin concentrations detected in soil samples taken from the Corporate Center as a surrogate for dioxin concentrations that might be found in soil in the surrounding residential areas. Historical records and photographs were consulted to identify areas that had not been disturbed during construction of the buildings now located on the Corporate Center property. Soil samples were collected from four grassy areas with some trees. Detected total dioxin TEQ concentrations ranged from 0.077 to 583 ppt in surface soils (Table 2). The highest detected concentration was in a soil sample taken across the street from residential homes.
Samples were also collected from the Midland Plant site along internal plant haul routes, along the northeast plant perimeter at the intersection of South Saginaw and Bay City roads, and along the Salzburg Road haul route (Dow 1999). Table 2 presents the range of concentrations detected in these areas.
| Soil Samples Location | Range of TEQ Detected | MDEQ Residential Direct Contact Criterion(ppt) | ATSDR Screening Level(ppt) | ATSDR Action Level(ppt) |
| Northeast Perimeter | 6-1,068 | 90 | 50 | 1,000 |
| Salzburg Road | 10-2,663 | 90 | 50 | 1,000 |
| Dow Plant Site | 170-17,030 | 90 | 50 | 1,000 |
| Dow Corporate Center | 77-583 | 90 | 50 | 1,000 |
Total dioxin TEQs were detected on the Dow plant site at concentrations ranging up to 17,000 ppt. Detected concentrations of total dioxin TEQs ranged up to 1,068 ppt on the northeast plant perimeter on the west side of South Saginaw Road (Figure 1). Properties located directly across the road are primarily commercial with residential homes located immediately behind and to the east. Detected concentrations of total dioxin TEQs ranged up to 2,663 ppt in samples taken along the Salzburg Road haul route (Figure 1). Most of the surrounding property is privately owned, but currently unoccupied.
No additional soil samples for dioxin and furan analysis have been collected in the Midland community since 1998. When considered together, the data gathered in the 1996 and 1998 soil studies indicate that detected total dioxin TEQ concentrations are generally very high on the Dow plant site and decrease as a function of distance from the Dow plant perimeter. However, data are available for only one community soil sample within a ¾ mile radius of the location on the northeast plant perimeter where total dioxin TEQ concentrations were detected at 1,068 ppt.
Dow has submitted an application to the MDEQ for renewal of their operating license under Part 111 of NREPA. Under corrective action, Dow is required to address any on-site or off-site releases of hazardous wastes. The current application identifies several issues that need to be addressed as part of corrective action (MDEQ 2002).
Human Exposure Pathways
To determine whether people are or could be exposed to contaminants associated with a property, ATSDR and MDCH evaluate the environmental and human components that lead to human exposure. An exposure pathway contains five major elements: 1) a source of contamination, 2) contaminant transport through an environmental medium, 3) a point of exposure, 4) a route of human exposure, and 5) a receptor population. An exposure pathway is considered a complete pathway if there is evidence that all five of these elements are, have been, or will be present at the property.
| Source | Environmental Transport and Media | Chemicals of Concern | Exposure Point | Exposure Route | Exposed Population | Time Frame | Status |
| The Dow Chemical Company | Incinerator Emissions, Soil Erosion, Loss and Track-Out | Chlorinated Dioxins and Furans | Midland Community Soil | Incidental Ingestion, Dermal Contact, Inhalation | Residents of Midland | Past Current Future | Complete Complete Complete |
Chlorinated dioxins and furans have been detected in soil samples taken from the Dow plant site, from the plant perimeter, from haul routes leading from the plant to near-by landfills, from the Dow Corporate Center, and in soil samples taken from parks, playgrounds and other locations within the Midland community. The likely source of these contaminants is chemical manufacturing activities on the Dow plant site. Emissions from the on-site incinerator complex, wind blown transport of contaminated soil, and loss or track-out of contaminants during waste transport are all possible mechanisms of transport from the Dow plant site to the surrounding community. Residents of Midland and the surrounding communities could be exposed to dioxins and furans in the soil through incidental ingestion, direct dermal contact, and inhalation of soil and dust. Chlorinated dioxins and furans are very persistent in soil, therefore, exposure is likely to have occurred in the past and will continue in the absence of any remedial action.
Demographics
The city of Midland is the county seat of Midland County, Michigan and encompasses an area approximately 28 square miles. The population of Midland was approximately 38,090 in 1990. Twenty-five percent of the population in 1990 was children under the age of 17 years (U.S. Census Bureau 1990).
Residential neighborhoods are located in close proximity to the northeast perimeter of the Dow plant and within a quarter of a mile from a soil sampling location where total dioxin TEQs were detected at concentrations greater than the ATSDR action level of 1,000 ppt. No soil data are available for these residential neighborhoods.
Health Effects
Dioxins are a group of 210 chlorinated chemicals with similar structures and chemical properties. This group of chemicals, which includes chlorinated dioxins, furans, and some polychlorinated biphenyls, is often referred to collectively as simply "dioxins" or "dioxin-like compounds." When found in the environment, dioxins are usually a mixture of several of these chemicals. Dioxins are not intentionally produced and have no known use. Not all dioxins have the same toxicity or ability to cause illness and adverse health effects. However, it is assumed that dioxins and dioxin-like compounds cause adverse health effects through a similar biological mechanism of action. Further, the available science indicates that the health effects resulting from exposure to multiple dioxin-like compounds are additive, meaning that the health effects are greater than would be expected for a single compound.
The most toxic chemical in the group is 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD). Toxic equivalency factors (TEF) have been developed to compare the relative toxicity of other dioxins and dioxin-like compounds to that of 2,3,7,8-TCDD. The levels of other dioxin-like compounds measured in the environment are multiplied by a TEF to produce a 2,3,7,8-TCDD toxic equivalent (TEQ) concentration. The resulting TEQs for all dioxin-like compounds measured in a sample are then added together to determine the total dioxin TEQ concentration for that sample.
People who have been exposed to high levels of dioxins have developed chloracne, a skin disease with severe acne-like pimples. Chloracne can persist for years, sometimes clearing only to recur several years later. Changes in blood and urine that may indicate liver damage have also been seen in some people. Exposure to high concentrations of dioxins may cause long-term alterations in glucose (blood sugar) metabolism and slight changes in hormone levels (ATSDR 1998).
Exposure to low levels of dioxins in study animals has resulted in a wide variety of adverse health effects, such as cancer, liver damage, and disruption of the endocrine system. In many species of animals, dioxins weaken the immune system and cause a decrease in the systems ability to fight infection. In other animal studies, exposure to dioxins has caused reproductive damage and birth defects. Some animal species, including monkeys, exposed to dioxins during pregnancy had miscarriages, and the offspring of animals exposed to dioxins during pregnancy often had birth defects including skeletal deformities, kidney defects, weakened immune responses, and neurodevelopmental effects (ATSDR 1998).
It is not known whether people exposed to low levels of dioxins will experience the same health effects seen in animal studies. However, based on the available information, dioxins are believed to have the potential to cause a wide range of adverse effects in humans, including cancer. The EPA (EPA 2000) has characterized the mixture of dioxins to which people are commonly exposed as "likely human carcinogens." The EPA has also characterized 2,3,7,8-TCDD as a "human carcinogen" (EPA 2000). The U.S. Department of Health and Human Services, National Toxicology Program 9th Report on Carcinogens (NTP 2001) lists 2,3,7,8-TCDD as a substance "known to be a human carcinogen."
ATSDR Interim Guidance
Because of the potential for adverse health effects in human populations exposed to environmental levels of dioxins, the ATSDR has developed interim policy guidelines to assist health assessors in identifying soil concentrations of potential concern (Attachments D and E). The guidelines recommend the tiered approach shown in the table below to evaluate dioxin concentrations in soil.
| Screening Level £ 50 ppt TEQs | Evaluation Level >50 ppt but < 1,000 ppt TEQs | Action Level ³ 1,000 ppt TEQs |
| Health effects are unlikely and further evaluation is not necessary, unless there are community health concerns. | Evaluation of site-specific factors, such as
| Potential public health actions considered, such as
|
The screening level of 50 ppt total dioxin TEQ is the environmental media evaluation guide (EMEG) for 2,3,7,8-TCDD. The EMEG was developed from the ATSDR minimum risk level (MRL) based on neurodevelopmental effects observed in the offspring of female rhesus monkeys exposed during pregnancy (ATSDR 1998). EMEGs are very conservative and protective values. Generally, if soil concentrations do not exceed the EMEG, ATSDR assumes that exposure is not likely to result in adverse health effects. However, if soil concentrations exceed the EMEG, this does not mean that adverse human health effects will always occur. Instead, soil concentrations greater than 50 ppt total dioxin TEQ indicate further site-specific evaluation is necessary (De Rosa et al. 1997a).
The action level of 1,000 ppt TEQ is a concentration of dioxin in soil at which various actions may be considered to prevent or limit exposure. The action level is based on the 1984 analysis by Kimbrough et al. of the carcinogenic potential of 2,3,7,8-TCDD. ATSDR recommends that the action level for soil be used in full consideration of site-specific factors that may affect total exposure to dioxins through all media and exposure pathways (De Rosa et al. 1997b).
ATSDR recommends that the action level be compared to the "maximum concentrations identified at the site" (De Rosa et al. 1997b). Dioxin TEQ concentrations greater than 1,000 ppt have been detected at two locations in the Midland community: 1,068 ppt dioxin TEQ was detected on the northeast perimeter of the Dow plant site and 2,663 ppt dioxin TEQ was detected along the Salzburg Road haul route.
Soil concentrations of dioxin TEQs that fall between the screening level and the action level warrant further site-specific study (De Rosa et al. 1997b). Many factors, such as those shown in the table above, can affect how much dioxin people are exposed to, how much is absorbed into the body, and whether or not adverse health effects will result. Most of the soil samples collected from the Midland community fall within 50 and 1,000 ppt, including all the samples taken from the Dow Corporate Center property that was intended to serve as a surrogate for the surrounding residential community.
Background Exposure to Dioxins
An important consideration when evaluating dioxin levels in soil is the level of exposure from all sources of dioxin, or the "background exposure." People can be exposed to dioxins from many sources other than contaminated soil. The general population is mainly exposed to dioxins through their diet by eating plants and animals that contain dioxins. People that live near or work at hazardous waste sites containing dioxins, waste incinerators, or manufacturing facilities that produces dioxins as a by-product may have additional dioxin exposures beyond their diet. When people are exposed to and absorb dioxins, they are stored in fatty body tissues where they may persist for months or years. The half-life (the time needed for the body to rid itself of half the contaminants absorbed) for dioxins in humans is 5 to 14 years. Because they remain for a long time, dioxins accumulate in the body and can cause health effects long after exposures have ended. The amount of dioxins accumulated over time is referred to as the "body burden." The best available science suggests that body burden levels of dioxins are closely associated with the likelihood of health effects. Therefore, many scientists recommend comparing dioxin body burden levels in at-risk populations to those associated with health effects observed in animal and human studies (EPA 2000, De Rosa et al. 1997b).
Because people may be exposed to dioxins from a variety of sources, and because all these exposures contribute to the body burden of dioxin accumulated over time, ATSDR recommends evaluation of the contribution of soil exposures to total exposures from all sources (De Rosa et al. 1997b). Evaluation of soil exposures would require sufficient knowledge of dioxin concentrations in soil, bioavailability of dioxin in site soils, and human behaviors that can affect exposures. Additionally, evaluation of all other sources of dioxin exposure would be required to evaluate the incremental contribution of soil exposures to the total body burden of dioxins in the at-risk population. Much of this information is not currently available for the Midland area.
ATSDR Child Health Initiative
Children may be at greater risk than adults from certain kinds of exposure to hazardous substances at sites of environmental contamination. They engage in activities such as playing outdoors and hand-to-mouth behaviors that increase their exposure to hazardous substances. They are shorter than adults, which means they breathe dust, soil, and vapors close to the ground. Their lower body weight and higher intake rate results in a greater dose of hazardous substance per unit of body weight. The developing body systems of children can sustain permanent damage if toxic exposures are high enough during critical growth stages. Prenatal exposures and those that occur in the first few years of life are more likely to cause permanent damage.
Fetuses, infants, and children may be especially sensitive to dioxin exposure because of their rapid growth and development. In animal studies, exposure to dioxin has caused reproductive damage and birth defects. Some animal species exposed to dioxins during pregnancy had miscarriages and the offspring of animals exposed to dioxins during pregnancy often had birth defects including skeletal deformities, kidney defects, weakened immune responses, and neurodevelopmental effects (ATSDR 1998).
Health Outcome DataOnly limited data are available to assess whether dioxin contaminated soil in the Midland area has affected the health of area residents. Brief summaries of the available information are presented below.
Cancer Incidence for the Midland Area
At the request of the MDEQ Air Quality Division, the MDCH Division for Vital Records and Health Statistics (DVRHS) compiled cancer incidence data for all cancers combined and cancers of the stomach, lung, bone and soft tissue, prostate, liver, and thyroid for zip codes 48640 and 48642, Midland and Bay Counties, and the entire state of Michigan. DVRHS used the Michigan Resident Cancer Incidence File (MRCIF) to compile the cancer incidence data. The MRCIF was started in 1985 and all hospitals and laboratories are required by law to report cancer cases to DVRHS. In addition, the state of Michigan has reciprocal agreements with 18 states that permit exchange of information on Michigan residents who are diagnosed and/or treated for cancer in other states.
DVRHS used the compiled MRCIF data to conduct cancer incidence rate and standardized incidence ratio (SIR) analyses for zip codes 48640 and 48642 in Midland and Bay Counties. Zip code 48640 encompasses the southwest area of Midland including the Dow plant site, while zip code 48642 includes property to the northeast of the Dow plant site. The SIR analyses were used to compare the number of incident cancer cases in these zip codes with the number that would be expected if the zip code populations had the same cancer incidence rates as the entire state of Michigan. The analyses showed no persistently elevated numbers of cancer cases for cancer of the stomach, lung, bone and soft tissue, prostate, liver, and thyroid in zip codes 48640 and 48642 as compared with Midland County, Bay County, and the entire state of Michigan from 1994 through 1998. The analyses also showed no elevated numbers of all cancers combined in zip code 48642 during the same period. However, the analysis showed higher-than-expected numbers of all cancers combined in zip code 48640 as compared with Midland County, Bay County, and the entire state of Michigan for individual years 1994 through 1998 and all years combined (MDCH 2001).
The analyses of all cancer cases combined are difficult to interpret, and elevation of all cancer cases combined does not indicate an elevation of a specific type of cancer. Specific cancer types can be addressed individually if incidence rates are different than expected or if community concern for specific cancer types increases.
Dow Cohort Mortality Study of Midland Workers
In 1997, Dow produced an internal epidemiological study of dioxin-exposed workers in the Midland plant (Dow 1997). Additional analyses of the data were conveyed to the MDEQ in April of 1998 (Dow 1998). The study population consisted of 2,187 male employees at the Dow Midland plant site who worked at any time between 1940-1983 in production areas of the plant where there was a potential for exposure to dioxin. Workers were classified by their cumulative potential for exposure based on job history (length of time in all jobs where exposure may have occurred, weighted by estimated intensity of exposure at each job), and causes of death were compared to those of the U.S. population and an internal "unexposed" group of Dow Midland employees. The results indicated that the incident rates for all causes of death were lower in the Dow exposed cohort than the general U.S. population (Dow 1997, 1998). This is not a remarkable finding since the Dow cohort was generally healthy male workers and was compared to the general U.S. population including all genders and socio-economic groups.
When exposed workers were compared to the Dow unexposed cohort, slight excesses were seen for mortality due to cancer of the prostate, stomach cancer, lymphatic, and hematopoietic cancer, and an ill-defined group of all other cancers. No significant excesses were observed for non-cancer causes of death. Analysis of cancer incidence using a 15-year lagged latency model demonstrated a significant linear trend with increasing exposure for cancers of the stomach and prostate at the highest cumulative exposure category. Analyses of the other causes of mortality by cumulative exposure showed no clear dose-response relationships. Unlike studies of other occupationally exposed groups, this study did not find an excess in respiratory cancer or in all cancers combined as compared to the U.S. population. The slight excesses in prostate, stomach and lymphatic/hemopoietic cancers reported is consistent with studies of other occupationally exposed groups, although (with the possible exception of stomach cancer) they did not appear to be dose related. There were two cases of soft-tissue sarcoma in the cohort, an excess also reported in some other studies (Dow 1997, 1998).
There are significant limitations in generalizing from the Dow study to the general population. First, the study is limited by the size of the exposed cohort, which is not sufficient to detect moderate increases in incidence rates for many of the cancer types of concern. Second, the individuals included in the cohort were all male. The effect of dioxin exposure on exclusively female cancer types cannot, therefore, be considered. Lastly, and of most importance, the assignment of workers to the "exposed" and "unexposed" groups and to the assumed level of exposure was based on job history, that is the work time spent in areas of the Dow plant where there was potential for dioxin exposure. Several confounding factors could effect the level of actual dioxin exposure for these individuals including non-occupational exposures (diet, exposure to contaminated community air and soil); interpersonal differences in work practices; and human variability in the uptake and elimination of dioxin. Biological tests for exposure such as dioxin levels in blood lipids would have provided a less subjective measure of exposure.
Birth Defects
DVRHS used the Michigan Birth Defects Registry (MBDR) to compile birth defects data. The MBDR is a statewide registry that has been in operation since 1992. Reporting by all hospitals and clinics is mandatory and requires the reporting of a predefined list of conditions that are diagnosed and/or treated within these facilities. Since not all birth defects are diagnosed at birth, cases diagnosed or treated before the age of two are reportable.
The analysis of the birth defects data for Midland county for the years 1992 through 1996 did not show any consistent pattern of excesses in any particular category or for birth defects overall. No excess was seen for those types of birth defects, such as anencephaly, spina bifada, and cleft lip, which had been reported as related to dioxin exposure. However, the relatively small number of births in Midland County (about 1,000 per year) and the rarity of birth defects (roughly 5% of all live births), along with the newness of the MBDR, make it difficult to calculate reliable statistics or detect moderate changes in risk for Midland County. In addition, the registry does not track incidence rates for conditions diagnosed after 2 years of age, nor does it track functional changes in behavior and cognition which have been identified as the most sensitive effects of prenatal exposure to dioxins.
The data necessary to determine if dioxin-contaminated soil in the Midland area poses a public health risk are not available; therefore the site poses an indeterminate public health hazard. ATSDR classifies sites of environmental contamination into the indeterminate category when the data to make a final decision are lacking.
Several soil-sampling studies have been conducted in the Midland area by the EPA, the MDEQ, and by the Dow Chemical Company. The results of these studies indicate that dioxin TEQs have been detected in soil on the Dow plant site at concentrations up to 17,000 ppt. Total dioxin TEQs above the ATSDR action level have been detected in off-site soil at the Dow plant perimeter at concentrations up to 1,068 ppt and along the Salzburg Road haul route at concentrations up to 2,663 ppt. Total dioxin TEQs detected in most of the soil samples collected from the Midland community fall between the ATSDR screening level and the action level, including all the samples taken from the Dow Corporate Center property intended to serve as a surrogate for the surrounding residential community. Therefore, the ATSDR guidance recommends a site-specific evaluation to determine if dioxin contamination in Midland soil presents a public health hazard (De Rosa et al. 1997a, 1997b).
Soil data are not available for all of the Midland community and it is likely that some areas are more heavily contaminated that others, particularly the northeast quadrant most impacted by historical emissions from the Dow incinerators. Of particular concern are those areas of the community closest to the Dow plant site to the east and north. No data are available concerning dioxin concentrations in these areas of Midland.
Additional site-specific information including an assessment of background exposures is necessary to evaluate the incremental contribution of soil exposure to the total dioxin body burden for the residents of Midland. This information is not currently available.
If any citizen has additional information or health concerns regarding the Midland, Michigan petitioned health assessment/consultation, please contact the Michigan Department of Community Health, Division of Environmental and Occupational Epidemiology, at 1-800-648-6942.
