Posted by Anne LeHuray, October 11, 2015
In a rare win for science, the subscription news service Chemical Watch reports that
The European General Court has annulled part of the mandatory classification of the substance CTPHT [coal tar pitch, high temperature], following an appeal by 18 companies.
Its ruling means CTPHT is no longer classified as a substance with category 1 acute and chronic aquatic toxicity…
In its written opinion, which is available here, the Court took note of the very low aqueous solubility of coal tar pitch, which is in contrast with the assumption made by European Chemical Agency’s (ECHA’s) Risk Assessment Committee (RAC) that all of the individual PAHs dissolved in water and were therefore available to aquatic biota. According to the Court’s opinion, the highest tested actual solubility of coal tar pitch was 0.0014%, but the European Commission accepted ECHA’s individual polycyclic aromatic hydrocarbon (PAH) constituent method as the basis of the “category 1 acute and chronic aquatic toxicity” classification. The PAH constituent method resulted in a calculated solubility of 9.2%. The Court concluded
…such a value is not realistic, given that the maximum rate is 0.0014%.
According to Chemical Watch, the Court’s ruling is a landmark case in the EU because, apparently for the first time, it sets limits on the discretionary powers of EU government agencies when assessing chemical risks.
The ruling has significance to how the US EPA assesses PAHs as well.
How EPA Tries to Quantify PAH Risks
Results of chemical analysis of samples of air, water, soil or sediment from the environment often include results for a number of individual PAH compounds- usually the 16 PAHs included on EPA’s priority pollutant list established in approximately 1980. Although analytical results provided by laboratories report individual PAHs separately, PAHs do not exist as individual compounds except in laboratory settings. Whatever the source of PAHs (heating anything organic produces PAHs, from cooking food to composting to forest fires to burning fossil fuels), the compounds always are found as complex chemical mixtures containing perhaps hundreds of individual PAHs in addition to other substances.
In the late 1980s and early 1990s when the Superfund Program was ramping up its activities, the US EPA developed policies to help risk managers make decisions about potential risks to human health in remediation of legacy waste sites. In 1993 EPA released its Provisional Guidance for Quantitative Risk Assessment of Polycyclic Aromatic Hydrocarbons. This guidance recommended weighting concentrations of individual PAHs by an estimated “relative potency” of each and calculating estimated cancer risk values adding weighted total PAHs. For evaluation of possible ecological risks, “consensus standard” sediment quality guidelines (SQGs) were developed using a similar approach of adding concentrations of individual PAH compounds.
While these policies established consistent decision-making tools for clean up programs, the scientific basis for the policies was weak. It was quickly recognized that both the relative cancer potency and the SQG approaches were poor predictors of actual health or environmental risks when applied to real world materials that contained PAH mixtures.
In the environment, it was found that sediment toxicity rarely correlates with PAH concentrations. One of the reasons for this is that PAHs have low solubility in water and, therefore, have low bioavailability. In response to the failure of SQGs to be useful predictors of PAH toxicity, EPA issued in 2003 Procedures for the Derivation of Equilibrium Partitioning Sediment Benchmarks (ESBs) for the Protection of Benthic Organisms: PAH Mixtures followed in 2009 by Evaluating Ecological Risk to Invertebrate Receptors from PAHs in Sediments at Hazardous Waste Sites. The procedure involves a three-tiered process for determining whether the PAH mixture present in specific sediments are associated with actual toxicity. Methodologies for and the utility of this tiered approach have now been demonstrated in many locations with PAH-contaminated sediment, including at US Department of Defense legacy sites in the Anacostia River and at many other legacy clean-up sites.
PAHs are commonly identified as “toxic” in the aquatic environment. The toxic effect associated with PAHs is principally narcosis; that is, narcotic effects (Burgess, 2009). Narcotic effects (e.g., stupor, unconsciousness) are the basis of EPA’s ecological risk guidance documents.
In estimating potential risks to human health, bioavailability of different PAH mixtures is important. As explained in a conference presentation by Dr. Lynn Flowers (Flowers, 2005) “the US EPA does not have a general framework for addressing the toxicology of PAH mixtures.” The individual assessments and the 1993 Provisional Guidance currently available via EPA’s Integrated Risk Information System do not reflect the current understanding of PAHs or PAH mixtures. In 2010, the US EPA’s Science Advisory Board reviewed the relative potency approach first outlined in the 1993 guidance document and recommended that EPA concentrate on developing methods for evaluating risks associated with “whole mixtures.” What this means is that EPA and environmental scientists studying the issue have long understood that assessing possible health risks by adding “relative potency” factors of individual PAHs can be misleading. The trend in PAH risk assessment has been moving towards evaluating whole PAH mixtures.
The Overturned ECHA Method was EPA’s RPF Approach
According to the EU Court’s opinion, the government agencies evaluating the data for coal tar pitch were not satisfied with the existing data for whole mixtures of the Class 2 chemical and used an alternative approach. The Court described the alternative approach this way:
According to that approach, the 16 PAH constituents of CTPHT, which have been defined as priority substances by the United States Environmental Protection Agency (EPA), and for which sufficient effect and exposure data were available (‘the 16 PAH constituents’), were analysed separately in accordance with their aquatic toxicity effects. By applying a method consisting in finding the sum of the results obtained by the attribution of multiplication factors to the different PAHs in order to attach more weight to the highly toxic constituents of CTPHT (the summation method), that analysis showed, according to the RAC’s opinion, that CTPHT had to be classified as Aquatic Acute 1(H400) and Aquatic Chronic 1 (H410).
The Court’s description matches EPA’s Relative Potency Factor approach.
EPA’s Risk Assessments Need to Get the Science Right
Decades ago, when EPA was laying the foundation for how to assess chemicals and chemical risk, perhaps it was reasonable to use whatever data were available, regardless of quality or fitness for purpose or risk of bias in assessments. Perhaps. With several decades of research now available, one thing that’s been learned is that the toxicology and chemistry of individual PAHs studied in the lab do not reliably predict the toxicology and chemistry of real world PAH mixtures. This is important because PAHs are everywhere. In space dust (here and here) and wood smoke and fossil fuels and food. Today, the European Food Safety Authority (EFSA) is using an equivalent of EPA’s Relative Potency Factor approach to PAH risk assessment to regulate foods. EFSA reports that
…eighteen[EU] Member States submitted almost 10,000 results for PAH levels in different food commodities. An evaluation of these data performed by EFSA in June 2007 and updated in June 2008 demonstrated that benzo[a]pyrene [the index compound for PAHs] could be detected in about 50% of the samples.
But, as the EU General Court just ruled and EPA’s Science Advisory Board panel for the review of PAH mixtures found in 2010, the RPF approach gives misleading results – usually in the direction of overestimating risk (sometimes vastly overestimating). One of the SAB’s recommendations was that EPA fund a program of toxicology testing of representative real-world PAH-containing materials to to inform and ultimately replace the RPF approach. EPA followed through by funding the National Toxicology Program to establish a PAH mixtures project. NTP accepted the project, and seems to have expanded its focus beyond just PAH mixtures. Let’s hope PAH mixtures don’t get lost in the expansion.