Why the basis matters

Trace-element results from ICP-MS, ICP-OES or AAS are very often reported on a dry weight (DW) basis, because samples are freeze-dried or oven-dried before digestion. People, however, eat food fresh. Ingestion rates in dietary exposure calculations are fresh-weight portions, so the estimated daily intake (EDI) and target hazard quotient (THQ) need concentrations on a wet weight (WW) basis.

Mixing the two is one of the most common errors in ecotoxicology papers. Because fish muscle or vegetables contain 70–95 % water, the difference is not a rounding issue — it is a factor of 4 to 20.

The conversion formula

With moisture content expressed as a percentage of the fresh mass:

C_WW = C_DW × (100 − moisture %) / 100
C_DW = C_WW × 100 / (100 − moisture %)

The term (100 − moisture %) / 100 is the dry matter fraction. For a sample with 75 % moisture it is 0.25.

Units do not change during conversion: µg/g DW becomes µg/g WW. Remember that 1 µg/g = 1 mg/kg.

Worked example: cadmium in fish muscle

A fish muscle sample contains 2.0 mg/kg Cd on a dry weight basis, with 75 % moisture.

C_WW = 2.0 × (100 − 75) / 100 = 0.50 mg/kg WW

Exposure parameters: ingestion rate IR = 0.0546 kg/day, body weight BW = 70 kg, USEPA oral RfD for Cd (food) = 0.001 mg/kg bw/day.

Correct (wet weight):

EDI = 0.50 × 0.0546 / 70 = 0.000390 mg/kg bw/day
THQ = 0.000390 / 0.001  = 0.39

Wrong (dry weight used directly):

EDI = 2.0 × 0.0546 / 70 = 0.00156 mg/kg bw/day
THQ = 0.00156 / 0.001   = 1.56

The same sample is either "no appreciable risk" (THQ < 1) or "potential health risk" (THQ > 1), depending only on whether the conversion was done. A reviewer who spots this will question every number in the paper.

Where moisture content comes from

Measured per sample — the best option. Weigh the fresh sample, dry it to constant mass, weigh again: moisture % = (fresh − dry) / fresh × 100.
Measured per batch or species — acceptable when individual values are missing; state it in the methods.
Literature values — last resort. Moisture varies with species, tissue, season and fat content, so cite the source and treat the result as an estimate.

Never guess a moisture value silently. If it is unknown, report the result on a dry weight basis and say that dietary risk could not be calculated on a fresh basis.

Which indices need which basis

EDI, THQ, HI, cancer risk — wet weight, because ingestion rates are fresh portions.
Comparison with food limits (for example maximum levels for Cd, Pb, Hg in food) — usually wet weight; check the regulation.
Pollution indices for sediments and soils (CF, PLI, Igeo) — dry weight, consistent with background and reference values.
Bioaccumulation factors (BAF, BSAF) — organism and medium must be on a stated, consistent basis; never mix DW tissue with WW tissue in one ratio.

Order of operations with non-detects

If your dataset contains values such as <0.02 or ND, substitute them first (for example with LOD/2) in the entered unit and basis, and only then convert to wet weight. The detection limit scales exactly like the measurement. Details are in our article on non-detects in heavy metal risk assessment.

What to report

A transparent methods section states:

the basis of the raw data (DW or WW) and of every reported index,
moisture content per sample or the source of assumed values,
the conversion formula,
the units of RfD (mg/kg bw/day) and CSF ((mg/kg bw/day)^-1).

Doing it in Labreador

The Risk Assessment calculator lets you choose the concentration basis of your data. When the data are dry weight, enter moisture content and the app converts to wet weight before calculating EDI, THQ and HI. Units such as µg/g, mg/kg or ng/g are recognised on import and converted explicitly, and both the PDF and CSV report list the basis and conversion that were used. Everything runs locally in your browser — no data leave your computer.