Synthetic pyrethroids are a group of chemicals which act as neuropoisons, acting on the axons in the peripheral and central nervous systems. They are believed to interfere with sodium channels and the permeability of nerve cells so affecting the transmission of nerve impulses. Various pyrethroids are commonly used as insecticides in a variety of applications.

Some examples of pyrethroids commercially available include allethrin, resmethrin, d-phenothrin and tetramethrin (for insects of public health importance); cypermethrin. deltamethrin, fenvalerate, cyhalothrin, cyfluthrin, lambda-cyhalothrin and permethrin (mainly for agricultural insect pests). Cypermethrin is used in cage fish farming for sea lice treatments and in sheep dip. Thus entry into the aquatic environment may be through diffuse or point sources.

Synthetic pyrethroids are not routinely monitored for in the UK marine environment.

The pyrethroids tend to be of low solubility and adsorb readily to suspended solids and sediments. Once adsorbed, they are thought to be less bioavailable. However, at present no methods are available to analyse or assess the bioavailable fraction of the pyrethroids present.

Pyrethroids are generally rapidly removed from the water column (via degradation and sorption) with half-lives of a few days. The half-lives of pyrethroids adsorbed to suspended solids and sediments tends to be considerably longer, i.e. several months.

An exhaustive literature review on the toxicity of synthetic pyrethroids to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (Zabel et al 1988, WHO 1989). The most sensitive groups of organisms have been identified.

Both aquatic invertebrates (especially insects and crustaceans) and fish have been found to be highly sensitive to these chemicals. In addition, based on their mode of action, effects on aquatic mammals from certain pyrethroids could be expected. However, data on such effects do not appear to be available.

As pyrethroids adsorb readily on sediments, there is a need for sediment standards. There are some concerns over bioavailability and the effects on benthic and sediment dwelling organisms. This is an area where investigations are on-going and the results so far are uncertain.

Of the pyrethroids available for use in the UK, only two (permethrin and cyfluthrin) currently have statutory EQSs for the protection of aquatic life, although standards for a few other pyrethroids (e.g. cypermethrin) are under development.

Zabel et al (1988) reviewed data on the toxicity of permethrin and cyfluthrin to marine organisms and a summary of these conclusions is presented below. In addition, cypermethrin is a pyrethroid which is being increasingly used (such as in formulations for use against ecotoparasites on sheep and for sea lice treatments in cage fish farming). Further information on this is also presented below.

Invertebrates were found to be more sensitive to permethrin than fish. The lowest reported LC50 value was 0.02 µg l^-1 for newly-hatched estuarine mysids (as the test result concentrations were not analysed there was some uncertainty about these). The LC50s for Crangon septemspinosa and Pennies duorarum of 0.13  µg l^-1 and 0.22 µg l^-1, respectively, were based on measured concentrations in the water.

Available data on the toxicity to aquatic organisms was limited to two species of fresh water invertebrates and three species of freshwater fish. Invertebrates were the more sensitive organisms. Acute LC/EC50 values of >=5 ng l^-1 were found for freshwater organisms. No data were available on the toxicity of cyfluthrin to saltwater organisms.

Available data on the toxicity of cypermethrin to saltwater organisms indicate that acute LC50s for crustaceans and fish range, in general, from 0.05 µg l^-1 upwards (WHO 1989), although an LC50 as low as 0.005 µg l^-1 was reported for the mysid shrimp Mysidopsis bahia (Hill 1985). In addition, the LC50 for the lobster Homarus americanus could be as low as 0.003 µg l^-1, based on a geometric mean of available toxicity data (McLeese et al 1980).

SEPA Policy No. 30 provides EQSs for cypermethrin on its use in cage fish farming. The SEPA Fish Farming Manual describes the application of these EQSs.

The log Kows of these chemicals suggest that bioaccumulation should be high. However, while there appears to be uptake of some of these chemicals as a result of rapid metabolism, bioaccumulation does not appear to be a problem (WHO 1989).

Potential effects include:

• acute toxic effects on invertebrates, in particular crustaceans, and fish at concentrations above EQS values of 0.001 µg l^-1 for cyfluthrin and 0.01 µg l^-1 for permethrin in the water column. An EQS for cypermethrin in under development;
• accumulation in sediments where concerns exist about the effects of sediment dwelling organisms;
• identification of permethrin as an endocrine disrupting substance.

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