Research on the effects of PACs may roughly (and without strict borders) be divided into a field more oriented towards applied research where the ultimate ambition is to define at what PAC concentrations toxic effects occur, and a field of more basic research where the main purpose is to understand underlying toxic mechanisms. Applied research forms the basis for design of standardised protocols for toxicity tests, which in turn are used in regulatory work such as in risk assessment or establishment of threshold values for PACs in the sea. Ideally, there would be ongoing communication between the two fields, so that when scientists in basic research discover previously unexplored endpoints that are more sensitive to PACs than those currently included in standardised protocols for toxicity testing, scientists in the applied field would ensure that the protocols are updated. If the applied research does not incorporate relevant findings from the basic research there is a risk that toxicity tests in the applied field will be performed not using the most sensitive species/life stages of species, not using the most sensitive endpoints, and maybe not using the relevant exposure times needed to detect effects. The risk is then that data obtained from the standardised tests will be used as a true description of the PAC toxicity whereas in fact there is a high risk that the toxicity will be underestimated.
Another problem when assessing the toxicity of PACs is that the focus today, both in research and in environmental legislation, is limited to the set of 16 US EPA PAHs. Currently, the only PACs regulated in any environmental legislation are selected from these 16 PAHs although it has been pointed out that this is far from sufficient if we want to understand the fate and predict toxicity of PACs in the environment (Andersson and Achten 2015b, Stout et al. 2015).
IMO has set threshold values on some of the components in scrubber water that must not be exceeded if the water is to be released into the sea and PAH is one of them (IMO; Resolution MEPC.340(77) 2021). The threshold concentration for PAHs is expressed as phenanthrene equivalents (PAHphe) in the discharged scrubber water and is set to a maximum of 50 µg/L above the concentration in the surrounding water. The value should be normalised to a wastewater flow rate through the scrubber unit of 45 tons/MWh. It is not possible to trace which ecotoxicological data were used when setting the threshold value, and it is also worth noting that the value was the same in 2015 when the number of ships with scrubbers were around 250 as compared to close to 5000 in 2022 (Det Norske Veritas (DNV- GL)). The use of PAHphe was suggested by for an estimation of the combined toxicity effect of multiple PAHs (Fisher et al. 2011), but such an approach has been questioned by other scientists (Billiard et al. 2008, Le Bihanic et al. 2014a). PAHphe was developed using the concept of toxic equivalency quotients (TEQs) which are applied to express the toxicity of complex mixtures of compounds. One of the compounds, in this case phenanthrene, is used as a reference and given the toxic equivalency factor (TEF) of 1, and all other compounds receive a TEF expressing their toxicity in relation to the reference substance. By multiplying the TEF with the concentration of each compound, a TEQ, in this case PAHphe, is obtained. However, the use of toxic equivalents builds on the concept that all compounds included are toxic through the same mode of action but have different potency. This is the case for e.g., dioxins, but PAH toxicity occurs through several different modes of action. So, researchers active in the field of mechanistic studies on PAH toxicity strongly advice against the use of TEF and TEQ (Billiard et al. 2008, Le Bihanic et al. 2014a).
There are national and international threshold values for PAH in seawater which must not be exceeded if the sea area should be considered to have a good environmental status. The EU water framework directive has developed Environmental Quality Standards (EQS), which specify threshold levels for the maximum allowed concentrations (MAC), and the allowed annual average concentration (AA) of a set of 44 compounds, including 8 PAHs, in coastal sea water (Directive 2013/39/EU). The latest version of the EQS is from 2013, and no updates have been made since then. The eight PAHs with an EQS are: naphthalene, anthracene, fluoranthene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3-c,d]pyrene, and benzo[g,h,i]perylene, all of them belonging to the 16 US EPA PAHs.