AERA guide | Section 3: Toxicity assessment

Pollutants concentrations estimated using the procedures described above are compared to health benchmarks, which are values developed from scientific studies to estimate potential cancer and non-cancer human health risks. The ingestion intake values are combined with ingestion-based toxicity benchmark values to estimate cancer and non-cancer risks. Estimated air concentrations are compared inhalation health benchmark (IHB) values. For chronic resident or farmer exposure scenarios, the air exposure concentration is the annual average air concentration. For acute exposures, the air exposure concentration is a maximum hourly air concentration.

Inhalation health benchmarks (IHBs) are developed through scientific review of toxicity and exposure data. IHBs are generally derived as air concentrations likely to be without appreciable risk of harmful effects on sensitive humans. The IHBs for carcinogenic air toxics used by MPCA are developed so the additional lifetime cancer risk of a lifetime exposure to the IHB concentration is equal to or less than 1 chance in 100,000 (or 10-5).

Non-cancer effects are predicted using IHBs that are estimates of continuous inhalation exposure likely to be without appreciable risks of deleterious effects during a lifetime.

Exposures to air concentrations somewhat higher than the IHBs may also be without appreciable risk of harmful effects, but there is not enough information to know how much higher, if any, would be considered safe.

The MPCA consults with the Minnesota Department of Health as to which inhalation health benchmark (IHB) values to apply in AERAs. The MDH is charged in Minnesota rule to develop IHBs called health based values (HBVs) or health risk values (HRVs). These IHBs are “intended for use by public agencies or private entities in Minnesota as one set of criteria in evaluating risks to human health by chemical emissions to the ambient air”. The Statement of Need and Reasonableness (SONAR) for the HRV Rules provides a thorough description of the scientific methods and principles used to develop HRVs.

If there is not a value developed by the MDH, the MPCA and MDH have agreed upon a hierarchy of IHB information sources. The list below identifies the hierarchy of information sources for the IHB values used in AERAs.

  1. MDH Health-Based Values and Risk Assessment Advice for Air
  2. MDH Health Risk Values
  3. USEPA Integrated Risk Information System (IRIS)
  4. California Environmental Protection Agency Reference Exposure Levels and Cancer Potency Values
  5. Provisional Peer Reviewed Toxicity Values (PPRTVs) derived by USEPA's Superfund Health Risk Technical Support Center (STSC) for the USEPA Superfund program

Sources of IHBs that are not included in this hierarchy may be discussed qualitatively in an AERA. The qualitative information may be considered for risk management decisions when additional context is needed.

Updated values according to this hierarchy are input into the Risk Analysis Screening Spreadsheet (RASS) by MPCA staff approximately every year and posted to the AERA website. In some cases, it may be recommended to contact an MPCA risk assessor directly to receive an updated RASS. If a HHRAP-based multi-pathway analysis is being conducted, the toxicity values in the software need to be updated to correspond to the most current RASS inhalation toxicity values. A MPCA risk assessor should be contacted to get the most up-to-date ingestion toxicity values.

Some estimated cancer risks from exposure to air toxics may be elevated if the exposure occurs during the early years of life, before the age of 16. The MPCA follows MDH guidance to ensure that AERAs are protective of early life sensitivity to carcinogens. This guidance suggests that some unit risks or cancer-based air concentrations may require an adjustment to become protective of potential exposures to air toxics in early life. Other unit risks or cancer-based air concentrations may already have this adjustment incorporated.

As a screening level estimate, the MPCA suggests multiplying the summed facility-specific cancer risks (except for those estimates based on an already-adjusted IHB) by 1.6. If this screening level default adjustment results in cancer risk estimates that are above facility risk guidelines, contact the MPCA risk assessment staff. They will provide an updated list of unit risks that have already been adjusted for early life exposures. Once this information is received, these air toxics may be eliminated from the sum multiplied by 1.6. This default factor is described in more detail in the MDH Risk Assessment Advice for Incorporating Early-Life Sensitivity into Cancer Risk Assessment for Linear Carcinogens.

At a screening level, the MPCA RASS automatically sums all individual pollutant hazard quotients to determine one total hazard index across all non-cancer endpoints (e.g., neurological, respiratory, reproductive). This “summation” practice follows both MDH and USEPA guidance.

If a project proposer undergoes a reasonable amount of refinement in other areas (e.g., more refined emissions estimates or air dispersion modeling) and is still unable to calculate a non-cancer hazard index below facility risk guidelines, hazard indices may be separated and summed by non-cancer health endpoints. The estimated risks for cancer endpoints are summed regardless of the specific disease association.

In a real human system, the individual air toxics may interact in a manner that implies additivity (summation of toxic responses), a manner that implies a greater than summed interactive toxic response (synergy), or in a manner that implies less than a summed toxic response (antagonism). There are few data, however, that address the variety of potential interactions. In some cases, there may be two or more emitted air toxics with data available suggesting a synergetic response. In this case, if these air toxics are emitted at risk driver levels, MPCA may request a qualitative discussion of the potential underestimation due to the potential for a synergistic toxic response. Discussions of potential antagonism may also be included in the qualitative section of the AERA.

Acute IHBs with developmental endpoints are considered ceiling values not to be exceeded, and are identified as such in the RASS. Although many chemical exposures can have adverse effects to a pregnant woman and her fetus, chemicals that are developmental toxicants may directly harm a fetus. Unfortunately, most chemicals have not been tested for developmental effects; for many chemicals there is uncertainty regarding time of exposure and mass of a chemical necessary to generate developmental effects. Those chemicals for which sufficient scientific evidence was available to develop an IHB for developmental effects are noted in the RiskCalcs tab of the RASS.

The MPCA will review RASS workbooks to note whether air toxics are emitted that have acute IHBs for the developmental health endpoint (i.e., ceiling value), and note if the maximum modeled air concentration exceeds the ceiling value.

Diesel exhaust particulate

Cancer risks and non-cancer hazard quotients are currently estimated differently for diesel exhaust particulates. Non-cancer health effects are assessed for the complex mixture known as “diesel exhaust particulates” using an IHB value from the AERA toxicity value hierarchy. However, potential cancer-related diesel exhaust health effects are not assessed as a diesel exhaust particulate mixture, but are assessed from individual chemical constituents (e.g., dioxins/furans, PAHs) adsorbed on or absorbed within the particles. The different methods are used because of the uncertainty in cancer unit risk factors for the diesel exhaust particulate mixture. A portion of this uncertainty stems from the continued modifications to both engines and fuel formulations invoked to reduce diesel related emissions. Once a unit risk factor for diesel exhaust particulate mixtures is found acceptable by MDH, the chemical constituent-based assessment approach will be replaced with using a cancer unit risk factor based on the complex mixture.

MDH review of Air toxics without IHBs

In reviewing an AERA, MPCA staff, a member of the public, or the project proposer may find that there is an air toxic emitted without an IHB within the MPCA/MDH toxicity value information source hierarchy. If this is the case, MPCA may request the MDH to review available information to assess the potential to develop a value for that air toxic. The air toxics currently under review are listed and described on the MDH Chemicals Under Review webpage. The process to review scientific literature and develop an IHB is lengthy; in some circumstances an IHB may be recommended by MDH for use only on a given project so that an AERA may be completed and the additional risk estimate using a newly developed inhalation toxicity value would be included later.