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Frequently Asked Questions

This page presents many questions asked by site users and the applicable responses. Please search this page for answers to your questions prior to contacting technical support staff. Researching the questions and answers posted here will greatly reduce the time it takes for you to solve many problems that arise from calculating and using this SL site.

This page presents many questions asked by site users and the applicable responses. Please search this page for answers to your questions prior to contacting technical support staff. Researching the questions and answers posted here will greatly reduce the time it takes for you to solve many problems that arise from calculating and using this SL site.

1. The screening levels (SLs) presented on this site are for the Superfund/RCRA programs. They are risk-based concentrations derived from standardized equations combining exposure information assumptions with EPA toxicity data. SLs are considered by the Agency to be protective for humans (including sensitive groups) over a lifetime; however, SLs are not always applicable to a particular site and do not address non-human health endpoints, such as ecological impacts. The SLs contained in the SL table are generic; they are calculated without site-specific information. They may be re-calculated using site-specific data.

2. Why are SLs used?

   They are used for site "screening" and as initial cleanup goals, if applicable. SLs are not de facto cleanup standards and should not be applied as such. The SL's role in site "screening" is to help identify areas, contaminants, and conditions that require further federal attention at a particular site. Generally, at sites where contaminant concentrations fall below SLs, no further action or study is warranted under the Superfund program, so long as the exposure assumptions at a site match those taken into account by the SL calculations. Chemical concentrations above the SL would not automatically designate a site as "dirty" or trigger a response action; however, exceeding a SL suggests that further evaluation of the potential risks by site contaminants is appropriate. SLs are also useful tools for identifying initial cleanup goals at a site. In this role, SLs provide long-term targets to use during the analysis of different remedial alternatives. By developing SLs early in the decision-making process, design staff may be able to streamline the consideration of remedial alternatives.

3. How do SLs differ from cleanup standards?

   SLs are generic screening values, not de facto cleanup standards. Once the Baseline Risk Assessment is completed, site-specific risk-based remediation goals can be derived using the BLRA results. The selection of final cleanup goals may also include ARARs and TBCs, as well as site-specific risk-based goals. In the Superfund program, this evaluation is carried out as part of the nine criteria for remedy selection outlined in the National Oil and Hazardous Substances Pollution Contingency Plan (NCP). Once the nine-criteria analysis is completed, the SL may be retained as is or modified (based on site-specific information) prior to becoming established as a cleanup standard. This site-specific cleanup level is then documented in the Record of Decision.

4. How often do you update the SL Table?

   It is anticipated that the SLs will be updated approximately semiannually. However, the beta release version may initially be updated sooner, to correct any errors or accommodate significant user feedback. Please take note of the "What's New" page to identify when toxicity values are updated.

5. Can I get a copy of a previous SL table?

   We do not distribute outdated copies of the SL table. Each new version of the table supersedes all previous versions. If you wish to maintain previous versions of the SLs for a long-term project, you can download the entire table and save multiple versions with a time-stamp.

6. How can I get the calculator results or the other web pages to print on one page?

   First, under your browser print options, rotate the page into the landscape position. Next, make sure the margins are as small as possible. Also, it may be possible to change your browser settings to make the viewable print size smaller. You can also cut and paste the results into a spreadsheet or database for further formatting or use the Output to File Option from the search page and format the results. A PDF file is provided at the top of each page that is compressed to fit on standard paper. To watch a brief video that explains how to get results into a spreadsheet. click here (large file) or here for smaller file.

7. Where else can I go for toxicity studies (values) not on this site?

   The EPA toxicity value hierarchy is explained in the User's Guide of this website. For chemicals not listed in the hierarchy, toxicity information may be obtained by contacting the U.S. EPA Superfund Health Risk Technical Support Center at (513) 569-7300 or the Agency for Toxic Substances and Disease Registry (ATSDR) Information Center at 1-888-422-8737. Consult with your regional risk assessor when considering toxicity values not listed on these tables. For occupational exposure standards, try NIOSH, WHO, or OSHA. For information on nerve agents, contact DENIX.

8. Where can I find out about WATER9, CHEMDAT8, and CHEM9?

   These programs help estimate various chemical-specific parameters such as diffusivity in air and water. WATER9 is an analytical model for estimating compound-specific air emissions from wastewater collection & treatment systems. CHEMDAT8 is a Lotus 1-2-3 spreadsheet that includes analytical models for estimating VOC emissions from treatment, storage and disposal facility (TSDF) processes. CHEM9 is a compound properties processor that is based upon an EPA compound database of over 1000 compounds. It provides the capability to estimate compound properties that are not available in the database, including the compound volatility and the theoretical recovery (fraction measured (Fm)) for EPA test methods 25D and 305.

9. I can't find the chemical in which I am interested. Why isn't it in your database? Are there other places where I should look to find the information that I need?

   The Generic Tables are not completely alphabetical. Some chemicals are listed together under a broader checmical group.

   If you are trying to locate various PAHs or PCBs, they are listed in the table under Polynuclear Aromatic Hydrocarbons and Polychlorinated Biphenyls, respectively. Also, dioxin congeners may be compared with the SL for congener 2,3,7,8-TCDD, once the appropriate Toxicity Equivalence Factors have been applied.

   Chemical groups are in bold type in the tables and chemicals in those groups are indented. Your chemical may be listed in one of the following chemical groups:

   • Cyanides
   • Dioxins
   • Furans
   • Lead Compounds
   • Mercury Compounds
   • Polychlorinated Biphenyls (PCBs)
   • Polynuclear Aromatic Hydrocarbons (PAHs)

   If you still cannot find the chemical in the database, it means that we have no EPA toxicity value for it. The SL table only includes chemical species for which we have toxicity values.

   Consult with your regional risk assessor when searching for toxicity values not listed on these tables.

   There are many other useful toxicological/risk assessment sites on the internet. In many cases, the data may be available but will require a literature search.

   The calculator allows the user to calculate SLs for a chemical not in our database. Select "Test Chemical" in the pick list and one can enter chemcial-specific information for any chemical not already listed.

10. Can I get copies of supporting documents for interim toxicity constants which are coded "P" in the SL Table?

    EPA's OSRTI sponsored the development of provisional peer-reviewed toxicity values (PPRTVs) for contaminants without IRIS. PPRTVs (coded "P" on the SL Table) are developed for use on Superfund or hazardous waste sites. In March 2004, OSRTI posted the PPRTV database on the internet on a trial basis and solicited feedback from stakeholders. Concerns were raised about the potential for misuse of these values and for misunderstanding of what these values represent. Consequently, EPA has removed the PPRTVs from the Internet. We are currently investigating other options for distributing this information. If you have a critical issue during this interim period, contact Dave Crawford via email at crawford.dave@epa.gov.

11. For manganese, IRIS shows an oral RfD of 0.14 mg/kg-day, but the SL Table uses 0.024 mg/kg-day. Why?

    The IRIS RfD includes manganese from all sources, including diet. The explanatory text in IRIS recommends using a modifying factor of 3 when calculating risks associated with non-food sources, and the SL table follows this recommendation. IRIS also recommends subtracting dietary exposure (default assumption in this case is 5 mg). Thus, the IRIS RfD has been lowered by a factor of 2 x 3, or 6. The table now reflects manganese for "non-food" sources.

12. Can the oral RfDs in the SL Table be applied to dermal exposure?

    Not directly. Oral RfDs are usually based on administered dose and therefore tacitly include a GI absorption factor. Thus, any use of oral RfDs (or CSFs) in dermal risk calculations should involve removing this absorption factor. Consult the Risk Assessment Guidance for Superfund, Part A, Appendix A, for further details on how to do this. (See also RAGS Part E.) Note that the SL table displays the GIABS used in dermal SL calculations.

13. The exposure variables table in the SL background document lists the averaging time for non-carcinogens as "ED*365." What does that mean?

    ED is exposure duration, in years, and * is the computer-ese symbol for multiplication. Multiplying ED by 365 simply converts the duration to days. In fact, the ED term is included in both the numerator and denominator of the SL algorithms for non-cancer risk, canceling it altogether. See RAGS for more information.

14. Where did the inorganic lead SL value in the Table come from?

    EPA has no consensus RfD or CSF for inorganic lead, so it is not possible to calculate SLs as we have done for other chemicals. EPA considers lead to be a special case because of the difficulty in identifying the classic "threshold" needed to develop an RfD.
    
    EPA therefore evaluates lead exposure by using blood-lead modeling, such as the Integrated Exposure-Uptake Biokinetic Model (IEUBK). The EPA Office of Solid Waste has also released a detailed directive on risk assessment and cleanup of residential soil lead. The directive recommends that soil lead levels less than 400 mg/kg are generally safe for residential use. Above that level, the document suggests collecting data and modeling blood-lead levels with the IEUBK model. For the purposes of screening, therefore, 400 mg/kg is recommended for residential soils. For water, we suggest 15 ug/l (the EPA Action Level in water), and for air, the National Ambient Air Quality Standard.
    
    However, caution should be used when both water and soil are being assessed. The IEUBK model shows that if the average soil concentration is 400 mg/kg, an average tap water concentration above 5 ug/L would yield more than than a 5% probability of exceeding a 10 ug/dL blood-lead level for a typical child. If the average tap water concentration is 15 ug/L, an average soil concentration greater than 250 mg/kg would yield more than than a 5% probability of exceeding a 10 ug/dL blood-lead level for a typical child.

    For more information see Addressing Lead At Superfund Sites.

15. Where did the cancer toxicity values for carcinogenic PAHs come from?

    The PAH SFOs are all calculated relative to benzo[a]pyrene, which has an IRIS slope factor. The relative factors for the other PAHs can be found in Provisional Guidance for Quantitative Risk Assessment of Polycyclic Aromatic Hydrocarbons (PDF) (28 pp, 1MB) . The Toxicity Equivalency Factors (TEFs) are listed in Section 2.3.5 of the User's Guide. The PAH IURs are all from California EPA.

16. Why is there no oral RfD for mercury? How should I handle mercury?

    IRIS gives oral RfDs for mercuric chloride and for methylmercury, but not for elemental mercury. Therefore, the SL Table follows suit. Consult your toxicologist to determine which of the available mercury numbers is suitable for the conditions at your site (e.g., whether mercury is likely to be organic or inorganic.)

17. The cadmium numbers are labeled "food" and "water." Which do I use if I have another medium, such as soil?

    The cadmium RfDs on IRIS are based on the same study. The food RfD incorporates a 2.5% absorption adjustment; the water RfD incorporates a 5% absorption adjustment. For another medium such as soil, the risk assessor should choose the number whose absorption factor most closely matches the expected conditions at the site. For example, if the expected absorption of cadmium from soil is 3%, the food-based number would be a good approximation.

18. The slope factors for benzene are actually ranges, yet the SL table shows only a single number. Which number was chosen and why?

    The upper end of the slope factor range was chosen. This is because the SL Table is a screening tool, and the consequences of screening out a chemical that could pose a significant risk are more serious than the consequences of carrying the chemical through to the next step of the risk assessment. (At each step of the risk assessment, the risk is further refined using site-specific analysis. Chemicals can always be eliminated from the risk assessment at a later step than the initial screening, if appropriate.)

19. What toxicity values are used for TCE?

    TCE currently has no EPA consensus toxicity value, and multiple estimates of TCE risk may be appropriate. Consult your regional risk assessor to determine whether your region or state has recommended TCE values.

    The toxicity hierarchy discussed in this FAQ was followed and the resulting toxicity values, except for the RfC, for TCE are from the California Environmental Protection Agency/Office of Environmental Health Hazard Assessment’s toxicity values.

    After evaluating the Tier 3 sources for noncarcinogenic toxicity values, we have elected not to choose any noncancer toxicity values for TCE. Rather, we will allow cancer-risk considerations to dominate the evaluation of TCE as they are protective of noncancer risks as well.

20. IRIS presents 2 types of toxicity values for vinyl chloride yet the SL table shows only a single number. Which number was chosen and why?

    The vinyl chloride calculations were based on the examples given in the Toxicological Review for vinyl chloride, which appears on IRIS. IRIS presents "continuous lifetime exposure during adulthood" and "continuous lifetime exposure from birth" slope factors and inhalation unit risks. Because the equations used on this website show the individual lifetime segments, the "continuous lifetime exposure during adulthood" toxicity values are chosen.

    The examples in the Toxicological Review indicate that, during childhood, both pro-rated and non-pro-rated risks should be generated using the lower slope factor or IUR. When estimating the risk using this method and considering the lifetime segments during childhood and adulthood, it is clear that the cancer risks early in life are higher than those that would be generated if the typical pro-rated risks were simply generated using the lifetime CSF or IUR. This finding is consistent with the IRIS assessment's statements that cancer risk is increased during early life.

    Over the course of a 70-year lifetime, the risk generated using the pro-rated and non-pro-rated segments, along with the lower CSF or IUR, generally exceeds the risk generated using only pro-rated exposure and the lifetime CSF or IUR. However, the former risk estimates trend closer and closer to the latter as life advances, and converge at about the 70-year mark.

21. 2,4/2,6-dinitrotoluene mixture has a cancer slop factor, why don't the individual isomers use the same slope factor?

    It was determined for this website that the IRIS toxicological profile did not adequately address this issue.

22. Do the fish tissue SLs apply to wet-weight or dry-weight data?

    The fish SLs represent the concentration that can be consumed at the rate indicated in the Technical Background Document. Therefore, wet or dry weight is not an inherent assumption of the SL numbers. Rather, users of the Table should consider whether their population of interest is more likely to consume the fish using a preparation method that is better simulated by a wet or dry weight. (For example, consumption of raw or fried fish would be more likely represented by wet weight, whereas consumption of smoked or dried fish might be better represented by dry weight.) In other words, the use of a site-specific sample as wet or dry weight should be governed by its representativeness for the population of interest.

23. Why do some of the numbers on the SL Table exceed a million parts per million (1E+06 mg/kg)? That's not possible!

    For certain low-toxicity chemicals, the SLs exceed possible concentrations at the target risks. Many years ago, these SLs were rounded to the highest possible concentration, or 1.0E+06 ppm. This type of truncation has been discontinued so that Table users can adjust the SLs to a different target risk whenever necessary. For example, when screening chemicals at a target HQ of 0.1, noncarcinogenic SLs may simply be divided by 10. Such scaling is not possible when SLs are rounded. Users who are interested in truncation can also consult the Soil Screening Guidance for a discussion of "Csat," the saturation concentration, which reflects physical limits on soil concentrations.

    SLs may also exceed a non-risk based 'ceiling limit' concentration of 1.0E+05 mg/kg ('max') for relatively less toxic inorganic and semivolatile contaminants. The ceiling limit of 1.0E+05 mg/kg is equivalent to a chemical representing 10% by weight of the soil sample. At this contaminant concentration (and higher), the assumptions for soil contact may be violated (for example, soil adherence and wind-borne dispersion assumptions) due to the presence of the foreign substance itself.

24. Why isn't oral/inhalation route-to-route extrapolation used to generate toxicity factors on the Screening Table?

    Previous versions of regional screening tables did contain some route-to-route extrapolation, because of the scarcity of inhalation toxicity factors. However, this was not optimal due to the uncertainty associated with making such adjustments (e.g., point-of-entry, first-pass, and route-specific effects may not be adequately considered by simple extrapolations). With the increasing availability of Tier III toxicity values, generic route-to-route extrapolation has been discontinued. Chemical-specific route-to-route extrapolation may be used by Tier I, II, or III sources after thorough consideration of the chemical-specific issues.

25. Previous Regional Tables used Inhalation Reference Doses (RfDi) and Slope Factors (SFI). Why does the new table use RfCs and IURs?

    In the past, some regional tables converted RfCs to RfDs and IURs to SFIs for inhalation. This was initially done because risk equations once relied upon RfDs and SFIs in units of mg/kg/day and 1/mg/kg/day, respectively. However, as the inhalation guidance has evolved, RfCs and IURs, in units of mg/m³ and m³/ug respectively, have become the recommended toxicity factors. Please see Methods for Derivation of Inhalation Reference Concentrations (RfCs) and Application of Inhalation Dosimetry or PDF (409 pp, 21 MB) for more information. Also please see the FAQ concerning route-to-route extrapolation.

26. How were the toxicity values provided in IRIS on Chromium used to calculate Chromium Screening Levels?

    It is recommended that valent-specific data for Chromium be collected when Chromium is likely to be an important contaminant at a site, and when Cr6 may exist. In the absence of valent-specific data, screening levels for total Chromium are provided. For Chromium (VI) (Cr6), IRIS shows an air unit risk of 1.2E-2 per (ug/m3). However, the supporting documentation in the IRIS file states that this toxicity value is based on an assumed 1:6 ratio of Cr6:Cr3. Because of this assumption and in an effort to be transparent, RSLs based on this cancer toxicity value are presented as "Chromium, Total (1:6 ratio Cr VI:III)" numbers.

    In the RSL Table, the Cr6 specific value (assuming 100% Cr6) is derived by multiplying the IRIS Cr6 value by 7. This is considered to be a conservative and protective assumption, and is also consistent with the State of California's interpretation of the Mancuso study that forms the basis of Cr6's toxicity values.

    If you are working on a chromium site, you may want to contact the appropriate regulatory officials in your region to determine what their position is on this issue.

    The MCL of 100 µg/L for "Chromium (total)", from the EPA's MCL listing is applied to the "Chromium, Total (1:6 ratio Cr VI:III)" analyte on this website.

27. What are the sources of toxicity values used on this site?

    In 2003, EPA’s Superfund program revised its hierarchy of human health toxicity values (PDF) (4 pp, 228 K), providing three tiers of toxicity values . Three tier 3 sources were identified in that guidance, but it was acknowledged that additional tier 3 sources may exist. The 2003 guidance did not attempt to rank or put the identified tier 3 sources into a hierarchy of their own. However, when developing the screening tables and calculator presented on this website, EPA needed to establish a hierarchy among the tier 3 sources. The toxicity values used as “defaults” in these tables and calculator are consistent with the 2003 guidance. Toxicity values from the following sources in the order in which they are presented below are used as the defaults in these tables and calculator.

    1. EPA’s Integrated Risk Information System (IRIS)

    2. The Provisional Peer Reviewed Toxicity Values (PPRTVs) derived by EPA’s Superfund Health Risk Technical Support Center (STSC) for the EPA Superfund program. (Note that the PPRTV website is not open to users outside of EPA, but assessments can be obtained for use on Superfund sites by contacting Dave Crawford at Crawford.Dave@epa.gov.)

    3. The Agency for Toxic Substances and Disease Registry (ATSDR) minimal risk levels (MRLs)

    4. The California Environmental Protection Agency (OEHHA) Office of Environmental Health Hazard Assessment's Chronic Reference Exposure Levels (RELS) from December 18, 2008 and the Cancer Potency Values (PDF) (7 pp, 91.9k) from December 17, 2008.

    5. The EPA Superfund program’s Health Effects Assessment Summary. (Note that the HEAST website of toxicity values for chemical contaminants is not open to users outside of EPA, but values can be obtained for use on Superfund sites by contacting Dave Crawford at Crawford.Dave@epa.gov).

       Users of these screening tables and calculator wishing to consider using other toxicity values, including toxicity values from additional sources, may find the discussions and seven preferences on selecting toxicity values in the attached Environmental Council of States paper useful for this purpose (ECOS website), (ECOS paper (Word Document).

       When using toxicity values, users are encouraged to carefully review the basis for the value and to document the basis of toxicity values used on a CERCLA site.

       Please contact a Superfund risk assessor in your Region for help with chemicals that lack toxicity values in the sources outlined above.

28. Why is the tapwater screening level for Perchlorate of 26 µg/l different from the preliminary remedial goal (PRG) of 15 µg/l calculated by the Office of Solid Waste and Emergency Response in its January 8, 2009, guidance?

    As described in the OSWER memorandum, the Agency has now issued an Interim Drinking Water Health Advisory (Interim Health Advisory) for exposure to perchlorate of 15 µg/L in water. A health advisory provides technical guidance to federal, state, and other public health officials on health effects, analytical methods and treatment technologies associated with drinking water contamination. The Interim Health Advisory for perchlorate was developed using EPA’s RfD of of 0.7 ug/kg-day and representative body weight, as well as 90th percentile drinking water and national food exposure data for pregnant women in order to protect the most sensitive population identified by the National Research Council (NRC) (i.e., the fetuses of pregnant women who might have hypothyroidism or iodide deficiency).

    The NCP (40 CFR 300.430(e)(2)(A)(1)) provides that when establishing acceptable exposure levels for use as remediation goals (for a Superfund site), consideration must be given to concentration levels to which the human population, including sensitive subgroups, may be exposed without adverse effects over a lifetime or part of a lifetime, incorporating an adequate margin of safety. As a result of the publication of the Interim Health Advisory for perchlorate, OSWER recommends that where no federal or state applicable or relevant and appropriate (ARAR) requirements exist under federal or state laws, 15 µg/L (or 15 ppb) is recommended as the PRG for perchlorate when making CERCLA site-specific cleanup decisions where there is an actual or potential drinking water exposure pathway. However, where State regulations qualify as ARARs for perchlorate, the remediation goals established shall be developed considering the State regulations that qualify as ARARs, as well as other factors cited in the NCP (see 40 CFR 300.430(e)(2)(i)(ff)). Final remediation goals and remedy decisions are made in accordance with 40 CFR300.430 (e) and (f) and associated provisions.

    Preliminary remediation goals are the starting points in the development of final cleanup levels at sites. As at all sites addressed under the NCP, these goals may be modified, depending on physical characteristics of a site, State laws and guidance, and other site specific factors, such as additional exposure routes.

    One can derive a Drinking Water Equivalent Level of 24.5 ppb using EPA’s reference dose (RfD) of 0.7 ug/kg-day and an assumption that all exposure to perchlorate comes from ground water. The table shows a slightly higher number due to rounding of calculator input values.

29. What is the preferred citation for information taken from this website? United States Environmental Protection Agency Regions 3, 6, and 9. (Insert date accessed). Regional Screening Levels for Chemical Contaminants at Superfund Sites.

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