PUBLIC HEALTH ASSESSMENT
HILL AIR FORCE BASE
HILL AIR FORCE BASE, DAVIS AND WEBER COUNTIES, UTAH
|Location||2000 Population||% Increase from 1990||Caucasian (%)||African-American (%)||Hispanic (%)||Male (%)||Under age 10 (%)||Over age 65 (%)||Population Density (# per square mile)|
What is meant by exposure?
ATSDR's public health assessments are driven by exposure or contact. Chemicals released intothe environment have the potential to cause harmful health effects. Nevertheless, a release doesnot always result in exposure. People can only be exposed to a chemical if they come in contactwith that chemical. If no one comes into contact with a chemical, then no exposure occurs, thusno health effects could occur.
The route of a chemical's movement is the pathway. ATSDR identifies and evaluates exposure pathways by considering how people might come into contact with a chemical. An exposure pathway could involve air, surface water, groundwater, soil, dust, or even plants and animals. Exposure can occur by breathing, eating, drinking, or by skin contact with a substance containing the chemical.
How does ATSDR determine which exposuresituations to evaluate?
ATSDR scientists evaluate base-specific conditionsto determine whether people are being exposed tobase-related contaminants. When evaluating exposure pathways, ATSDR identifies whetherexposure to contaminated media (soil, water, air, waste, or biota) is possible through ingestion,dermal (skin) contact, or inhalation.
If exposure is possible, ATSDR scientists then consider whether contamination is present atlevels that might affect public health. ATSDR selects chemicals for further evaluation bycomparing them against health-based comparison values. Comparison values are developed byATSDR from available scientific literature concerning exposure and health effects. Comparisonvalues are derived for each of the media and reflect an estimated chemical concentration that isnot expected to cause harmful health effects for a given chemical, assuming a standard dailycontact rate (e.g., amount of water or soil consumed or amount of air breathed) and standardbody weight.
Comparison values are not thresholds for harmful health effects. ATSDR comparison valuesrepresent chemical concentrations many times lower than levels at which no effects wereobserved in experimental animals or human epidemiologic studies. If chemical concentrationsare above comparison values, ATSDR further analyzes exposure variables (e.g., duration andfrequency) for health effects, including the toxicology of the chemical, other epidemiologystudies, and the weight of evidence.
Some comparison values used by ATSDR scientists include ATSDR's environmental mediaevaluation guides (EMEG), reference dose media evaluation guides (RMEG), and cancer riskevaluation guides (CREG). EMEGs, RMEGs, and CREGs are non-enforceable, health-basedcomparison values developed by ATSDR for screening environmental contamination for furtherevaluation. Risk-based concentrations (RBCs) and soil screening levels (SSLs) are health-basedcomparison values developed by EPA Region III to screen areas not yet on the NationalPriorities List (NPL), respond rapidly to citizens inquiries, and spot-check formal baseline riskassessments.
More information about the ATSDR evaluation process can be found in ATSDR's Public HealthAssessment Guidance Manual at http://www.atsdr.cdc.gov/HAC/HAGM/ or by contactingATSDR at 1-888-42-ATSDR. For reference, Appendix A defines some of the technical termsused in this public health assessment and a List of Acronyms is available after the Table ofContents.
If someone is exposed, will they get sick?
Exposure does not always result in harmful health effects. The type and severity of health effectsthat occur in an individual as the result of contact with a chemical depend on the exposureconcentration (how much), the frequency and duration of exposure (how long), the route orpathway of exposure (breathing, eating, drinking, or skin contact), and the multiplicity ofexposure (combination of chemicals). Once exposure occurs, characteristics such as age, sex,nutritional status, genetics, lifestyle, and health status of the exposed individual influence howthat individual absorbs, distributes, metabolizes, and excretes the chemical. Taken together,these factors and characteristics determine the health effects that can occur as a result ofexposure to a chemical in the environment.
Considerable uncertainty exists regarding the true level of exposure to environmentalcontamination. To account for that uncertainty and to protect public health, ATSDR scientiststypically use high-end, worst-case exposure level estimates to determine whether harmful healtheffects are possible. These estimated exposure levels are usually much higher than the levels towhich people are really exposed. If the exposure levels indicate harmful health effects arepossible, a more detailed review of exposure, combined with scientific information from themedical, toxicologic, and epidemiologic literature about the health effects from exposure toharmful substances, is performed.
Exposure Assumptions and Methodology
To evaluate whether indoor air, surface water, or crop contaminants measured in Hill AFB andthe surrounding communities are associated with any unhealthy effects, ATSDR derivedexposure doses based on a hypothetical exposure of a resident to each contaminant. Whenderiving the doses, ATSDR used conservative assumptions about duration of exposure tocontaminants and quantity of contaminated indoor air, crops, or surface water inhaled or ingestedeach day. Assumptions used to estimate exposure doses for indoor air inhalation, surface waterdermal contact, surface water ingestion, and ingestion of contaminated crops are reviewedbelow.
Indoor Air Exposure Dose Estimation: Because some uncertainty exists about how long thecontaminants have been in the homes, ATSDR conservatively assumed that an adult wasexposed to indoor air contaminants for 30 years. In all likelihood, families stationed at HillAFB and those living in the surrounding communities have lived in these homes for lessthan 30 years. We also assumed that the exposure is constant (24 hours a day for 365 daysover the 30 year period). We next assumed that a person exposed to the contaminated airwas an adult, inhaling 20 cubic meters of air per day and weighing 70 kilograms (157pounds). These assumptions create a very conservative estimate of exposure, most likelyhigher than a exposure dose for an average individual in the community. Furthermore,ATSDR assumed that residents living in the housing units were exposed to the highest levelof contamination in the air; therefore, ATSDR used the highest (or maximum) measuredconcentrations of contaminants in indoor air. This is another conservation assumptionbecause we would not expect people to be exposed to the highest concentration all the time.Together, these conservative estimates allow ATSDR to safely evaluate the likelihood, ifany, that indoor air contaminants in the home could cause harm to its residents.
Surface Water Dermal Contact Exposure Dose Estimation: In order to calculate dermalexposure doses ATSDR assumed that both adults and children would be touching the water.The average body weight for an adult and a child are 70 kilograms and 16 kilograms,respectively. We next assumed that the only body parts touching the water were hands andlegs, providing a surface area of 3,670 cm2 for children and 6,320 cm2 for adults. Wefurther assumed that the exposure time for dermal contact was 1 hour per day, 365 days peryear. The maximum concentration for each detected chemical was used in the calculations.Each of these assumptions is very conservative, resulting in an exposure dose greater thanwhat would truly be expected in this situation.
Surface Water Ingestion Exposure Dose Estimation To calculate ingestion exposure doses,ATSDR assumed that both adults and children would be ingesting the water. The averagebody weight for an adult and a child are 70 kilograms and 16 kilograms, respectively. Wefurther assumed that the exposure time was 1 hour per day for 365 days per year, and thatthe water ingestion rate was 0.05 liters per hour for both children and adults. The maximumconcentration for each detected chemical was used in the calculations. Each of theseassumptions is very conservative, resulting in an exposure dose greater than what wouldtruly be expected in this situation.
Crop Ingestion Exposure Dose Estimation Exposure doses were calculated for ingestion ofTCE through fruit and vegetables. First, we assumed that the average body weight for anadult is 70 kilograms. We further assumed an adult consumes 35 grams of each type of fruitper day and that all fruit eaten in these communities are homegrown. The maximumdetected concentration of TCE for eight different types of fruit was used in our exposuredose calculations. Each of these assumptions is very conservative, resulting in an exposuredose greater than what would truly be expected in this situation. We calculated exposuredoses for consumption of peaches, plums, walnuts, apples, apricots, nectarines, plums, andgrapes. We then added the exposure doses for all the fruits.
- How a chemical enters a person's blood after the chemical has been swallowed, has come into contact with the skin, or has been breathed in.
- Acute Exposure:
- Contact with a chemical that happens once or only for a limited period of time. ATSDR defines acute exposures as those that might last up to 14 days.
- When the effect of the mixture can be estimated from the sum of exposure levels or the effects of the individual components,the mixture is said to be additive (i.e. 2 + 2 = 4).
- Adverse Health Effect:
- A change in body function or the structures of cells that can lead to disease or health problems.
- Water held under pressure in porous rock.
- The Agency for Toxic Substances and Disease Registry. ATSDR is a federal health agency in Atlanta, Georgia, that deals with hazardous substance and waste issues. ATSDR gives people information about harmful chemicals in their environment and tells people how to protect themselves from coming into contact with chemicals.
- Background Level:
- An average or expected amount of a chemical in a specific environment. Or, amounts of chemicals that occur naturally in a specific-environment.
- A group of diseases that occur when cells in the body become abnormal and grow, or multiply, out of control
- See Comprehensive Environmental Response, Compensation, and Liability Act.
- Chronic Exposure:
- A contact with a substance or chemical that happens over a long period of time. ATSDR considers exposures of more than one year to be chronic.
- Completed Exposure Pathway:
- See Exposure Pathway.
- Comparison Value (CV):
- Concentrations or the amounts of substances in air, water, food, and soil that are unlikely, upon exposure, to cause adverse health effects. Comparison values are used by health assessors to select which substances and environmental media (air, water, food and soil) need additional evaluation while health concerns or effects are investigated.
- Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA):
- CERCLA was put into place in 1980. It is also known as Superfund. This act concerns releases of hazardous substances into the environment and the cleanup of these substances and hazardous waste areas. ATSDR was created by this act and is responsible for looking into the health issues related to hazardous waste areas.
- A belief or worry that chemicals in the environment might cause harm to people.
- How much or the amount of a substance present in a certain amount of soil, water, air, or food.
- See Environmental Contaminant.
- Dermal Contact:
- A chemical getting onto your skin. (see Route of Exposure).
- The amount of a substance to which a person may be exposed, usually on a daily basis. Dose is often explained as "amount of substance(s) per body weight per day".
- The relationship between the amount of exposure (dose) and the change in body function or health that result.
- The amount of time (days, months, years) that a person is exposed to a chemical.
- Environmental Contaminant:
- A substance (chemical) that gets into a system (person, animal, or the environment) in amounts higher than that found in Background Level, or what would be expected.
- Environmental Media:
- Usually refers to the air, water, and soil in which chemicals of interest are found. Sometimes refers to the plants and animals that are eaten by humans. Environmental Media is the second part of an Exposure Pathway.
- U.S. Environmental Protection Agency (EPA):
- The federal agency that develops and enforces environmental laws to protect the environment and the public's health.
- Coming into contact with a chemical substance. (For the three ways people can come in contact with substances, see Route of Exposure.)
- Exposure Assessment:
- The process of finding the ways people come in contact with chemicals, how often and how long they come in contact with chemicals, and the amounts of chemicals with which they come in contact.
- Exposure Pathway:
- A description of the way that a chemical moves from its source (where it began) to where and how people can come into contact with (or get exposed to) the chemical.
ATSDR defines an exposure pathway as having five parts:
- Source of contamination,
- Environmental media and transport mechanism,
- Point of exposure,
- Route of exposure, and
- Receptor population.
When all five parts of an exposure pathway are present, it is called a Completed Exposure Pathway. Each of these five terms is defined in this Glossary.
- The rate of transfer of fluid or gas across a surface.
- How often a person is exposed to a chemical over time; for example, every day, once a week, twice a month.
- Hazardous Waste:
- Substances that have been released or thrown away into the environment and, under certain conditions, could be harmful to people who come into contact with them.
- Health Effect:
- ATSDR deals only with Adverse Health Effects (see definition in this Glossary).
- Indeterminate Public Health Hazard:
- This category is used in Public Health Assessment documents for areas where important information is lacking (missing or has not yet been gathered) about base-related chemical exposures.
- Swallowing something, as in eating or drinking. It is a way a chemical can enter your body (See Route of Exposure).
- Breathing. It is a way a chemical can enter your body (See Route of Exposure).
- Lowest Observed Adverse Effect Level (LOAEL):
- The LOAEL is the lowest dose at which an adverse health effect is seen in a particular study. The LOAEL is often used to derive RfDs.
- The National Priorities List. (Part of Superfund.) A list kept by the U.S. Environmental Protection Agency (EPA) of the most serious uncontrolled or abandoned hazardous waste areas in the country. An NPL area needs to be cleaned up or is being looked at to see if people can be exposed to chemicals from the base.
- No Observed Adverse Effect Level (NOAEL):
- The NOAEL is the highest dose from a study that did not find any adverse health effects. The NOAEL is often used to derive MRL and RfDs.
- No Apparent Public Health Hazard:
- The category is used in ATSDR's Public Health Assessment documents where exposure to base-related chemicals may have occurred in the past or is still occurring but the exposures are not at levels expected to cause adverse health effects.
- No Public Health Hazard:
- The category is used in ATSDR's Public Health Assessment documents for areas where there is evidence of an absence of exposure to base-related chemicals.
- The top of the zone of saturation that bottoms on an impermeable horizon above the level of the general water table in the area.
- Public Health Assessment. A report or document that looks at chemicals at a hazardous waste area and tells if people could be harmed from coming into contact with those chemicals. The PHA also tells if possible further public health actions are needed.
- A line or column of air or water containing chemicals moving from the source to areas further away. A plume can be a column or clouds of smoke from a chimney or contaminated underground water sources or contaminated surface water (such as lakes, ponds and streams).
- Point of Exposure:
- The place where someone can come into contact with a contaminated environmental medium (air, water, food or soil). For example:
the area of a playground that has contaminated dirt, a contaminated spring used for drinking water, the location where fruits or vegetables are grown in contaminated soil, or the backyard area where someone might breathe contaminated air.
- A group of people living in a certain area; or the number of people in a certain area.
- Public Health Assessment(s):
- See PHA.
- Public Health Hazard:
- The category is used in PHAs for areas that have certain physical features or evidence of chronic, base-related chemical exposure that could result in adverse health effects.
- Public Health Hazard Criteria:
- PHA categories given to an area which tell whether people could be harmed by conditions present at the area. Each are defined in the Glossary. The categories are:
- Urgent Public Health Hazard
- Public Health Hazard
- Indeterminate Public Health Hazard
- No Apparent Public Health Hazard
- No Public Health Hazard
- Receptor Population:
- People who live or work in the path of one or more chemicals, and who could come into contact with them (See Exposure Pathway).
- Route of Exposure:
- The way a chemical can get into a person's body. There are three exposure routes:
- Breathing (also called inhalation),
- Eating or drinking (also called ingestion),
- Or getting something on the skin (also called dermal contact).
- The Superfund Amendments and Reauthorization Act in 1986 amended CERCLA and expanded the health-related responsibilities of ATSDR. CERCLA and SARA direct ATSDR to look into the health effects from chemical exposures at hazardous waste areas.
- Source (of Contamination):
- The place where a chemical comes from, such as a landfill, pond, creek, incinerator, tank, or drum. Contaminant source is the first part of an Exposure Pathway.
- Special Populations:
- People who may be more sensitive to chemical exposures because of certain factors such as age, a disease they already have, occupation, sex, or certain behaviors (like cigarette smoking). Children, pregnant women, and older people are often considered special populations.
- Superfund Site:
- See NPL.
- When the effect of the mixture is greater than estimated for additivity, the mixture is said to be synergistic (i.e. 2 + 2 = 6).
- Harmful. Any substance or chemical can be toxic at a certain dose (amount). The dose is what determines the potential harm of a chemical and whether it would cause someone to get sick.
- The study of the harmful effects of chemicals on humans or animals.
- Urgent Public Health Hazard:
- This category is used in ATSDR's Public Health Assessment documents for areas that have certain physical features or evidence of short-term (less than 1 year), base-related chemical exposure that could result in adverse health effects and require quick intervention to stop people from being exposed.
VOC Air Modeling
Indoor air sampling was limited in communities around Hill AFB. ATSDR applied the Johnson and Ettinger (1991) model to estimate indoor air concentrations in those areas not sampled. Groundwater contamination surrounding Hill AFB only involves the shallow aquifer at depths less than 150 feet. The majority of contaminants in these groundwater plumes are volatile organic compounds that have the ability to volatilize into vapor. This vapor can, in turn, move from the groundwater, through soil, and eventually seep into basements and affect the indoor air.
Rather than simulating the many complex factors that affect how toxic chemicals disperse in air, ATSDR evaluated a simple and overestimated exposure situation: What would be the estimated indoor air concentration of a VOC contaminant for a house located directly above a groundwater plume with a VOC concentration equal to the highest level measured at Hill AFB? Though obviously unrealistic, this scenario provides an extreme upper bound estimate of what the actual ambient air concentrations might have been. We used the Johnson and Ettinger indoor air model and Hill AFB environmental data to estimate indoor air concentrations in residences in the Patriot Hills Housing Area, Sunset, Clinton, Roy, Layton, Clearfield, Riverdale, and South Weber.
Johnson and Ettinger Model (1991)
In September 1988, the U.S. Environmental Protection Agency (EPA) developed the Johnson and Ettinger Model to estimate indoor air concentrations and associated health hazards from subsurface vapor intrusion into buildings. This model is a screening-level model that incorporates mechanisms for estimating the transport of contaminated vapors from either subsurface soils or groundwater into the spaces directly above the source of contamination (Environmental Quality Management, Inc. 2000). Soil properties, chemical properties of the contaminant, and structural properties of the building are entered into the model. When an initial concentration is entered into the model an incremental risk is produced. This risk, in turn, can be converted into an air concentration.
The Johnson and Ettinger model is a first-tier screening tool that is based on severalassumptions. As a result it has limitations.
- The model does not consider the effects of multiple contaminants.
- Its calculations do not account for preferential vapor pathways due to soil fractures, vegetation root pathways, or the effects of a gravel layer beneath the floor slab.
- The groundwater model does not account for the rise and fall of the water table due to aquifer discharge and recharge.
- The model also assumes that all vapor will enter the building, implying a constant pressure field is generated between the interior spaces and the soil surface.
- It neglects periods of near zero pressure differential.
- Soil properties in the area of contamination are assumed to be identical to those in the area above the contamination.
All but the most sensitive parameters have been set to either an upper bound value or the medianvalue. As a result, the model is very conservative when predicting indoor air concentrations.
For predicting indoor air concentrations in homes at or near Hill AFB, ATSDR entered themaximum groundwater and soil concentrations for each VOC into the Johnson and Ettingermodel. Incremental risks obtained for each compound for vapor intrusion into indoor air werethen converted into predicted concentrations and compared to a reference value for thatcompound. Based on this strategy, ATSDR found that none of the predicted incremental risks orair concentrations exceeded reference values and thus were not at levels that are known to causeadverse health effects.
Table D-1 lists the estimated indoor air concentrations that ATSDR predicted for the VOCsconsidered in this analysis. We emphasize that these are conservative estimates because ourinitial modeling application assumed that the maximum concentration of VOCs detected in theplume entered the home.
Potential trichloroethene exposures. As Table D-1 shows, the estimate of the indoor airconcentrations of TCE was generally lower than the lowest observed adverse effect level(LOAEL: 50,000 ppb). As a result, ATSDR can conclude that indoor air concentrations of TCEin the homes near plumes at Hill AFB do not reach unhealthy levels as a result of the operationsat Hill AFB.
Potential benzene exposures. As Table D-1 shows, the estimate of the indoor air concentrationsof benzene was far lower than the lowest observed adverse effect level (LOAEL: 780 ppb). As aresult, ATSDR can conclude that indoor air concentrations of benzene in any of the homes nearplumes at Hill AFB do not reach unhealthy levels as a result of the operations at Hill AFB.
Potential tetrachloroethylene exposures. As Table D-1 shows, there were three estimates ofindoor air concentrations of PCE. All were lower than the NOAEL (200 ppb). As a result,ATSDR can conclude that indoor air concentrations of PCE in any of the homes near plumes atHill AFB do not reach unhealthy levels as a result of the operations at Hill AFB.
Potential 1,2-dichloroethane exposures. As Table D-1 shows, the estimates of the indoor airconcentrations of DCA were far lower than the no observed adverse effect level (NOAEL:50,000 ppb). As a result, ATSDR can conclude that indoor air concentrations of DCA in any ofthe homes near plumes at Hill AFB do not reach unhealthy levels as a result of the operations atHill AFB.
Potential 1,1-dichloroethene exposures. As Table D-1 shows, the estimates of the indoor airconcentrations of 1,1-DCE were far lower than the no observed adverse effect level (NOAEL:25,000 ppb). As a result, ATSDR can conclude that indoor air concentrations of 1,1-DCE in anyof the homes near plumes at Hill AFB do not reach unhealthy levels as a result of the operationsat Hill AFB.
Potential carbon tetrachloride exposures. As Table D-1 shows, the estimates of the indoor airconcentrations of carbon tetrachloride were far lower than the no observed adverse effect level(NOAEL: 5,000 ppb). As a result, ATSDR can conclude that indoor air concentrations of carbontetrachloride in any of the homes near plumes at Hill AFB do not reach unhealthy levels as aresult of the operations at Hill AFB.
Potential vinyl chloride exposures. As Table D-1 shows, the estimates of the indoor airconcentrations of vinyl chloride were far lower than the lowest observed adverse effect level(LOAEL: 10,000 ppb). As a result, ATSDR can conclude that indoor air concentrations of vinylchloride in any of the homes near plumes at Hill AFB do not reach unhealthy levels as a result ofthe operations at Hill AFB.
Potential methylene chloride exposures. As Table D-1 shows, the estimates of the indoor airconcentrations of methylene chloride were far lower than the no observed adverse effect level(NOAEL: 200,000 ppb). As a result, ATSDR can conclude that indoor air concentrations ofmethylene chloride in any of the homes near plumes at Hill AFB do not reach unhealthy levelsas a result of the operations at Hill AFB.
Summary of ATSDR's Findings
ATSDR analyzed available environmental sampling data for Hill AFB. This analysis found thatthe chemicals detected in the groundwater contamination plumes in these areas never reachedconcentrations at off-base locations at levels thought to be associated with adverse health effects,even when considering extremely conservative exposure assumptions. Since the maximumconcentration of groundwater contamination does not appear to pose a public health hazard,current groundwater concentrations (thought to be lower than in the past due to remediationefforts), also are not expected to pose past, present, or future public health hazards to off-baseresidents.
The Johnson and Ettinger Model predicts that the indoor air of existing homes above even themost contaminated plumes will not reach concentrations likely to cause adverse health effectsdue to the groundwater contamination. The measured values for all of the contaminants indicatethat the indoor air concentration from all sources will not reach concentrations that are known tocause adverse health effects. In addition, the majority of the measured indoor air concentrationsare within the range measured in offices and homes across the country, suggesting that thegroundwater plumes do not adversely affect the indoor air.
|Community||VOC||Maximum Groundwater Concentration (ppb)||Incremental Risk (unitless)||Model Concentration (ppb)||LOAEL (ppb)||NOAEL (ppb)|
|Patriot Hills Housing Area||Benzene||550||6.1E-07||57||780|