Risk assessment Risk assessment is the process by which one estimates the probability of some adverse effect, such as that of a contaminant in the environment. Environmental risk assessments are conducted to help protect human health and the environment and are often used to assist in meeting regulations such as those stipulated by CERCLA in the United States. Risk assessments may take place retroactively, i.e., when assessing the contamination hazard at a superfund site, or predictively, such as when planning waste discharges. The complex nature of chemicals mixtures in the environment presents a challenge to risk assessment. The<unk>  approach was developed to help assess the<unk> ity of DLCs and other environmental contaminants with additive effects and is currently endorsed by the World Health Organization Human health Human exposure to dioxins and DLCs is a cause for public and regulatory concern. Health concerns include endocrine, developmental, immune and carcinogenic effects. The route of exposure is primarily through the ingestion of animal products such as meat, dairy, fish, and human breast milk. However, humans are also exposed to high levels of “natural dioxins” in cooked foods and vegetables. The human diet accounts for over 95% of the total uptake of TEQ. Risks in humans are typically calculated from known ingestion of contaminants or from blood or adipose tissue samples. However, human intake data is limited, and calculations from blood and tissue are not well supported. This presents a limitation to the<unk>  application in risk assessment to humans. Fish and wildlife DLC exposure to wildlife results from various sources including the atmospheric deposition of emissions (e.g. waste incineration) over terrestrial and aquatic habitats and contamination from waste effluents. Contaminants then bioaccumulate up the food chain. The WHO has derived<unk> s for fish, bird, and mammal species, however differences among taxa for some compounds are orders of magnitude apart. Compared to mammals, fish are less responsive to mono-ortho PCBs. Limitations The<unk>  approach DLC risk assessment operates under certain assumptions which attach varying degrees of uncertainty. These assumptions include: *Individual compounds all act through the same biologic pathway *Individual effects are dose-additive *Dose-response curves are similarly shaped *Individual compounds are similarly distributed throughout the body<unk> s are assumed to be equivalent for all effects, all exposure scenarios and all species, although this may not be the reality. The<unk>  method only accounts for<unk> ity effects related to the AhR mechanism - however, some DLC<unk> ity may be mediated through other processes. Dose-additivity may not be applicable to all DLCs and exposure scenarios, particularly those involving low doses. Interactions with other chemicals that may induce antagonistic effects are not considered and those may be species-specific. In terms of human health risk assessments, estimates of relative potency from animal studies are assumed to be predictive of<unk> ity in humans, although there are species-specific differences in the AhR. Nevertheless, In vivo mixture studies have shown that WHO 1998<unk>  values predicted mixture<unk> ity within a<unk>  of two or less A probabilistic approach may provide an advantage in the determination of<unk>  because it will better describe the level of uncertainty present in a<unk>  value The use of<unk>  values to assess abiotic matrices such as soil, sediment, and water is problematic because<unk>  values are primarily calculated from oral intake studies.
Toxic equivalency factor
Fractional kill
Equivalent dose
Dose area product
Median lethal dose
Maximum acceptable toxicant concentration
Therapeutic index
Dose profile