Considering Background Concentrations

This section provides information about the role of background concentrations in your site-specific analysis. It provides tips to help you interpret background concentrations as well as locate relevant information sources.

While a sampling study can indicate that environmental contamination exists, it will not always tell you the contaminant source. This is because some contaminants originate from multiple sources that all can contribute to a measured environmental concentration.

Ultimately, you will need to evaluate the public health implications of exposure to measured or predicted levels of contamination, regardless of whether contaminants are naturally occurring or result from human (anthropogenic) activities. Yet, understanding the contributions from background concentrations is an important element of your site-specific analysis.

Background is a widely used term, but it does not have a single definition. Two definitions of background are commonly used:

  1. Naturally occurring ambient levels of contaminants in the environment that have not been influenced by humans (e.g., metals naturally found in soils).
  2. Anthropogenic levels of contaminants in the environment because of human-generated, non-site-related sources (e.g., benzene in ambient air as a result of a city’s motor vehicle traffic, radiation in sediments that resulted from fallout from past use and testing of nuclear weapons).
Understanding Contributions from Background Concentrations

ATSDR’s primary goal is to evaluate exposures to individuals rather than document the attribution of a source. Thus, ATSDR will evaluate sites even without a confirmed link between a contaminant source and the site. In some cases, the presence of contaminants may not be related only to the activities at a particular site. For example, you might find that part of the arsenic in residential soil downwind from a smelter is naturally occurring. In other cases, contaminants might be primarily associated with a given source. For example, you may be able to identify an association between polychlorinated biphenyl (PCB) levels observed in fish to a manufacturing plant’s discharges. You also may be able to confirm contaminants came from multiple sources, such as tetrachloroethylene (PCE) concentrations in drinking water downgradient from a commercial/industrial zone that could be related to merging plumes from multiple dry-cleaning facilities. If site documentation provides reliable insights on sources of contamination, that is an important detail to include in your documents. Health assessors need to use caution when attributing contamination to a particular source, because such attributions can be controversial at some sites.

Tips for Interpreting Background Concentrations

When site references describe background contamination, be sure you understand what that means in the context of your site. General rules for interpreting sampling data when considering background concentrations include the following:

  • If statistical analyses demonstrate that the average contamination levels in your exposure unit are significantly higher than background, you can generally conclude that some source — either the site you are evaluating or some other source — has contaminated the media of concern.
  • If statistical analyses demonstrate that the average contamination levels in your exposure unit are consistent with background concentrations, you can typically conclude that local sources have not significantly impacted the media of concern. Another possibility is that limitations in the sampling prevent identification of statistically significant trends.
  • If statistical analyses demonstrate that the average contamination levels in your exposure unit are lower than background, you can again typically conclude that local sources have not significantly impacted the media of concern. The sample results may be truly lower than background. But if that is an unexpected result, health assessors should consider the possibility that the site sampling data are biased low or that the background levels are biased high.

If measuring background is a specific objective of a sampling program, then principal investigators should have the intent of their background sampling efforts built into the program’s DQOs. When identifying appropriate background data, select high-quality data that are most representative of the site. For instance, when identifying background data for metals in soils, use soils that have similar physical characteristics as site soils, such as sandy or loamy.

In the PHA process, site-specific background data are preferred. When they are not available, regional, state, or national background data should be considered, though they may be less representative of site-specific conditions.

Information Sources for Background Concentrations

Some sources of background data include site investigation reports, data from nearby sites, state and local environmental agencies, or other state and local organizations. No single reference documents background concentrations for all contaminants in all media. These featured sources (see table below) might include reasonable estimates of background concentrations for your site. Also consider accessing similar state sources.

Background Concentrations Featured Sources

Background Concentrations Featured Sources
Source Name
Source Description
ATSDR Toxicological Profiles Each profile has a chapter titled “Potential for Human Exposure” that provides contaminant concentrations reported in the literature. The chapters often provide both levels of environmental contamination measured near sources of significant releases and levels believed to represent background concentrations.
EPA Regional Officesexternal icon Most EPA regional offices can provide sediment sampling data gathered through joint EPA/U.S. Army Corps of Engineers efforts to assess the nature and quality of sediments in Federal Navigation Channels.
EPA’s Background Indoor Air Concentrations of Volatile Organic Compoundsexternal icon This source provides background levels of volatile organic compounds in homes in North America from 1990–2005.
EPA’s National Air Toxics Assessment (NATA)external icon Provide background concentrations for different air toxics emissions.
EPA’s National Air Toxics Trends Station (NATTS) Networkexternal icon The NATTS Network is a resource to learn about levels of air toxics currently measured in the air.
State Geological Surveysexternal icon Provide potential sources of state soil data.
U.S. Department of Agriculture Natural Resources Conservation Service (NRCS)external icon NRCS provides access to soil maps and survey data for states across the nation.
U.S. Geological Survey (USGS)external icon Background levels for surface waterexternal icon will vary from one watershed to the next and groundwaterexternal icon will vary from one aquifer to the next. USGS often reports background data for specific watersheds and aquifers. USGS is also a source for background soil dataexternal icon.
Page last reviewed: April 14, 2022