Skip directly to search Skip directly to A to Z list Skip directly to site content




Figure 1: Location map of the Tucson International Airport Superfund Site

Figure 2: Location of Larger Plume A and Smaller Plume B

Figure 3: Close-up Map of Plume B

Figure 4: Results of 1994 and 1998 Private Well Studies Conducted by Pima CountyDepartment of Environmental Quality

Comparison Values

Listed below are some of the comparison values used by ADHS to select chemicals which meritdetailed site specific evaluation. In addition, other non-ADHS values are listed which aresometimes used to provide a meaningful frame of reference for environmental chemical data.For convenience, the list also includes some of the common abbreviations used for common unitsof measure. Following the list is a brief description of each value.

CREG = Cancer Risk Evaluation Guide
MRL = Minimal Risk Level
EMEG = Environmental Media Evaluation Guide
RMEG = Reference Dose Media Evaluation Guide
RfD = Reference Dose
RfC = Reference Dose Concentration
RBC = Risk Based Concentration
DWEL = Drinking Water Equivalent Level
LTHA = Lifetime Health Advisory
MCL = Maximum Contaminant Level
HBGL = Human Health Based Guidance Level
PRG = Permissible Remediation Goal (Action Level)
PEL = Permissible Exposure Limit
TLV = Threshold Limit Value
ppm = parts per million
ppb = parts per billion
kg = kilogram (1000 grams)
mg = milligram (0.001 grams)
ug = microgram (0.000001 grams)
L = liter
m3 = cubic meter (referring to 1000 liters of air)

Cancer Risk Evaluation Guides (CREGs) are estimated contaminant concentrations expectedto cause no more than one excess cancer in one million persons exposed over a lifetime. CREGsare calculated from USEPA's cancer slope factors or cancer potency factors using standardassumptions for exposure rates. These cancer estimates are commonly used because low-dose chemical exposure lab studies typically are not in the scientific literature and the true risk isunknown (and may be as low as zero).

Minimal Risk Levels (MRLs) are estimates of daily human exposure to a chemical (usually interms of milligrams chemical per kilogram of body weight per day) that are unlikely to beassociated with any appreciable risk of adverse noncancer effects over a specified duration ofexposure. MRLs are calculated using data from human and animal studies and are usuallyreported for one of three time frames: acute (up to 14 days exposure), intermediate (15 - 364 daysexposure), and chronic (1 year or more of exposure). MRLs are developed by the federal Agencyfor Toxic Substances and Disease Registry (ATSDR) and are published in ATSDR'sToxicological Profiles.

Environmental Media Evaluation Guides (EMEGs) are media-specific concentrations ofchemicals calculated from ATSDR minimal risk levels using standard body weight and ingestionassumptions. EMEGs may be developed for specific timeframes of exposure duration such asacute, intermediate, or chronic (see MRLs). Chemical amounts below an EMEG are considered tobe harmless to public health while amounts above an EMEG require detailed site-specificevaluation.

Reference Dose Media Evaluation Guide (RMEG) is the concentration of a contaminant in air,water, or soil that corresponds to USEPA's RfD for that contaminant when standard assumptionsof body weight and intake rates are taken into account.

Reference Dose (RfD) is USEPA's estimate of the daily exposure to a contaminant unlikely tocause noncarcinogenic adverse health effects. Like the ATSDR MRL, the RfD is a doseexpressed in mg/kg/day.

Reference Concentration (RfC) is a concentration of a substance in air which USEPA considersunlikely to cause non-cancer adverse health effects over a lifetime of exposure.

Risk-Based Concentrations (RBCs) are media-specific concentrations calculated from RfDs,RfCs, or USEPA's Cancer Slope Factors. They represent concentrations of a contaminant that areconsidered unlikely to cause adverse health effects over a lifetime of chronic exposure.

Drinking Water Equivalent Levels (DWELs) are based on USEPA's oral RfD and representcorresponding concentrations of a substance in drinking water that are estimated to havenegligible deleterious effects in humans at an intake rate of 2 liters per day for life, assuming thatdrinking water is the sole source of exposure.

Lifetime Health Advisories (LTHA) are calculated from the DWEL and represent theconcentration of a substance in drinking water estimated to have a negligible deleterious effect inhumans over a lifetime of 70 years, assuming 2 liters per day consumption for a 70 kilogramadult, and taking into account other probable sources of exposure. In the absence of chemicalspecific data, the assumed fraction of total intake from drinking water is 20%. Lifetime healthadvisories are not derived for compounds considered potentially carcinogenic for humans.

Maximum Contaminant Levels (MCLs) are legally enforcible contaminant concentrations indrinking water that USEPA deems protective of public health (considering the availability andeconomics of water treatment technology) over a 70 year lifetime at an exposure rate of 2 liters ofwater per day.

Health Based Guidance Levels (HBGLs) are calculated by ADHS to limit excess lifetimecancer risk to one-in-one million (10-6) for known human carcinogens and to one-in-one-hundred-thousand (10-5) for possible and probable human carcinogens. HBGLs are consideredindividually protective of human health, including sensitive groups, over a lifetime. Chemicalconcentrations that exceed the applicable HBGL may not necessarily represent a health hazard. Rather, when contaminant concentrations exceed the HBGL, further evaluation may be necessaryto determine whether a contaminant poses an unacceptable health hazard to humans.

Permissible Remediation Goals (PRGs), or Action Levels, are chemical- and media- specificlevels of contamination which, when exceeded, automatically trigger a regulatory response orremedial action of some kind.

Permissible Exposure Limit (PEL) is an 8-hour time-weighted average concentration of asubstance in workplace air designed by the Occupational Safety and Health Administration(OSHA) to provide that, to the extent feasible, chemical exposures in the workplace do not impairthe health or functional capacity of workers throughout their working life. The PEL may beexceeded for brief periods, but the sum of the exposure levels averaged over 8 hours is not toexceed to PEL.

Threshold Limit Value (TLV), developed by the American Conference of GovernmentalIndustrial Hygienists (ACGIH), is "the time-weighted average concentrations for a normal 8-hourworkday and a 40-hour workweek, to which nearly all workers may be repeatedly exposed, dayafter day, without adverse effect." Many of ACGIH's TLVs were adopted by OSHA for use asPELs. Note that TLVs and PELs, which were designed to protect healthy workers, are usuallymuch higher than the public health based values of ATSDR and USEPA, which were designed toprotect the health of the general population, including subgroups such as the very young and theelderly.



a permeable rock stratum below the earth's surface through which groundwatermoves; generally capable of producing water for a well.

chemicals of concern

chemicals whose concentrations are above the appropriate screening level.

detection limit

the minimum concentrations that must be accurately and precisely measured bythe laboratory and/or specified in the quality assurance plan.


the amount of a contamination that is absorbed or deposited in the body of anexposed organism for an increment of time. A total dose is the sum of dosesreceived by a person from a contaminant in a given interval resulting frominteraction with all environmental media that contain the contaminant. Units ofdose and total dose are often converted to units of mass per volume ofphysiological fluid or mass of tissue.


an event that occurs when there is contact at a boundary between a human beingand the environment with a contaminant for a specific concentration for aninterval of time: the units of exposure are concentration multiplied by time.

exposure pathway

the process by which an individual is exposed to contaminants that originatefrom some source of contamination and are categorized as inhalation, dermal,and/or ingestion exposures.


the period between stimulus application and response onset.

parts per million

a common basis of reporting water analysis. One part per million (ppm) equals1 pound per million pounds of water.

public health assessment

an evaluation of relevant environmental data, health outcome data, andcommunity concerns associated with a site where hazardous substances havebeen released.

route of exposure

means by which the contaminant actually enters or contacts the body, such asingestion, inhalation, dermal contact, and dermal absorption.

volatile compounds

compounds amenable to analysis by the purge and trap techniques. Usedsynonymously with purgable compounds.


the conversion of a liquid or solid into vapors.


ADHS Arizona Department of Health Services
ADEQ Arizona Department of Environmental Quality
AMC American Cancer Society
ATSDR Agency for Toxic Substances and Disease Registry
COC chemical of concern
HBGL Health-based Guidance Levels
MCL maximum contaminant level
MRL minimum risk level
NA not applicable
ND non-detect
NS not sampled
PCDEQ Pima County Department of Environmental Quality
ppm parts per million
SLE Systemic Lupus Erythematosus
TCE trichloroethylene
TIAA Tucson International Airport Area
USEPA United States Environmental Protection Agency
VOCs volatile organic compounds
µg/L micrograms per liter

Tables A1-A19, the ATSDR Review of Eight TIAA Health Studies Related to Trichloroethylene (TCE) Contamination, and the ADHS Toxicological Profile for TCE were not available in electronic format for conversion to HTML at the time of preparation of this document. To obtain a hard copy of the document, please contact:

Agency for Toxic Substances and Disease Registry
Division of Health Assessment and Consultation
Attn: Chief, Program Evaluation, Records, and Information Services Branch E-56
1600 Clifton Road NE, Atlanta, Georgia 30333

Table of Contents The U.S. Government's Official Web PortalDepartment of Health and Human Services
Agency for Toxic Substances and Disease Registry, 4770 Buford Hwy NE, Atlanta, GA 30341
Contact CDC: 800-232-4636 / TTY: 888-232-6348

A-Z Index

  1. A
  2. B
  3. C
  4. D
  5. E
  6. F
  7. G
  8. H
  9. I
  10. J
  11. K
  12. L
  13. M
  14. N
  15. O
  16. P
  17. Q
  18. R
  19. S
  20. T
  21. U
  22. V
  23. W
  24. X
  25. Y
  26. Z
  27. #