Dioxins Program

This page last reviewed July 21, 2015

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Dioxins in California

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Dioxins are highly toxic chemicals that are formed during the combustion (burning) of materials and the manufacture of certain chlorinated chemicals. Dioxins can be emitted from a variety of sources including waste incineration, some chemical manufacturing processes, cars and trucks, and other industrial sources that burn fuel.  Dioxins can also be emitted from other sources such as forest fires and residential wood burning.  For the year 2000, U.S. EPA determined that on a national level, the top three sources of dioxin included: backyard barrel burning of refuse (burn barrels), (33% of total dioxin emissions), medical waste/pathological incineration (25% of total dioxin emissions), and municipal waste combustion (5% of total dioxin emissions).

Dioxins can be inhaled directly or can contaminate vegetation that are a food source for animals and humans.  The Air Resources Board (ARB) has identified dioxins as a toxic air contaminant (TAC) and they are listed as hazardous air pollutants by the United States Environmental Protection Agency (U.S. EPA). Many studies, including U.S. EPA's Reassessment of Dioxins, have shown that dioxins can cause cancer and other health problems including birth defects and liver damage.

Air Monitoring 

In 2002, ARB initiated the California Ambient Dioxin Air Monitoring Program (CADAMP) to collect comprehensive information on the ambient levels of dioxins, furans, biphenhyls, and diphenylethers in populated urban areas.  View more detailed information, including the monitored results, and data analysis

What has ARB done to Reduce Dioxin Exposure in California?

The ARB has taken aggressive steps to reduce the public’s exposure to known sources of dioxins and other TACs.  In 1990, the ARB adopted the Dioxin Airborne Toxic Control Measure for Medical Waste Incinerators (link to ATCM) to reduce emissions of dioxins from medical waste incinerators by 99 percent.  At that time, medical waste incinerators were one of the largest known air sources of dioxins in California.  As a result of the control measure, the number of medical waste incinerators in the state dropped sharply.  Currently, there are only two small medical waste incinerators operating in the State.

In 2001, the ARB approved an Airborne Toxic Control Measure (ATCM) to reduce air emissions of dioxins and other toxic substances from outdoor residential waste burning.  Beginning January 1, 2004, no household trash or garbage can be burned outdoors at residences.  View more information on the ATCM. In 2003, U.S. EPA approved California’s State Plan (Plan) for Municipal Waste Combustors (MWC).  This Plan establishes emission limits for dioxins and other toxic air contaminants, includes requirements for testing, monitoring, recordkeeping and reporting, and operator training and certification.  Three MWC facilities exist in California, all of which employ state-of-the art air pollution control technology.

In 2003, testing was conducted on a catalytic and thermal oxidizer used for soil remediation.  These units are designed to remove chlorinated compounds from contaminated soil.  The potential for dioxin formation can result in the presence of these chlorinated compounds.  Results of the testing showed that dioxins emissions were well below the emission limits required by other sources such as medical and municipal waste incineration.  Additionally, many local air districts permit these units and require permit conditions on them.  Most catalytic and thermal oxidizers in California are used to clean up fuel contamination where chlorinated compounds are not present in the soil.  However, in some cases if chlorinated compounds are present, districts may require dioxin testing from a facility or manufacturer if they suspect that dioxins could be an issue. 

Facilities that emit dioxin are also subject to California’s AB 2588 “Hot Spots” Program.  The “Hot Spots” Program requires facilities to provide an emissions inventory of their TAC’s.   If a facility is determined to be high priority for potential health impacts, they are required to conduct a health risk assessment.  If these risks are significant, the facility must notify nearby residents and businesses of the risk assessment results.   View more information on California’s Hot Spots Program.

ARB, along with local air districts programs, have taken aggressive steps to minimize dioxins emissions from the largest stationary sources.  ARB will continue to evaluate other sources as additional information becomes available.

Mobile Sources

While ARB was addressing the significant contributors to dioxin emissions through the implementation of the above-mentioned ATCMs, staff were also investigating mobile sources’ potential role in the release of dioxins.   Past studies have shown mobile sources to be a relatively minor contributor to dioxin emissions.  However, diesel-fueled vehicles have been found to emit more dioxins than gasoline fueled vehicles.    

With the advent and popularity of advanced diesel emission control technology for meeting increasingly more stringent standards and for retrofit application to in-use vehicles and engines, concerns have also been raised that dioxin emissions may increase with the use of diesel particle filters (DPF) and selective catalytic reduction (SCR) systems. The premise is that these devices may provide an environment much more conducive to dioxin formation than the combustion chamber alone.  Additionally, certain compounds (i.e., chlorine and copper) in fuels, lubricants, or catalytic surfaces can be precursors to dioxin formation in diesel engines.  As a result, ARB decided to focus its efforts on clean diesel technologies to determine whether DPF and SCR systems could result in increased dioxin emissions. Recently, the U.S. EPA launched a similar effort to investigate dioxin emissions from clean diesel engines.

Overall, ARB staff found no evidence that DPF and/or SCR systems increased dioxin emissions from diesel engines, unless chlorine or copper are added to the air/fuel stream such as in the use of a fuel-borne catalyst for emission reduction.  Potential sources of chlorine include lubricating oil, fuel, and ambient air; sources for copper include brakes.  However, no increases in dioxin emissions were noted during normal operations.  The U.S. EPA also recently confirmed that no increases in dioxins were caused by the application of advanced aftertreatment controls on new diesel engines. Although there do not appear to be any remaining critical knowledge gaps with respect to dioxin emissions from the current diesel engine, ARB staff will continue to track and evaluate mobile source oriented dioxin information as it becomes available.