South Coast Air Quality Management District
South Coast Air Quality Management District
BOARD MEETING DATE: November 17, 2000 AGENDA NO. 28
Procedures for Calculating Toxic Risk Reduction from Vehicle Emissions
SYNOPSIS:
On June 16, 2000 the Governing Board directed staff to develop procedures for calculating toxic risk reduction from vehicle emissions. The calculations are to be based on the unit risk factors as approved by the California Office of Environmental Health Hazard Assessment (OEHHA). This report presents the staff recommended procedure. The procedure entails determining the emission rates for carcinogenic chemicals or surrogates, and the calculation method to yield a total risk weighted emission rate for each vehicle engine family for toxic chemicals. The relative toxicity from different vehicles can then be compared. Emission rates are to be determined by CARB approved methods.
COMMITTEE:
Mobile Source: August 25, 2000; September 22, 2000; October 27, 2000; Reviewed
RECOMMENDED ACTION:
Barry R. Wallerstein, D.Env.
Executive Officer
BACKGROUND
The South Coast AQMD Governing Board on June 16, 2000 adopted three fleet rules. These included Rule 1192 – Clean On-Road Transit Buses, and Rule 1193 – Clean On-Road Residential and Commercial refuse Collection Vehicles. The Board adopted Rule 1186.1 – Less Polluting Sweepers and Rule 1194 – Commercial Airport Ground Access on August 18, 2000, and Rule 1196 – Clean On-Road Heavy duty Public Fleet Vehicles on October 20, 2000. These Rules require fleets with 15 or more vehicles to acquire less polluting or alternative fuel vehicles when procuring or leasing vehicles for operation in the District. The rules are part of a number of independent actions to reduce public exposure to both criteria and toxic pollutants.
As part of an allowance for conventionally-fueled vehicles under Rules 1192, 1193, 1194, 1196 and 1186.1, the Governing Board approved an approach that would allow for use of such vehicles that are equivalent to alternative fueled vehicles. This approach considers the equivalency of emissions of particulate, nitrogen oxides, and toxic risk. The Board directed staff to develop procedures for calculating the equivalent toxic risk of exhaust emissions from vehicles using conventional fuel and alternate fuels. Equivalent toxic risk is to be calculated based upon unit risk factors (URF) as approved by the California Office of Environmental Health Hazard Assessment (OEHHA), and in consultation with the CARB and OEHHA.
Public Process
As directed in the Board resolution, staff consulted with both the CARB and OEHHA staff. A technical working group comprising representatives from industry, the environmental community, and experts in the field, was established to gather input on technical aspects of the procedure. Additionally, three public consultation meetings were held to solicit input on developing the calculation procedure. Numerous helpful comments, critiques, and suggestions were obtained in this process.
PROPOSAL
The staff has consulted with CARB and OEHHA staff in developing the toxicity comparison procedure. A letter from these agencies is attached to the staff report as Appendix A. The following lists key components of the staff proposal.
Implement the calculation procedure as an interim method.
Monitor ongoing studies on characterizing the toxic emissions of conventional and alternative fueled vehicles, and consider co-funding studies to gather additional data on emissions and data on the toxicity of exhausts. These studies could include toxicity screening bioassays, such as genotoxicity assays and cellular toxicity tests, as well as animal toxicity tests.
Develop guidelines on how to analyze and interpret any data collected, and how to incorporate such data into the procedure to calculate toxic risk.
Consider the potential for using uncertainty factors in determining the equivalency cancer risk.
Calculation Method
Cancer Risk
Although there remain data gaps in adequately characterizing vehicle toxic emissions, the relative cancer risks of vehicle emissions can be estimated using an approach based on determining risk weighted emissions levels for the fuels and engine technologies under consideration. The risk weighted emission is determined by multiplying the emission rate of the toxic constituents in the exhaust by their respective cancer potency factor. The cancer potency factor is the unit risk factor (URF) developed by OEHHA.
This approach is similar to that used for risk assessments under the toxic hot spots program, where emission rates for known toxic substances are used to estimate ambient levels, and the ambient levels are multiplied by the URF, if available, to estimate a risk for cancer for an exposed individual at a specific site. The risks for each substance are then added to give an overall risk estimate. In the proposed method, the dispersion and estimation of ambient levels is left out, as the purpose of this procedure is to compare risks of emissions, and not to estimate a cancer risk at a specific geographic site. In any event, it is assumed the dilution and dispersion would be the same for similar vehicles using different fuels. Thus, rather than a risk of cancer, a risk weighted emission is calculated that can be used to directly compare the relative risk of emissions from similar vehicles using different fuels.
The calculation procedure for carcinogenic risk is shown schematically below.
When available, the URF for the whole exhaust is used. If there is no URF for the whole exhaust, then speciating the substances in the exhaust is done and the URFs for the carcinogens present are applied.
The emissions of toxic substances should be in units of average mass emissions per mile under standard testing conditions for vehicle certification.
The calculation can be used to directly compare the risk weighted emissions from conventional fueled and alternate fueled vehicles.
Non-Cancer Risk
For non-cancer risks and non-reproductive risks such as increased mortality and increases in respiratory disease, staff proposes to use mass equivalency of particulate emissions. Numerous epidemiological studies have found such effects that are directly proportional to the mass of exposure to particulate matter. Staff notes that there is already a provision for mass equivalence of emissions for particulate matter and nitrogen oxides when comparing conventional fueled to alternatively fueled vehicles contained in the adopting resolutions for the fleet rules.
Data For Calculations
Emissions data should by gathered using test protocols approved by CARB.
Replicate sampling and analyses should be conducted.
Laboratories certified by the Sate of California should be used for conducting chemical analyses.
Analytical methods that give the lowest detection limits and are generally available should be used.
For alternate fueled vehicles, the toxics to be analyzed include those compounds known to cause cancer and that may be present in the exhaust. At a minimum, this would include the following:
Acetaldehyde
Formaldehyde
Benzene
1,3-Butadiene
Metals: e.g., hexavalent chromium, nickel
Polycyclic aromatic hydrocarbons (PAH): those known to cause cancer
Nitro-aromatics: those known to cause cancer
Nitrosamines: those known to cause cancer
Staff will be available to work with applicants who are interested in equivalency demonstrations to develop testing protocols, including selection of engine families, lab methods, data analyses, etc.
Future Evaluations Of Procedures
Due to data constraints, staff considers the current proposal as an interim calculation procedure. As more data become available, staff recommends that the calculation procedure be periodically evaluated and revised, as appropriate. Therefore, staff recommends that in addition to monitoring several ongoing vehicle emission studies, consideration be given to co-funding a comprehensive speciation studyconsultation with stakeholders to better characterize the toxic species present in treated diesel exhaust and alternative fuel exhaust. Consideration should also be given to co-funding studies to gather additional data on the toxicity of emissions. These studies could include genotoxicity assays, cellular toxicity tests, and animal toxicity tests.
Staff proposes to work with CARB, OEHHA, and interested stakeholders to determine the studies that may be appropriate to conduct. As part of this process, staff also proposes to develop guidelines on how best to analyze the data collected, and how to incorporate any such analyses into the calculation procedure. Staff intends to report back to the Governing Board on study findings periodically, beginning July 2001.
POLICY ISSUES
When OEHHA developed a cancer potency factor for diesel exhaust, particulate matter was used as the measure for exposure to whole exhaust. The URF for diesel particulate matter thus incorporates the carcinogenic potency for all the toxic substances in the exhaust. In contrast to diesel emissions, exhausts from alternate fueled vehicles do not have a URF associated with the total exhaust. As a result, each toxic substance in the exhaust is identified in estimating a total cancer risk. Due to this limited data availability, several comments were made during the working group meetings and the public consultation meetings relating to the accuracy of staff’s recommended approach. The major issues are summarized below.
Completeness of Comparison
Some commented that there is no scientifically valid way to compare risks from different fuels, because there are no comparable health effects studies on alternative fuel emissions as there are for diesel exhaust. It is further argued that this lack of comparable information on different exhausts prevents the ability to make any meaningful comparisons of risks.
Staff notes, however, that assessments of risk from carcinogens and other toxic substances emitted from a variety of sources are routinely done with the same data constraints. That is, emissions of toxic agents are determined, and unit risk factors are used to estimate risks to exposed individuals, from both existing and new sources of emissions. When there is data on health effects of the whole emissions mixture, then that data is used. Examples of such mixtures with cancer risk estimates include cigarette smoke, coke oven emissions, and diesel exhaust. There is ample scientific and regulatory precedent to this approach, and this is the approach followed when assessing risks under the Toxic Hot Spots program.
Because there is no risk factor for whole exhaust from alternatively fueled vehicle emissions, the comparison of cancer risk may not be complete. Comments on this issue were received from CARB, OEHHA, and others. The main reasons are that speciating carcinogens only assesses the risk from the chemicals measured that are known carcinogens and have URFs assigned. It also does not take into account any interactions or synergistic effects that may be present. However, the approach prepared herein is consistent with methods used to assess risks on stationary sources with similar circumstances.
Appropriateness of Using the Diesel PM URF
Others stated that using the URF for diesel exhaust to estimate the relative potency of
diesel emissions, and using the URFs for individual toxic species that are present in other
exhausts, such as from CNG, is not a fair comparison, and suggested that a speciation
approach (that is, measuring emissions of individual toxic species and adding the risk
weighted emissions) be used for diesel exhaust as well. In this way, it would be a more fair
and equitable comparison of emissions.
Representatives from OEHHA, however, pointed out that if the risk from each known carcinogen were added, the estimated risk from exposure to diesel exhaust would be a factor of 10 or more lower than that estimated by OEHHA from epidemiology studies. There may be an underestimation of risk from diesel exhaust using this approach.
Additional comments were that the URF for diesel particulate is not applicable to current and future vehicle exhaust. It was stated that different fuel formulations and the use of treatment systems, such as particulate filters, result in lower emissions of toxic substances. Further, the emissions are substantially different than those in the health studies that OEHHA and the Scientific Review Panel used to establish the URF. Since the composition of current and future emissions may not be comparable to historic emissions, any risk factors derived from previous studies would not be relevant to current emissions or to emissions using future diesel technology.
Staff has concerns that although emissions rates for toxic components in diesel exhaust are becoming lower, it is not as clear that the profile of emissions is substantially altered. The same substances that are of health concern are found in the newer analyses as were found in older analyses. Many of these substances are known or suspected to cause cancer. These substances include benzene, formaldehyde, 1,3-butadiene, and PAH compounds. There are little data comparing the profiles of these substances in exhausts from different fuels and in vehicles with exhaust treatment systems.
Appropriateness of Using the CNG Speciated URFs
Commentors also suggested that the speciation approach applied to alternative fuel emissions may miss any interactive or synergistic effects of the mixture of substances in the exhaust, and the risk may be underestimated.
Staff appreciates these differences. However, there are no data to suggest that the relative carcinogenic potency of the mixtures of toxic chemicals in exhausts of alternative fueled vehicles are more or less than that which would be estimated using an additive approach. In fact, regulatory precedent for estimating risks from mixtures of carcinogenic chemicals emitted from a facility is to add the risks of the components of the mixture, unless there is evidence on the health effects of the mixture. Nevertheless, staff will continue to monitor the health literature in this regard.
Despite the limitations discussed above, CARB and OEHHA have advised staff that this is the only possible way to evaluate the carcinogenic potential for CNG emissions at this time, but it does not provide a complete comparison of risk to diesel exhaust.
Uncertainty Consideration
Staff received comments from OEHHA and others related to the consideration of uncertainty in determining the equivalence of toxic risk of emissions. This issue relates to the determination of how close the toxic risk from conventionally fueled vehicles needs to be to the risk from alternatively fueled vehicles to be considered equivalent. Should the risk be the same or lower, or is there some sort of uncertainty factor that could be used to account for the limitations of data related to the risk calculations and emissions measurements?
Staff recommends that the potential and appropriateness of including uncertainty be pursued with CARB, OEHHA, and other stakeholders. Staff will report back to the Board with recommendations on whether to use of an uncertainty factor in comparing toxic risk.
ALTERNATE APPROACHES
Several commentors provided suggestions regarding alternative methods for estimating air toxics equivalency. These methods and their associated pros and cons are described in the attached staff report.
RESOURCE IMPACTS
Implementation of the proposed procedure is not expected to have an impact on AQMD fiscal resources. It will, however, require staff resources to evaluate data and analyses related to comparing equivalent risks from vehicles submitted to the AQMD. Funding will also be needed for any AQMD contributions to future vehicle emissions testing. Should such a study take place, a separate Board action will be required. It is anticipated that the workload can be addressed with the existing level of staffing.
Staff Report
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