How to Conduct PBT and vPvB Assessment

Little Pro on 2019-02-08

PBT substances are substances that are Persistent, Bioaccumulative and Toxic (PBT). vPvB substances are substances that are very Persistent and very Bio-accumulative (vPvB). PBT substances and vPvB substances pose big threats to human health and the environment. Both EU and USA have set criteria for identifying a substance as PBT or vPvB. In this article, we will summarize how to conduct PBT and vPvB assessment.

Information Required for PBT and vPvB Assessment

The following info is usually required for PBT and vPvB assessment.

Item Screening level High tier studies
  • Ready biodegradability (OECD 301, OECD 306, OECD 310)
  • Inherent biodegradability (OECD 302B or OECD 302C)
  • Abiotic degradation such as hydrolysis, photolysis, etc;
  • Biodegradation QSAR models
  • OECD TG 309: Simulation test – aerobic mineralisation in surface water - preferred
  • OECD TG 308: Aerobic and anaerobic transformation in aquatic sediment systems
  • OECD TG 307: Aerobic and anaerobic transformation in soil
  • Field studies
  • Octanol-water partitioning coefficient logKow (i.e. If logKow<4.5, it is unlikely for a substance to reach a BCF value of 2000)
  • QSAR prediction
  • Bioaccumulation study in aquatic species (i.e. fish);
  • Toxicokinetic studies with mammals or;
  • Other information such a bioaccumulation study in terrestrial species or biomagnification info in the food chain.
  • Acute aquatic toxicity data
  • Mammalian toxicity data
  • Chronic aquatic toxicity data

P and vP Assessment

Tests on ready and inherent biodegradability are usually conducted first to assess the P potential of a substance at a screening level. If a substance is readily biodegradable ( ≥70% biodegradation measured as DOC or ≥60% biodegradation measured as ThCo2/ThOD) or inherently biodegradable (≥70 % mineralisation based on DOC removal within 7 d or ≥70% mineralisation based on O2 uptake within 14 days), we can usually conclude that the substance does not meet P criteria

In an inherent biodegradability test (OECD 302B or OECD 302C), lack of degradation (<20% degradation) may provide sufficient information to confirm that the P-criteria are fulfilled without the need for further simulation testing.

When biodegradability test data is not available, QSAR predictions can be used as part of weight-of-evidence approach. Predictions that the substance is not rapidly degradable would support the conclusion that the substance is potentially P/vP. In the contrary, predictions indicating that the substance could degrade rapidly would support the conclusion that the substance is not persistent.

When there is not sufficient info to assess whether a substance is P or not P at a screening level, degradation simulation studies in appropriate environmental media are performed to make confirmatory assessment. The simulation tests provide definitive degradation half-life that can be compared directly to the persistence criteria set out in REACH annex XIII (i.e, 40 days for fresh water and 120 days for fresh water sediment).

Simulation studies on ultimate degradation in surface water(OECD 309) are usually the preferred simulation studies unless the substance is highly insoluble in water.  The soil and sediment degradation simulation tests may be needed when results from simulation tests in water do not exceed the P/vP criteria but there are indications that the substance or its degradation products could persist in soil and sediment, meeting the respective P criteria.

B and vB Assessment

If an organic substance has a logKow less than 4.5 and no specific mechanism of uptake apart from hydrophobic partitioning is known, then the substance can be considered as not B and not vB. The assumption behind this is that the uptake of an organic substance in aquatic organisms is driven by its hydrophobicity. For organic substances with a log Kow value below 4.5 it is assumed that the B criterion, i.e. a BCF value of 2000 (based on wet weight of the organism, which refers to fish in most cases), is not exceeded. If measured logKow is not available, QSAR predictions can also be used as part of weight-of-evidence approach. 

If there is not sufficient information to make B or vB assessment at a screening level (i.e, logKow>4.5), a flow-through bioaccumulation test with fish according to OECD TG 305-I or OECD TG 305-II is usually conducted due to the best possibilities of reliably comparing the results from such test with the B/vB criteria.  If the measured BCF is higher than 2000 or 5000 the substance can be assigned to be B or vB.

In addition,  elimination rates and half-lives from mamallian toxicokinetic studies are acknowledged as useful metrics indicative of the bioaccumulation potential.

T Assessment

If a substance meets the criteria for classification as carcinogenic (category 1A or 1B), germ cell mutagenic (category 1A or 1B), or toxic for reprodu]ction (category 1A, 1B or 2)specific target organ toxicity after repeated exposure (STOT RE category 1 or 2) or has acute aquatic toxicity data E(L)C50 < 0.01 mg/L, we can directly conclude that the substance meets T criteria. 

If any EC50 or LC50 from acute aquatic toxicity studies < 0.1 mg/L, then the substance is a potential T candidate. Chronic aquatic toxicity studies shall be conducted if the substance has been confirmed to be P or B. If NOEC or EC10 from chronic aquatic toxicity studies (daphnia, fish, algae) < 0.01 mg/L, the substance will be confirmed as a T substance.

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