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Step 2: Toxic Air Contaminant Emissions Inventory and Source Testing

A Toxic Air Contaminant emissions inventory is a list of information about each Toxic Air Contaminant (TAC) that your facility emits. The full list of TACs regulated under the Cleaner Air Oregon program that you are responsible for reporting in your TAC emissions inventory is found in OAR 340-245-8020 Table 2. The TAC emissions inventory is required in order to complete the risk assessment for your facility, and must include the following:
  • A list of all emissions units that emit TACs. Emissions units that emit TACs are called Toxics Emissions Units, or TEUs, as defined in OAR 340-245-0020;
  • Annual and acute (24 hour maximum) production activity and/or fuel and material usages for each TEU;
  • All control devices, along with their associated capture and removal efficiencies;
  • Annual and acute emissions for all TACs, estimated using emission factor and/or material balance calculations;
  • Reference materials needed to verify the estimated emissions – e.g., source test data, emission factor references, Safety Data Sheets (SDS), and/or engineering estimate calculations materials.

CAO Emissions Inventory requirements are listed in OAR 340-245-0040.

​The CAO rules provide DEQ with the authority to require the owner or operator of any permitted or unpermitted source to submit an emissions inventory for the following purposes:

  1. Completing a risk assessment [OAR 340-245-0040(1)] – All new facilities are required to submit an inventory, and complete a risk assessment, prior to obtaining a Simple or Standard Air Contaminant Discharge Permit. Some General Permittees will also be required to complete an assessment for a new permit assignment. (See 'Step 1' FAQ section) When an existing facility is called into CAO, the facility is required to submit an inventory within 90 days of the notification date.
  2. Periodic state-wide emissions inventory [OAR 340-245-0040(2)]. As part of the CAO program DEQ may require all permitted air quality facilities to submit a toxics air contaminant emissions inventory every three years, with the intent to align with the National Emissions Inventory (NEI) reporting cycle, (e.g., 2020, 2023, 2026, etc.). For more information on this process:

​The emissions inventory should be provided to DEQ electronically using the AQ520 Form. DEQ recommends submitting the following accompanying materials to expedite the review process:

  • Cover letter detailing the approaches and methods used to calculate the emissions for your facility

  • Process flow diagram, clearly indicating the emission points that correlate to the Toxics Emissions Units (TEUs) in your submitted inventory

  • Source testing data summary reports and/or raw data

  • All calculations used to estimate emissions from your facility, explicitly stating assumptions made to estimate emissions

  • All reference materials required to verify calculations and/or appropriateness of methods and assumptions

  • All Safety Data Sheets (SDS) required to verify emission calculations

  • All technical specification drawings for TEUs and controls required to verify calculations

Provide all calculations as Excel workbooks, unless otherwise approved by DEQ. Please provide all of these materials to DEQ electronically. For files larger than 20MB, please contact DEQ for instructions for transfer by FTP.

It is important to communicate closely with your DEQ point of contact while preparing the inventory to ensure what is submitted will be approvable. Failure to submit all the necessary materials needed to verify the emissions estimates in your inventory may result in a follow-up request from DEQ, longer approval times, and/or enforcement action.

All Emissions Units (EUs) included in the operating or construction permit for a facility must be included in the emissions inventory as Toxics Emissions Units (TEUs) unless they do not emit any Toxic Air Contaminants (TACs) listed in OAR 340-245-8020 Table 2. If you are unsure if an Emissions Unit emits any of the TACs listed in Table 2, please contact DEQ. DEQ may provide a facility with TAC emissions information requiring that an EU be included in the CAO emissions inventory as a TEU.

Please note that TEUs that meet the criteria outlined in OAR 340-245-0060(3) for Exempt TEUs must be included in the emissions inventory, but emissions calculations for these TEUs are not required.

TEUs included in the TAC emissions inventory must be designated in the same manner as they are designated in the operating or construction permit. If the owner/operator would like to request a different designation they may do so in writing and the request must comply with the following:

  • A TEU may not be designated in such a way as to avoid CAO requirements.
  • An individual TAC emissions-producing activity that exhausts through multiple openings must be designated as an individual TEU.
  • Where multiple TAC emissions-producing activities exhaust through a common opening, stack, or control device, each activity may be designated as a single TEU or may be considered separate TEUs.
  • The list of TEUs is not limited to emission units in an existing source's operating permit and must include all TAC emitting processes and activities (e.g., a specific activity currently included as 'Aggregate Insignificant' in the current permit that emits TACs).

​The AQ520 Emissions Inventory Form includes a total of six production activity/material usage levels for a facility to report. For the purposes of performing a risk assessment, the CAO rules outline two time periods that apply to risk assessments and emission inventories:

  • Annual: Production activities or material usage over a one-year period for continuous operation.

  • Acute: Maximum production activities or material usage over a 24-hour period.

For each of these two time periods (annual and acute), AQ520 includes three types of estimated activity levels:

  • Actual:

    • Existing facilities must submit the actual production or usages in the calendar year preceding the year the facility is called in.

    • New facilities must submit emissions based on reasonably anticipated actual production or usage.

  • Requested PTE: Production or usage levels that a facility chooses as the basis for the risk calculations used for establishing Source Risk Limits and other CAO permit requirements.

  • Capacity: Maximum production or usage levels from a stationary source under its physical and operational design. These levels can be used for the risk assessment as PTE.  These levels must be used to determine if a source meets the criteria to be considered de minimis [OAR 340-245-0050(7)].

A facility may be required to include startup/shutdown emissions in the CAO emissions inventory based on OAR 340-214-0310. Please contact DEQ if you have questions.

​​DEQ does not currently have a database of approved emission factors available – below are a number of links to EPA and other state agencies resources that may be useful in establishing emission factors for activities at your facility.

USEPA Resources:

Other Resources:

Yes, facilities are encouraged to submit emission factors that represent the best available data for their permitted activities and operations. Industry research groups offer sources of emissions data, but be aware that DEQ requires the reference materials for each emission factor in order to confirm the selections are applicable.

Many sources of emission factors provide a 'rating' to indicate the uncertainty around the data collection, number of detections, etc. If possible, select emission factors with the highest ratings. If limited emission factors are available, select emission factors that most closely represent the process and materials of the Toxics Emissions Unit (TEU) at your facility.

If a wide range exists for an emission factor you may submit the value that you feel represents emissions at your facility (i.e., mean, median, or max value), but be aware this emission factor may be used for enforceable compliance demonstrations in your permit.

If emission factors have a poor rating or are otherwise not a reasonable representation of the process, source testing may be the best way to adequately estimate emissions.

If you are unsure if an emission factor is applicable for use at your facility, please contact DEQ. DEQ may have resources available or may indicate that source testing is required.

Documentation must be provided for all submitted emission factors. Final emission factors are subject to DEQ approval after review of documentation.

When reporting emissions for a chemical compound that is composed of one or more TACs, please provide the emissions for the compound itself in the emissions inventory. For example, 10.0 pounds of emissions of Lead Chromate (CAS 7758-97-6) per year from a coating operation should be reported in your emissions inventory, rather than separate emissions for 6.4 pounds of Lead (7439-92-1) and 3.6 pounds of Chromate (11104-59-9) per year.

​Estimating emissions from stationary diesel engines (i.e., diesel compression ignition engines) presents unique challenges related to assessing Diesel Particulate Matter (DPM) and the other Toxic Air Contaminants (TACs) present in these emissions. DPM is an aggregate pollutant, meaning that it is comprised of a number of chemical compounds including black carbon (soot) and multiple incomplete combustion byproducts; it also contains metal and Polycyclic Aromatic Hydrocarbon (PAH) TACs in variable quantities.

Diesel Particulate Matter emissions

DPM emissions must be included in the toxic emission inventory from diesel generators. In Oregon, source testing for particulate matter using DEQ Method 5 requires sampling of both the filterable (“front half") and condensable (“back half") Particulate Matter (PM) from an emissions source. Therefore, DEQ recommends including both the filterable and condensable PM when establishing an emission factor for risk assessment purposes. For newer Tier 2 federally compliant diesel engines still under manufacturer warranty, DEQ has approved the use of manufacturer emissions data for PM for the filterable fraction, and use of Hydrocarbon (HC) data as a conservative surrogate for the condensable fraction. For older engines a more conservative emissions factor may be required.

Toxic air contaminant emissions other than diesel particulate matter

Because of the compositional variability of DPM emissions, DEQ requires facilities to report emissions of DPM as well as emissions for all other TACs listed in OAR 340-245-8020 Table 2 for which emission factor data are available. Below is a table of recommended TAC emission factors for diesel internal combustion operations – these factors may represent conservative over-estimates of emissions from Tier 2 engines as they are based on testing performed before Tier 1 federal performance standards were implemented. These emissions data were taken from Ventura County Air Pollution Control District and South Coast Air Quality Management District AB 2588 reporting documents.

CAS No. Toxic Air Contaminant

Emission Factor

[lb/1000 gal]

7440-43-9Cadmium and compounds0.0015
1854-02-99Chromium VI, chromate, and dichromate particulate0.0001
7440-38-2Arsenic and compounds0.0016
7439-92-1Lead and compounds0.0083
7440-02-0Nickel and compounds0.0039
-PAHs (excluding Naphthalene)0.0362
50-32-8Benzo[a]pyrene[1]3.55 x 10-5
7440-50-8Copper and compounds0.0041
100-41-4Ethyl benzene0.0109
7647-01-0Hydrochloric acid0.1863
7439-96-5Manganese and compounds0.0031
7439-97-6Mercury and compounds0.002
7782-49-2Selenium and compounds0.0022
1330-20-7Xylene (mixture), including m-xylene, o-xylene, p-xylene0.0424
​-​Diesel Particulate Matter​See DPM section


[1] – Benzo[a]pyrene emission factor is derived from AP-42 Table 3.4-4 value using 138,000 BTU/gallon of diesel. These emissions are required to account for Noncancer health impacts – see below.
[2] – Ammonia value corresponds to equipment with Selective Non-Catalytic Reduction (SNCR) control; for equipment with Selective Catalytic Reduction (SCR) control substitute 1.4 lb/M gal, and for equipment without SNCR or SCR substitute 0.8 lb/M gal.

Noncancer health effects from Benzo[a]pyrene

In cases where facilities have reported PAH emissions in aggregate (as in the above table), DEQ has required benzo[a]pyrene emissions to be accounted for separately in order to capture noncancer health effects, both chronic and acute, of this TAC in the risk assessment. This is because the aggregate PAH Risk Based Concentration (RBC) in OAR 340-245-8010 Table 2​ ​only accounts for the cancer health effects.

Cold start emissions

Diesel emergency generators have higher emissions at start up (creating a 'black puff'). DEQ recommends using data on transient cold start emissions included in Section 3.4 (p.31) of this report on emissions from stationary diesel engines prepared by the California Energy Commission to develop TAC emissions from cold starts. These emissions should account for the transient increase in DPM and organic species present from incomplete combustion. Metals emissions should remain unaffected during this period. Please provide documentation on the methodology and assumed number of cold starts anticipated per emergency generator on both an annual and acute basis.

Operational considerations for modeling emissions from diesel engines influencing EF values

Emissions from diesel engines should be modeled based on fuel consumption because all of the TAC emission factors are based on pounds per thousand gallons of fuel used. DEQ has approved approaches that assume the worst-case manufacturer data for PM and HCs relative to engine loading, coupled with the highest fuel consumption rate (i.e., typically 100% load). In this way, the facility ensures that the modeled risk, and subsequent risk assessment, provides a worst-case scenario.

In general, DEQ recommends that facilities take a more conservative approach to estimating emissions from these TEUs as in many cases the emission factors and rates used to model risk from a facility will become enforceable limits in their permits. For facilities that have a large number of generators, DEQ may require source test validation of these emission factors and rates.

Modeling decisions about diesel engine operations should be made in conjunction with submitting the emissions inventory as they are critical for determining the DPM emission factor values used in the risk assessment.

​Some information in the emissions inventory can be considered confidential business information if it meets the criteria outlined in OAR 340-214-0130. All information that a facility claims as CBI should be identified clearly in the document title and on each page.  DEQ will review requests for CBI treatment against these criteria.

If DEQ has reviewed your submitted emissions data and determines that it does not represent the best available data, the following may occur:

  • DEQ requires you to obtain best available data by:
    • Performing source testing
    • Obtaining more appropriate emissions data available from alternate sources
  • DEQ may ultimately revise, with factual basis, your inventory with approvable emissions data [OAR 340-245-0030(4)(a)]

DEQ may require source testing if DEQ determines that submitted emissions data or control efficiencies do not represent best available data or are not sufficiently accurate for risk assessment purposes. You may request source testing at your facility if you feel it is necessary to get accurate emissions data.

Existing facilities that choose to perform testing and submit DEQ-approved source testing data as the basis of emission factors in their inventory are also required to submit the final inventory, along with a modeling protocol and risk assessment work plan (if applicable), 150 days from the DEQ notice date. [OAR 340-245-0030(1)(a)(B)]

DEQ understands the costs and time involved in source testing, and will work closely with facilities to ensure the needs and requirements are clear.  CAO is a new program that requires information not asked for in the past and not always readily available.  DEQ will be compiling information about source tests over time so that we can provide the best resources possible for understanding emissions.

​Existing facilities planning to perform source testing to develop emission factors for use in their inventories should communicate these intentions as soon as possible with DEQ after the notice date, and by no later than 15 days prior to the 90-day inventory submittal deadline in order to be granted the 60 day submittal extension without penalty.

​​Generally, DEQ or EPA methods are preferred for conducting a testing program. Established DEQ sampling methods for various pollutants are listed within Appendix B of the DEQ Source Sampling Manual. This list is not all-inclusive and may not reflect current method updates. EPA methods can be found on the Emission Measurement Center (EMC) website. A facility may propose to use alternative methods published by other public agencies and organizations but must demonstrate the proposed method is applicable.

In all cases, the proposed methods must be submitted to DEQ as part of the source testing plan at least 30 days prior to testing; 45-60 days prior is recommended for complex sampling programs. DEQ must approve the sampling program and methodologies in order for the information to be included as part of the submitted inventory.

​​Appendix A of the DEQ Source Sampling Manual contains the minimum source test plan requirements and a list of required statements. The information listed in Table A-1 must be included (as applicable). In general, testing conditions required for CAO as part of completing your TAC emissions inventory may differ from conditions required for criteria pollutant testing, and in some cases, DEQ may require multiple operating scenarios be tested. For these reasons, a thorough discussion and detailed justification of testing conditions is required as part of your source test plan. 

Source tests may yield results that are below the In-Stack Detection Limit (ISDL) for a given pollutant if it is not present or only present at very low concentrations. The Source Sampling Manual states (section 2.7.a. General Duration & Volume Requirements):

“Unless otherwise specified by rule, permit condition, or source test plan approval letter, all toxic air contaminants and hazardous air pollutants (HAPs) sampling programs must ensure adequate sample volumes so that the mass recovered is at least five (5) times the limit of detection for the analytical method chosen. Alternatively, the ISDL must be less than or equal to one-fifth (1/5) the emission standard."

Because many of the toxic air contaminants have stringent Risk Based Concentrations (RBCs), accurately assessing emission estimates on a pound per year basis is required; therefore determining the required sample volumes is critical to ensure appropriate detection limits are established prior to testing. A detailed discussion on sample volumes is required as part of the submitted source testing plan and must be approved by DEQ.

​Each test replicate that is below the In-Stack Detection Limit (ISDL) should be reported as less than (<) the detection limit value in the final testing summary submitted to DEQ for review. DEQ may allow substitution at less than the ISDL for the CAO risk assessment. Determination criteria and alternative reporting values for the purposes of the risk assessment can be found in Appendix G of the Risk Assessment Recommended Procedures document.

Cleaner Air Oregon frequently asked questions for facilities 

The frequently asked questions (FAQs) on this page are provided to answer common questions about completing air toxics risk assessments for the Cleaner Air Oregon program, and ensure consistency across assessments. As the Cleaner Air Oregon process evolves, DEQ will update this page with common questions or unique scenarios. If there is a question you would like to see answered or have feedback on a response below, please contact us at

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