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Are Standardized Test Guidelines Adequate for Assessing Waterborne Particulate Contaminants? Farhan R. Khan,* Kristian Syberg, and Annemette Palmqvist* Department of Science and Environment, Roskilde University, Universitetsvej 1, PO Box 260, 4000 Roskilde, Denmark specific limitations of standardized test methods that, while designed for chemical solutes, fail to capture the intricies of particulate exposures:
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EXPRESSION OF DOSE Mass is the recognized dose metric for solutes, but is likely inappropriate for expressing the exposure concentration for particulates. Particle numbers and surface area can differ between treatments with similar overall weights. Furthermore, smaller-sized particles are generally considered to be more hazardous than larger ones due to increased surface to volume ratio.3 Alternative metrics which may be more suitable, such as particle number or total surface area, are not currently considered within risk assessment frameworks,2 but the ramifications of incorrectly expressed doses are significant when considering the importance of determining dose−response relationships, which underpin the analysis and interpretation of standardized test results.
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TEST CHOICES Guidelines for solutes are not directly transposable to particulates and many aspects require reconsideration. As current protocols only account for chemically induced effects, the potential nonchemical impacts of particulates are not captured. If particulates, as have been proposed,3 exert toxicity via mechanical interactions (e.g., digestive disturbances following ingestion leading to an inability to process food and/or blockages of feeding or swimming appendages resulting in increased energy cost for maintenance), then conceivably this is manifested in timeframes longer than the duration of an acute test. Chronic guidelines are available and for particles may constitute the more appropriate choice. Similarly, currently prescribed model organisms may not adequately reflect how particulates and biota interact in the environment. In selecting species for particulate tests, greater focus on the organisms’ feeding mode and behavior is required (see ‘Uptake route and site of action’).
ften the first step in identifying the ecotoxicological risk and producing regulatory guidance for new chemical substances is the performance of standardized toxicity tests. Protocols described within the test guidelines of the Organisation for Economic Cooperation and Development’s “Effects on Biotic Systems”1 outline methodologies to determine toxicities, defined by specific end points and model organisms within the aquatic environment. These procedures are equipped for the assessement of chemical solutes, but critical deficiencies in standardized test guidelines are increasingly apparent in the determination of toxicities resulting from the dispersion of particulates in water. Our aim is to highlight these shortcomings and promote discussion of how testing methods could be adapted to an increasingly common form of contaminant. In the past decade two of the most prominent research areas in ecotoxicology have been engineered nanomaterials (ENMs, one dimension
