1986-01-00 Air Toxics Update #1

This page last reviewed July 30, 2008


Air Toxics Update #1





One of the most difficult environmental health problems facing California at this time is exposure to toxic substances. Working with toxics differs significantly from working with traditional pollutants because there are a large number of substances that are potentially toxic, there is limited health effects data, there may be no threshold or safe levels with many toxics, and there are no accepted guidelines to follow.

A toxic substance is a chemical, physical or biological agent that interferes with our life processes and may endanger human health. The adverse effects on health from exposure to toxic substances can be just as diverse as the substances themselves. Cancer is one effect of wide concern; others are birth defects, neurological damage, damage to the body's natural defense system, and other fatal diseases.

Only in the last few years has it become known that many potentially toxic substances are present in measurable quantities in the air we breathe. Concentrations measured thus far in ambient air are less than those measured in the workplace, but have been measured at levels comparable to health standards set for food and water.

Airborne toxic substances can create both immediate and long-term health effects. Sudden accidental releases of airborne toxics such as the Bhopal, India disaster can create immediate and extremely serious health effects. These kinds of incidents are being addressed in a statewide effort coordinated by the Office of Emergency Services.

Determining the long-term health effects that can occur from prolonged low-level exposure to airborne toxic substances is being evaluated by the Air Resources Board and the Department of Health Services. The statewide program to identify and control toxics in ambient air is the focus of this Program Update.


A policy flexible enough to deal with the diverse nature of the problems posed by toxic air contaminants has evolved over a number of years. In 1977, the Air Resources Board (ARB) appointed an independent panel of several experts to review what was known about carcinogenic air pollutants in California. Based on the panel's recommendations, follow-up research was done to explore further the relationship of cancer and air pollution and to determine the extent of the problem in California.

In 1982, the Board conducted a number of public meetings to discuss the science related to testing carcinogenic compounds and possible regulatory approaches to identifying and controlling air toxics. The effort to develop a program to deal with ambient toxics culminated in 1983 when the Legislature adopted and the Governor signed California's air toxics law (AB 1807). This legislation became effective in January 1984, and defines California's air toxics program (Health and Safety Code sections 39650 et seq., Food and Agriculture Code Sections 14021 et seq.).


With the enactment of AB 1807, the Legislature created a statutory mandate for the identification and control of air toxics in California as a separate and complementary program for the control of traditional air pollutants. The legislative intent of the law states that California's program to control toxic air contaminants should:

  • Identify Toxic Air Contaminants
  • Determine Priorities for Control
  • Promote Advanced Control Technologies and Alternative Processes
  • Assist Local Air Pollution Control Districts
  • Provide a Consistent Level of Protection Throughout the State.

In addition, the legislation declares a clear public policy that emissions of toxic air contaminants should be controlled to levels that prevent harm to public health. It also states that while undisputed scientific evidence may not be available to determine the exact nature and extent of risk from toxic air contaminants, it is necessary to take action to protect public health.

The framework of the air toxics program is the two-step process which separates the risk assessment decisions needed for the identification of airborne toxics from the regulatory control decisions which are necessary for risk management. This separation allows the risk assessment decisions about the potential adverse health effects to be made without having to take into consideration simultaneously the regulatory decisions that must follow. The separation also allows the risk management decisions regarding the appropriate level of control to be made without becoming immersed in the science-policy decisions which are a part of the toxics identification process.

As a basis for the identification process, toxic air contaminants are defined as those air pollutants which may cause or contribute to an increase in deaths or serious illness or which may pose a present or further hazard to human health. Substances identified by EPA as hazardous air pollutants under Section 112 of the Clean Air Act must be included as toxics in the California program.

To ensure that those substances of most concern are evaluated first, the toxics law requires that the following criteria be used for prioritizing compounds.

  • Risk of Harm to Public Health
  • Amount or Potential Amount of Emissions
  • Manner of Usage
  • Persistence in the Atmosphere
  • Ambient Concentrations

ARB has developed a list of potentially toxic substances to enter the AB 1807 process using these mandated criteria.

Another important component of this legislation is the clarification of roles and responsibilities of governmental entities for identifying and controlling airborne toxics. The key players in the identification phase are the Department of Health Services (DHS), the Air Resources Board (ARB), the Department of Food and Agriculture, and a nine-member independent scientific committee called the Scientific Review Panel (SRP).

In the control measure phase the ARB, working closely with local air pollution control districts, is responsible for developing control measures for all identified toxic air contaminants except those used as pesticides. Pesticides are evaluated in a similar process by the Department of Food and Agriculture. Following the ARB adoption of measures to control a specific toxic compound, the local districts must adopt equal or more stringent regulations for the stationary sources in their jurisdiction. Regulations to control airborne toxic emissions from mobile sources are the responsibility of the ARB.


In January 1984, the ARB endorsed a prioritization scheme for ranking potentially toxic substances based on the criteria spelled out in the law. This approach placed compounds of concern in California in two categories -- one for substances of significant concern and the second for substances of potential concern because of insufficient information.

Early in 1986, ARB staff recommended changes to the ranking system adding a third category. The first category now contains those substances designated by the Board as toxic air contaminants, the second group includes those compounds currently under review or waiting to be reviewed in the toxics identification process. Category three contains compounds of current interest in California which have insufficient data to proceed at this time.


Once the ARB and DHS select a compound to enter the toxics identification phase, the ARB circulates a request for relevant information on the health effects of the compound from the public. Approximately two months later the ARB formally requests DHS to evaluate the available health effects information for the candidate compound and to prepare recommendations regarding effects.

As a basis for its recommendations, DHS reviews all available scientific data associated with the health effects of the compound and makes an assessment of the health risks posed by exposure to the substance. This evaluation includes a discussion of whether a threshold exposure level exists below which effects do not occur.

Simultaneous with preparation of the DHS health evaluation, ARB prepares a comprehensive exposure assessment including information on the compound's usage, emissions or potential emissions, persistence in the environment, ambient concentrations and present or potential public exposure. These two segments -- the exposure assessment and the helath effects evaluation -- become the risk assessment report which is the technical foundation for determininig if the compound should be listed as a toxic air contaminant in California. The ARB makes copies of the draft report available for public comment before it is submitted to the Scientific Review Panel.

The next step in the process is the submittal of the report including the public comments and staff responses to the SRP. The SRP reviews the scientific procedures and methods used, the health and exposure data, and the reports conclusions. The SRP can request that ARB or DHS staffs correct deficiencies in the report, so that when the report is presented to the ARB for formal identification, the Board can be confident that the information in the report is scientifically sound. At the conclusion of its review, the SRP prepares recommendations for consideration by the Board during its decision-making.

The final decision regarding the listing of a candidate substance as a toxic air contaminant is made by the Air Resources Board after a public hearing. If the Board concurs with the findings that a compound is a toxic substance and does pose a health risk, the compound is listed by regulation as a toxic air contaminant in the California Administrative Code.

If the scientific evidence indicates a threshold exposure level exists for the compound below which no significant adverse health effects are anticipated, this will also be specified by the ARB as a part of the regulation. The time period for this identification phase is approximately 14 months. The diagram on the previous page graphically presents the formal steps in this phase.


Once a compound has been listed as a toxic air contaminant, the control decision or risk management phase of the air toxics program begins. With the help of local air pollution control districts and in consultation with affected sources and the interested public, ARB prepares a report on the need and appropriate degree of regulation for an identified toxic air contaminant. Some of the same subjects included in the identification report -- emissions, exposure, persistence -- will be included in this regulatory needs report in expanded detail plus information on:

  • Numbers and Contribution of Sources
  • Controls for Such Sources
  • Substitute Compounds
  • Adverse Impacts
  • Magnitude of Risk.

This regulatory needs report is the basis for the Air Resources Board decisions on the kinds of toxics control measures needed to reduce emissions of airborne toxics from stationary sources. This report also may serve as the basis for adoption of regulations to control toxic emissions from mobile sources such as setting emission standards for vehicular sources or standards for motor vehicle fuels.

As a part of the regulatory decision-making, the law requires the Board to consider whether a threshold for significant effects has been identified for the compound. For substances which have a threshold, sources are required to operate in a manner that ensures the threshold is not exceeded. Where a threshold cannot be demonstrated, control measures are identified that reduce emissions to the lowest achievable level unless an alternative level of emission reduction is necessary to prevent an endangerment of public health.

It is expected that the preparation of each regulatory needs report will take an average of twelve months. Depending on the complexity of the investigation on a particular substance, it may be possible to also develop a control measure during this time, if appropriate. Within six months of the ARB decision to adopt a control measure for stationary sources, local air pollution control districts are required to adopt regulations that are at lease equally as effective. Local district new source review rules must also include requirements for the control of new or modified sources of toxic air contaminants. The diagram below graphically presents the sequence of this phase.


Compound Identification Phase

  • Selection of Candidate Substances
  • Request for Health Effects Information from Public
  • ARB Request to DHS for Health Effects Evaluation
  • Publication of the Risk Assessment Report Containing Exposure and Health Effects Information
  • Public Review and Comment on Draft Report
  • SRP Review of the Report and Public Comments - Revisions if deficiencies are found.
  • SRP Findings Prepared and Submitted to ARB
  • ARB Public Hearing and Decision on Candidate Toxic Air Contaminant

Control Measures Phase

  • Investigation of Control Strategies by ARB working with local air districts and the interested public.
  • ARB prepares draft report on the need and appropriate degree of control.
  • ARB holds public consultation meeting on draft regulatory needs report.
  • ARB public meeting or hearing on the regulatory needs report and decision on control strategies.
  • ARB adoption of control measures for stationary sources, if appropriate.
  • Local air districts propose stationary source regulations based on ARB adopted measures.
  • Local district public hearing and adoption of equally or more stringent regulations.
  • ARB development and adoption of mobile source regulations, if appropriate.

For further information on ARB's toxic program, please contact Stationary Source Division, Emissions Assessment's Branch Chief, P.O. Box 2815, Sacramento, CA 95812 (916) 322-6023.

January 1986


Air Toxics Updates