Trace-metal contamination rarely speaks for itself. A raw concentration of 25 mg/kg of zinc in sediment means very little until you ask the only question that matters: compared to what? Pollution indices exist to answer that question in a reproducible, comparable way. This post walks through the three indices most commonly used in sediment, soil and biota studies — CF, PLI and MPI — and what each one actually tells you.

Contamination Factor (CF)

The Contamination Factor is the simplest and the foundation of everything that follows. For each metal, it is the ratio of the measured concentration in the sample to a background (reference) concentration:

CF = C_sample / C_background

The background can be a local pristine site, a pre-industrial sediment layer, or a published geochemical baseline (e.g. Turekian & Wedepohl average shale). The interpretation, after Hakanson (1980), is:

CF < 1 — low contamination
1 ≤ CF < 3 — moderate contamination
3 ≤ CF < 6 — considerable contamination
CF ≥ 6 — very high contamination

CF is metal-specific. It does not tell you anything about the overall pollution status of a site — only how a single element compares to its reference.

Pollution Load Index (PLI)

Tomlinson et al. (1980) introduced PLI to collapse a vector of CF values into a single, site-level number. It is the geometric mean of the CFs of all metals considered:

PLI = ( CF_1 × CF_2 × … × CF_n )^(1/n)

Two properties make the geometric mean the right choice here. It is dominated by neither extreme highs nor extreme lows, and it goes to zero if any single CF is zero — a useful sanity property when one metal is below detection. Interpretation is binary and conservative:

PLI < 1 — no overall pollution (baseline-like)
PLI ≥ 1 — progressive deterioration of site quality

PLI is excellent for comparing sites along a gradient (e.g. upstream vs. downstream), because it produces one number per location.

Metal Pollution Index (MPI)

MPI is conceptually similar to PLI but is computed directly on concentrations, not on CFs:

MPI = ( C_1 × C_2 × … × C_n )^(1/n)

Because no reference is involved, MPI is not a contamination index in the strict sense — it is a summary of metal load. Its strength is that it lets you compare biota or matrices when a credible local background is unavailable or contested. It is widely used in fish-tissue studies, where "background concentrations" of essential metals are biologically meaningful and a CF would be misleading.

Choosing an appropriate baseline

Every CF-derived index inherits the quality of its baseline. Three rules of thumb:

Replicate the reference. A single reference sample sits somewhere inside its own natural variability; using it as "the" baseline propagates that noise into every CF you compute. Wherever possible, use the mean of multiple reference replicates for each metal.
Match the matrix. Sediment baselines for sediment samples; soil baselines for soils; species-specific baselines for biota. Cross-matrix references are not interchangeable.
State the source. Local pristine site, deep sediment core, or a published geochemical average are all defensible — but the choice changes the numbers and must be reported.

Reporting per-sample and per-group results

Field studies almost always involve replication: n samples per site, several sites per study. Per-sample CF/PLI/MPI values are the raw evidence, but the per-group mean ± SD is what belongs in a manuscript table. Reporting only individual values hides the within-site variability; reporting only group means hides outliers and the sample size behind each estimate. Best practice is to publish both, with sample standard deviation (n − 1) and explicit N.

Common pitfalls

Mixing units. CF is unitless, but only because numerator and denominator share units. Mixing dry weight and wet weight is a frequent silent error.
Treating BDL as zero. Below-detection-limit values should be imputed (commonly LOD/2) rather than set to zero, otherwise a single non-detect can drag PLI to zero.
Reading CF as toxicity. CF measures enrichment relative to background, not biological harm. A high CF for an essential nutrient (e.g. iron) is not necessarily a toxicological signal — pair indices with hazard-based metrics (THQ, HI) when human or ecological risk is the actual question.
Comparing across studies with different references. Two PLIs computed against different baselines are not directly comparable. Always report the reference values used.

In short

CF answers "how enriched is this metal at this site?"
PLI answers "how polluted is this site overall, relative to background?"
MPI answers "what is the total metal load here, regardless of reference?"

Used together, and reported with proper baselines and replication, they provide a clear, defensible picture of metal contamination without overstating what the numbers can support.

If you use Labreador to compute these indices for a publication, please cite it via the Cite Labreador button on the home page.

Understanding Pollution Indices: CF, PLI and MPI Explained