Bioaccumulation factors look trivial — divide one concentration by another. In practice most of the numbers reported in student theses and even in published papers are not comparable, because the denominators, the weight basis, and the handling of non-detects differ from study to study. This guide sets out the three factors you actually need, the assumptions behind each, and the traps that silently change your result by an order of magnitude.

The three factors, and when each applies

Factor	Formula	Reference compartment	Typical use
BCF — bioconcentration factor	C<sub>organism</sub> / C<sub>water</sub>	Water only	Controlled exposure, uptake from the dissolved phase
BAF — bioaccumulation factor	C<sub>organism</sub> / C<sub>water</sub>	Water, all routes	Field studies where diet also contributes
BSAF — biota-sediment accumulation factor	C<sub>organism</sub> / C<sub>sediment</sub>	Sediment	Benthic organisms, sediment-driven exposure

BCF and BAF share a formula but not a meaning. BCF is a laboratory concept: the organism is exposed to a dissolved substance and food-borne uptake is excluded by design (OECD TG 305). BAF is a field concept: the same ratio, but uptake from water, diet, and sediment ingestion are all inside the numerator. Reporting a field ratio as a "BCF" overstates what the number proves about the dissolved phase.

BSAF replaces water with sediment and, for hydrophobic organics, is usually normalised — lipid in the organism, organic carbon in the sediment:

BSAF = (C_organism / f_lipid) / (C_sediment / f_OC)

Without that normalisation, two fish of different lipid content in the same sediment will give different BSAFs for no toxicological reason. For trace elements lipid normalisation is not meaningful, so the unnormalised ratio on a dry-weight basis is the convention.

Units decide your answer

Three unit choices change the number more than any biological effect:

Dry vs wet weight. A fish muscle at 78 % moisture gives a dry-weight concentration roughly 4.5x the wet-weight value. Mixing a dry-weight tissue result with a wet-weight sediment result inflates BSAF by that same factor. Convert everything to one basis before dividing, and state which one you used.
Water concentration basis. BCF must use the dissolved fraction (filtered, typically 0.45 µm). Total water concentration includes metal bound to suspended particles that the gill never sees, which deflates BCF.
L/kg vs dimensionless. BCF in L/kg (µg/kg tissue over µg/L water) is standard. BSAF is dimensionless because both compartments are per kg. Never report a bare number without the unit — a "BCF of 500" is meaningless if the reader cannot tell whether it was dry or wet weight.

Non-detects: the LOD/2 convention

Trace-element datasets almost always contain values below the limit of detection. Three options exist and they are not equivalent:

Drop the sample. Biases the mean upward — you deleted only the low values.
Substitute zero. Biases downward and, worse, produces a division by zero if the denominator is the non-detect.
Substitute LOD/2. The pragmatic convention in ecotoxicology when censoring is limited (< ~15 % of values), and the one Labreador's Bioaccumulation module applies.

Whatever you do, flag it in the output table. A BSAF computed from an imputed denominator is not the same evidence as one from a measured denominator, and a reviewer will ask. If more than roughly 15 % of a variable is censored, LOD/2 stops being defensible and a censored-data method (Kaplan–Meier, ROS) is the honest choice.

Interpreting the value

The classic screening bands come from regulatory practice on organics, not from biology, so treat them as triage rather than truth:

BCF / BAF (L/kg)	Interpretation
< 100	Low potential
100 – 1000	Moderate; worth monitoring
1000 – 5000	Bioaccumulative
> 5000	Very bioaccumulative (REACH Annex XIII criterion for B/vB)

For BSAF, a value around 1 means the organism is roughly in equilibrium with the sediment; > 1 indicates net accumulation; < 1 indicates regulation or exclusion. Essential elements (Zn, Cu, Fe) are physiologically regulated, so a BSAF below 1 at high sediment load is an expected homeostatic result, not evidence of clean sediment. Non-essential elements (Cd, Pb, Hg) are the ones where a rising ratio is toxicologically meaningful.

A worked example

Sediment Cd = 2.4 mg/kg dw. Bivalve soft tissue Cd = 6.9 mg/kg dw.

BSAF = 6.9 / 2.4 = 2.9

Net accumulation, roughly threefold above the sediment. Now suppose the tissue result had been reported wet weight (85 % moisture) and used directly:

BSAF_wrong = (6.9 x 0.15) / 2.4 = 0.43

The same dataset flips from "net accumulator" to "excluder" purely through a unit slip. This is the single most common error in bioaccumulation reporting.

Checklist before you report

One weight basis across all compartments, stated explicitly
Dissolved (filtered) water concentration for BCF
Lipid/OC normalisation for organics; unnormalised dry weight for trace elements
Non-detect rule stated, and censoring rate reported
Sample size and dispersion (SD or 95 % CI), not a lone point estimate
Essential vs non-essential elements interpreted differently

Doing it in Labreador

The Bioaccumulation calculator in the Ecotoxicology hub computes BCF, BAF and BSAF from pasted or uploaded tables, applies the LOD/2 rule with explicit flagging, keeps dry/wet weight conversion visible, and returns per-element summaries with dispersion. Everything is processed in your browser — no sample data leaves the machine.

References

OECD (2012). Test No. 305: Bioaccumulation in Fish: Aqueous and Dietary Exposure. OECD Publishing.
ECHA (2017). Guidance on Information Requirements and Chemical Safety Assessment, Chapter R.11: PBT/vPvB assessment.
Ankley, G. T. et al. (1992). Bioaccumulation of PCBs from sediments by oligochaetes. Environmental Toxicology and Chemistry, 11(5), 615-629.
Helsel, D. R. (2012). Statistics for Censored Environmental Data Using Minitab and R, 2nd ed. Wiley.

Cite Labreador

If Labreador supported your analysis, please cite it: Labreador - Bioassay Analysis Platform (Version 1.6.0) [Computer software]. Zenodo. https://doi.org/10.5281/zenodo.21676846

BCF, BAF and BSAF: How to Calculate Bioaccumulation Factors Correctly