Dioxins

Available Reports and Work ProductsFact Sheet

The Program works with its Co-permittees to help reduce discharges of dioxins from Santa Clara Valley municipal storm drain systems.  Our overall strategy focuses on pollution prevention measures that address controllable sources by reducing or preventing the formation of dioxins or their release into the environment.

What Are Dioxins?

Dioxin Molecules"Dioxin-like compounds" typically refers to a group of chemical compounds with similar chemical structures that share certain biological characteristics.  The three main categories of these compounds are 1) polychlorinated dibenzo-p-dioxins (PCDDs), 2) polychlorinated dibenzofurans (PCDFs) and 3) certain polychlorinated biphenyl (PCB) congeners with dioxin-like potency that are often referred to as dioxin-like PCBs.  PCDDs and PCDFs (collectively referred to as "dioxins") are not commercially produced but are unintentional by-products of combustion in the presence of chlorine and chemical production processes involving chlorinated compounds.  In contrast, PCBs, including dioxin-like PCBs, were intentionally manufactured for a wide variety of applications, and have different sources and probably a different distribution in local watersheds.  The Program has a separate task that addresses PCBs, including dioxin-like PCBs.

Problem Statement and Regulatory Background

White CroakerThe Clean Water Act (CWA) requires that states develop water quality standards protective of human health and the aquatic environment.  Section 303(d) of the CWA requires development and periodic update of a list of "impaired" water bodies that do not meet the standards.  The “303(d) list” designates all segments of San Francisco Bay as impaired by certain dioxin compounds (the State of California opposed the original 303(d) listing of dioxins in the Bay but was overruled by USEPA).  The primary basis for the impairment finding is an interim advisory on the consumption of fish from the Bay issued by the California Office of Environmental Health Hazard Assessment.  The advisory was issued because the levels of dioxins and other pollutants (e.g., mercury and PCBs) in fish caught in the Bay may be unhealthy to people consuming the fish.  However, dioxin-like PCBs have been found to contribute most of the overall dioxin-like potency in Bay fish and addressing PCBs has been and remains the priority.  Since 1994, dioxins and other pollutants have been measured in Bay fish every three years.  The level of dioxin-like toxicity in Bay fish has not changed significantly over that time period.

Dioxin Reassessment

Exposure to dioxins at high enough concentrations may cause a number of human health effects, depending on a number of factors including concentration, frequency, and duration of exposure.  Potential impacts include skin rashes, liver damage, cancer and reproductive and developmental effects.  However, the level of health risk to humans from exposure to dioxins is controversial.  Beginning in the early 1990s the USEPA began undertaking a scientific reassessment of these human health risks.  The National Academy of Sciences (NAS) reviewed a draft of this "dioxin reassessment" and released comments and recommendations in 2006.  The NAS concluded that although USEPA presented a comprehensive review of the scientific literature, the dioxin reassessment:

  • May have overstated human cancer risk;
  • Understated the uncertainty about the health risks; and
  • Failed to adequately justify the assumptions used to calculate the risks.

USEPA is currently preparing a response to the NAS review.  It is not known when the issues with the dioxin reassessment will be resolved.  In the meantime the degree of human health risk associated with dioxin exposure remains uncertain.

Sources of Dioxins

truckThere are many potential sources of dioxins to environment, including the following:

  • Diesel exhaust - dioxins are found in diesel fuel and exhaust.
  • Wood burning - combustion of wood (e.g., in residential fireplaces, forest fires) results in dioxin emissions due to the presence of trace chlorine in wood.
  • 2,4-D - trace dioxins are found in this chlorinated phenoxy compound used as a systemic herbicide for broad-leafed weeds.
  • Pentachlorophenol - dioxins have been found as a contaminant in this fungicide used as a wood preservative.  Dioxins may be released to the environment during treatment of wood by pentachlorophenol and through volatilization from treated wood (e.g., wood utility poles).
  • Polyvinyl chloride (PVC) - combustion of PVCs during intentional burning/incineration and accidental fires results in dioxins emissions.
  • Garbage burning - many wastes contain chlorine, and their combustion results in dioxin emissions.
  • Medical waste incineration - combustion of medical waste (e.g., PVC products) results in dioxin emissions.
  • Paper bleaching - dioxins are formed when chlorine gas is used to bleach paper during the paper production process.  Although there are no paper mills in the Bay Area, dioxins emitted by paper mills in other parts of the country could potentially reach the Bay Area, since atmospheric dioxins migrate worldwide.

Based on highly uncertain emission estimates by the Bay Area Air Quality Management District, larger contemporary sources of dioxins in the Bay Area may include diesel exhaust, residential wood burning and use of the pesticide 2,4-D.  In addition, a relatively large amount of dioxins possibly remains in the Bay Area environment due to practices potentially carried out widely in the past.  Such activities include garbage burning, medical waste incineration and the use of certain chlorinated pesticides (e.g., pentachlorophenol).  Because a large portion of the mass of dioxins in the Bay Area environment may have been released many years ago and current sources may primarily be air emissions that are not directly under municipal control, addressing dioxin sources is highly challenging for local governments.

Program Accomplishments

The Program has periodically tracked regional, state and federal efforts that address dioxins and encourages Co-permittees to also track and understand these programs (see Summary of Selected
Regional, State and Federal Efforts Relevant to Addressing Dioxins).  One particularly relevant project is the Association of Bay Area Government’s Bay Area Dioxins Project.  This regional effort has identified approaches for Bay Area municipalities to reduce the release of dioxins to the environment and implemented demonstration projects for selected pollution prevention strategies.

The Program also developed and distributed a survey to identify dioxins pollution prevention actions currently implemented by Co-permittees and any related policies, resolutions or ordinances already adopted (see Dioxin Control Program - FY 2005-2006 Activities).  The survey revealed that Co-permittees currently implement a wide range of activities that help prevent the formation of dioxins or their release to the environment.  Measures implemented may help address potentially larger contemporary sources of dioxins in the Bay Area, including diesel exhaust, residential wood burning and use of the pesticide 2,4-D.  For example, most Co-permittees are implementing policies for purchasing alternative fuel vehicles and equipment (e.g., compressed natural gas, biodiesel, and electric).  Additional policies and ordinances limit or prohibit the use of wood-burning appliances and fireplaces.  Co-permittees also generally have well-established Integrated Pest Management programs that discourage the use of pesticides (e.g., 2,4-D).

In addition, the Program actively participates in regional collaborative efforts and work cooperatively with other stakeholders to address dioxins and other pollutants of concern, including the Bay Area Stormwater Management Agencies Association (BASMAA) and the San Francisco Estuary Regional Monitoring Program (RMP).  For example, Program staff is representing BASMAA on the RMP's dioxins strategy work group.  In addition, the Program collaborated with other BASMAA agencies to develop
Introduction to Dioxins for Bay Area Stormwater Management Agencies.  This report emphasizes issues related to urban runoff in the Bay area, including regulatory context, impacts, sources, pathways, review of relevant Bay Area, national and international studies, and qualitative review of potential stormwater controls.  The Program also participated in the Clean Estuary Partnership, which developed a Conceptual Model/Impairment Assessment Report on dioxins in San Francisco Bay.  The report provides a detailed analysis of the status of the impairment and associated uncertainties, and presents a conceptual model that describes sources, estimated loads, and processes that affect the fate of dioxin compounds in the Bay.