Screening Analysis Overview

Learn how to use ATSDR comparison values (CVs) and non-ATSDR screening levels to screen sampling data to identify potential contaminants of concern that require further evaluation for completed and potential exposure pathways. Concentrations that meet or exceed ATSDR CVs and/or non-ATSDR screening levels do not indicate that health effects are likely, but rather help you prioritize contaminants to evaluate further during the PHA process.

As you gather information and get familiar with the site and community health concerns, the nature and extent of contamination, and exposure pathways, you will begin performing another scientific component of the PHA process covered in this section: the screening analysis. Then, based on the results of the screening analysis and community health concerns at some sites, you may calculate exposure point concentrations (EPCs) and perform exposure calculations. Based on those results, you may need to conduct a more in-depth analysis to evaluate potential health effects and determine possible public health implications of site-specific exposures.

During the PHA process, health assessors typically review large amounts of sampling data for contaminants in media such as air, soil, and water. The screening analysis enables you to systematically and consistently sort through the data to identify potential contaminants of concern within completed and potential exposure pathways, by exposure unit if appropriate (see text box).

ATSDR conducts the screening analysis as a quick, easy mechanism for identifying contaminants that do and do not require further evaluation at a site. When comparing against screening levels, generally you begin with the list of contaminants found in potential or completed exposure pathways.

Important: The screening analysis is a method that provides a health protective initial screen, but it does not incorporate site-specific exposure scenarios that will need to be considered during the EPC and Exposure Calculations Evaluation.

ATSDR’s screening analysis is a health-protective process, which is more inclusive than exclusive, designed so that health assessors do not screen out any potential contaminants of concern (COCs).

To conduct the screening itself, you will compare maximum detected concentrations (except for some air screening evaluations [see text box on the right]) to the most appropriate screening level. This will allow you to identify

  1. contaminants whose concentrations are below screening levels and likely pose no health hazards,
  2. contaminants whose concentrations meet or exceed screening levels and require further evaluation, and
  3. contaminants without readily available screening levels.
Exceptions to Using the Max Concentrations for Screening

ATSDR typically uses the maximum concentration for screening; however, there are exceptions for some air screening evaluations. For example, depending on the site, it could be appropriate to average temporal sampling data over a time period corresponding to the duration of the screening level prior to making comparisons.

For contaminants with concentrations at or above screening levels, you will proceed to the EPC and Exposure Calculations Evaluation component of the PHA process.

Defining Exposure Units

Health assessors define exposure units for each potential contaminant of concern identified in a completed exposure pathway or potential exposure pathway. Based on the nature of the data set, you can define exposure units during the exposure pathway evaluation and either before or after the screening analysis, as long as you do so before estimating EPCs.

With large data sets, for example, the health assessor may decide to complete the media-specific screening on the entire data set to identify potential contaminants of concern at the site, then define appropriate exposure units for further analysis.

For smaller or more defined data sets, the health assessor may define the exposure units prior to conducting the screening analysis. Health assessors will use professional judgement to determine when to define exposure units; however, they must define them before determining EPCs.

ATSDR has developed Exposure Unit Guidance to help health assessors define exposure units. Ask the Associate Director for Science (ADS) group if you have questions about exposure units.

Special Considerations

Some site data sets require special considerations to assess exposures, as outlined below. ATSDR has resources available to help health assessors when screening sites with special considerations related to dioxins and hexavalent chromium.

Select Contaminant Groups

Some contaminants will require special consideration during the screening process. Examples include asbestos and lead, as well as individual contaminants within groups like polychlorinated biphenyls (PCBs), dioxins/furans, and polycyclic aromatic hydrocarbons (PAHs). Some special contaminants, such as dioxin and dioxin-like compounds can be screened in ATSDR’s Public Health Assessment Site Tool (PHAST); others might need to be evaluated outside of PHAST. If you have special contaminants at your site, consult with the Associate Director for Science (ADS) group to identify the appropriate subject matter experts (SMEs) who can guide you on the best approach for the screening analysis evaluation.


When evaluating possible health effects from exposures to radionuclides, consult with a health physicist to evaluate sampling results and select appropriate screening levels. If screening levels for radioactive materials are not available (which will often be the case), consult with a health physicist to determine if the reported results

  • are realistic, based on the methods of analysis (equipment used),
  • are plausible, based on the site history and description, or
  • meet standard, recognized quality control and quality assurance protocols (such as minimum detectable activity and uncertainty of the measurement).

In some circumstances, you may be able to develop a site-specific screening level with assistance from an ATSDR toxicologist or health physicist.

Biological Sampling Data

When available, biological sampling data (most commonly urine and blood) can provide additional perspective for the health assessor. Consult with an SME (such as an epidemiologist and a toxicologist) to determine appropriate exposure biomarker (i.e., an indicator that exposure to a contaminant occurred) methods and to define appropriate blood or urine screening levels (e.g., reference ranges) based on the available sampling data, measured contaminants, and site-specific conditions.

ATSDR and EPA have not established screening levels to help systematically evaluate contaminant concentrations measured in biological samples. Useful information sources on biomarkers include ATSDR’s Toxicological Profiles (sections related to biomarkers) and Case Studies in Environmental Medicine. CDC’s National Health and Nutrition Examination Survey (NHANES) also collects human exposure data for selected environmental contaminants. You can use these data to determine U.S. representative levels of specific contaminants. While these data are helpful, they reflect only national exposure levels. Note that biological sampling concentrations above NHANES levels do not mean that potential harmful health outcomes will occur (i.e., a biological concentration greater than an NHANES level does not equate to a health effect). ATSDR might also compare measured concentrations in biological samples to other national or cohort-based studies.


Sometimes during your screening analysis, you will find that no ATSDR CVs or non-ATSDR screening levels exist for your specific contaminant of interest. In some cases, you may identify that a screening level is available for a “surrogate” chemical. When appropriate, health assessors can use screening levels for surrogates that have similar chemical or radiological properties as the subject contaminant or properties that are even more toxic. Before using a surrogate screening level, health assessors need to confirm with the ADS group, a technical project officer (TPO), or a SME that those surrogates and screening levels are appropriate. If the surrogate is appropriate and used during the screening analysis, health assessors need to document the surrogate screening level and the source in their reports.


Odors in the environment can come from many sources, including human activities, animals, nature, vehicles, and industrial activities. During the screening analysis, ATSDR needs available environmental sampling data and screening levels based on a contaminant’s toxic effects, rather than on a contaminant’s odor-related effects, to help identify contaminants that need closer evaluation. Thus, ATSDR does not evaluate odors during the screening analysis. ATSDR can offer help in other ways such as recommending individuals stay indoors when environmental odors are strong and providing environmental health education about odors to health care providers in the area.

ATSDR Toxicological Profiles
Beakers with colored liquids with the text "Toxicological Profiles" on top

Each peer-reviewed Tox Profile reflects a comprehensive evaluation, summary, and interpretation of available toxicological and epidemiological information on a hazardous substance and provides information on contaminant-specific properties. More

ATSDR Case Studies in Environmental Medicine

These educational case studies are designed to increase health assessors’ knowledge of hazardous substances in the environment, and to help medical practitioners evaluate and care for potentially exposed patients. More

ATSDR Website on Environmental Odors
Boy pinching his nose

This suite of webpages offers tools, resources, and insight into engaging residents and partners about environmental odors in their community. More

CDC National Health and Nutrition Examination Survey (NHANES)
Large group of people standing and smiling at the camera

NHANES reports biomonitoring data for more than 350 environmental chemicals in the non-institutionalized, civilian U.S. population. More

Page last reviewed: April 14, 2022