Tire particles and 6PPD-quinone: measuring fish risk

Every rotation and braking event sheds a mixture of rubber, minerals and additives from tires. One additive, 6PPD, protects rubber against ozone; environmental transformation produces 6PPD-quinone, or 6PPD-Q. The compound is linked to acute toxicity in some salmonids when roadway runoff reaches streams. This is an important warning, but it does not justify treating every tire particle as one chemical or transferring one species’ sensitivity to all fish.

A rain-dependent pathway from road to stream

Particles accumulate on roads and are mobilised by rainfall. Some remain particulate while some compounds leach into water. Flow, antecedent dry period, storm intensity, drainage design, soils and treatment systems alter transport. Concentration can therefore change rapidly during a single event.

6PPD-Q is one studied hazard within a much broader mixture. Finding tire particles does not directly quantify 6PPD-Q, and measuring that compound does not describe all runoff toxicity. A defensible study combines targeted chemistry, hydrological context and, where appropriate, biological tests designed for the stated question.

Fish sensitivity differs sharply

Shankar and colleagues exposed several life stages of coastal cutthroat trout, Oncorhynchus clarkii clarkii, to 6PPD-Q under controlled conditions. Acute mortality occurred at concentrations ranging from tens to hundreds of nanograms per litre depending on stage, and swimming performance declined in some exposed juveniles. These values describe the tested protocol, stages and species; they are not a universal regulatory threshold.

The contrast with Foldvik and colleagues is informative. Atlantic salmon and brown trout alevins exposed for 48 hours to initial concentrations up to 12.16 micrograms per litre showed no mortality or substantial behavioural change. That does not establish an absence of chronic, reproductive or other life-stage effects. It demonstrates that taxonomic proximity alone cannot predict acute sensitivity.

Sample the event rather than the convenient day

Study design also needs a question that can be answered. Surveillance for occurrence, estimation of peak exposure and attribution of a mortality event require different timing and replication. A reference catchment may help, but only if land use, rainfall and hydrology are sufficiently comparable. Otherwise, the apparent control can introduce a second set of differences rather than isolate road influence.

A dry-weather grab sample may miss the primary exposure. Planning starts with roads, drains, outfalls, tributaries and sensitive habitat. It specifies the rainfall trigger, time from storm onset, sampling duration, flow and upstream-downstream locations. Sequential or composite samples may represent the hydrograph better than one bottle.

6PPD-Q analysis requires validated methods and contamination control. Container material, preservation, holding time, field blanks, duplicates and reporting limits belong in the record. Results below a limit should not be converted to zero. A concentration without flow does not provide transported mass.

Biological investigation documents species, life stage, season, temperature and co-occurring pressures. Mortality after rain still needs differential diagnosis: hypoxia, thermal change, pesticides, hydrocarbons, ammonia, infectious disease and trauma remain possible. Early, coordinated sampling by chemists, ecotoxicologists and fish-health professionals improves interpretation.

Intervene at source and along the pathway

Maintenance is part of exposure control. Sediment removal, vegetation management and bypass behaviour during intense storms can determine real performance long after construction. Monitoring should therefore include operational condition and storm volume, not only a concentration pair collected on one date. Benefits may extend beyond 6PPD-Q because the same pathway carries metals, hydrocarbons and suspended solids.

Where sensitive salmonids are present, prioritisation can combine road density, direct hydraulic connection, spawning or rearing habitat and documented storm timing. This screening identifies where analytical effort is most informative. It is not a substitute for measurement, and it should be revised as drainage networks or land use change.

Action need not wait for every mechanism to be resolved. Reducing direct runoff, slowing flow and promoting infiltration or treatment can decrease exposure to multiple road contaminants. Vegetated swales, filter media, ponds and specialised systems perform differently with design, maintenance, soil and climate. A facility should not be claimed effective for 6PPD-Q without appropriate inlet and outlet measurements.

The U.S. Environmental Protection Agency’s FY 2025–2028 plan coordinates analytical methods, research, risk assessment and work on alternatives to 6PPD. Substitution must preserve tire safety while avoiding a regrettable replacement or transformation product. It therefore needs toxicology, material performance and life-cycle thinking.

Communicate without turning a warning into a universal claim

For watershed managers, the useful question is local: which outfalls reach which habitats during which storms, and what species or life stages are present? Published studies guide priorities but cannot replace site data. Units, analytical matrix and biological stage should always accompany a number.

Public communication should distinguish 6PPD, 6PPD-Q and tire-wear particles. Lack of acute mortality in one species does not mean no ecological risk, while extreme sensitivity in another cannot be generalised. This precision supports targeted investment instead of diffuse alarm.

Vetofish can contribute to sampling design, fish surveillance and joint interpretation of health and environmental evidence. The principle is straightforward: measure during the relevant event, identify species and stage, then act on demonstrated pathways.