EPA-Expo-Box (A Toolbox for Exposure Assessors)
Organic compounds are generally defined as substances with structures based on carbon. Although some carbon-based substances are classified as inorganic, all substances classified as organic are carbon-based. Organic compounds comprise a highly variable group of ubiquitous substances with uses ranging from consumer products (e.g., ingredients in personal care products, building materials, food packaging, clothing, furniture) and drugs (e.g., Ibuprofen) to fuel components (e.g., hydrocarbons) and explosives (e.g., TNT). Many pesticides are also classified as organic – these are discussed in more detail in the Pesticides Module in the Chemical Classes Tool Set of EPA-Expo-Box.
Organic compounds can be natural, synthetic, or semi-synthetic. The source and method by which organic compounds are produced can affect their properties and expected exposure scenarios. For example, many natural organic compounds, such as lipids, proteins, and carbohydrates, are plant-based and have medicinal or nutritional benefits that are exploited through direct exposure to these compounds. On the other hand, some synthetic organic compounds are produced unintentionally, such as combustion byproducts like polycyclic aromatic hydrocarbons (PAHs) and dioxin released from industrial chlorinated processes, and exposures to these compounds might be largely indirect.
Organic compounds are often grouped according to structural similarities. Different “classes” of organic compounds refer to groups based on specific structural characteristics, including chemical bonds and functional groups. Similarities in chemical structure affect the properties of organic compounds, and thus their uses and human exposure considerations.
Importance of Physicochemical Characteristics
After contaminants are released to the environment, their transport, dispersion, and transformation are governed by certain physicochemical properties. Physicochemical properties data can help determine whether a contaminant is likely to remain in place, partition to other media, or transform physically, chemically, or biologically after release. Physicochemical properties data may be determined by measurements or estimated based on chemical structure. Sources of physicochemical values include:
- EPA ExpoCastDB
- EPA Aggregated Computational Toxicology Resource (ACToR)
- EPA Estimation Program Interface (EPI) Suite
- EPA High Production Volume Information System (HPVIS)
- EPA Superfund Chemical Data Matrix
- National Library of Medicine (NLM) Hazardous Substances Data Bank (HSDB)
- Organization for Economic Cooperation and Development (OECD) eChemPortal
The environmental fate, transport, and exposure of organic compounds can be estimated based on models that take into account certain physicochemical properties that can help predict partitioning behavior in the environment. Some physicochemical properties are also important to consider when designing appropriate analytical methods for classes of organic compounds. The text box to the right provides several sources of physicochemical values.
The behavior of organic compounds varies widely; however, some generalities can be made for subgroups of compounds. The behavior or organic compounds in the environment is affected by the physicochemical properties of the compound, including molecular weight and the presence of specific functional groups, such as alcohols, amines, carboxylic acids, and ketones. These properties and characteristics often affect a compound’s solubility, partitioning behavior, and reactivity in relatively predictable ways, allowing exposure assessors to easily conduct screening-level assessments to estimate which organic compounds might be of highest concern.
It is not the intent of this tool set to provide information relevant to all organic compounds. As mentioned above, organics comprise a highly variable group of substances with a large range of uses. Instead, this tool set focuses on tools for assessing exposures to five specific groups of organic compounds of concern, including:
- Dioxins, Furans, and PCBs
- Flame Retardants
- Perfluorinated Compounds (PFCs)
- Phthalates and Bisphenol A (BPA)
These compound groups are known to possess properties that increase the potential for human and ecological exposure and are addressed specifically in this tool set because of their relevance to developing policy and risk management efforts.
Many organic compounds identified above fall into a category known as persistent organic pollutants, or POPs, including PCBs, dioxins, furans, brominated flame retardants, and perfluorinated compounds (PFCs). These compounds have long half-lives in environmental media and are subject to long-range transport through the environment, meaning that exposure can occur long after the chemical is released and at locations far from the original source of the contaminant. Many persistent organic compounds also have a tendency to accumulate in biota, meaning that significant secondary exposures can occur through ingestion of foods from higher levels of the food chain, like fish, meat, and dairy products. Information specific to POPs is also provided in this tool set.
Volatile Organic Compounds, or VOCs, are another category that overlaps with the other groups described in this tool set. Similar to POPs, VOCs are defined by a physicochemical property that imparts unique fate and transport and exposure considerations—high vapor pressure at room temperature. Because of their high volatility, VOCs are common air pollutants in both outdoor and indoor environments. VOCs can be further categorized based on boiling point as very volatile organic compounds (VVOCs) and semi volatile organic compounds (SVOCs). Several substances including DDT, chlordane, and PCBs are classified as both SVOCs and POPs. Information specific to VOCs is also provided in this tool set.