Permethrin Risk Assessment On A Coastal Area Biology Essay


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Permethrin is a pesticide, so is used to preventing, destroying, repelling or mitigating any pest. It was first synthesized in 1973 and first marketed in 1977. However, it was only registered with the U.S. Environmental Protection Agency (EPA) and its tolerances were established in 1979 (Rusiecki et al, 2009). According to USEPA (2006), permethrin is applied to agricultural (only 30%), residential and public health uses. Relatively non-toxic to mammals, it is however very toxic to aquatic life. Indeed, the Environmental Quality Standard proposed in the UK is 0,01 mg/dm3. Moreover, this substance is classified in a List II with the Dangerous Substance Directive (74/464/EEC) (House et al, 2000).

This environmental toxicology report will study the impact of a spill of 4000 tonnes of permethrin on a coastal area (it is supposed to be diluted into 120*109 L of water). Determination of the physico-chemical properties and of the toxicity of the pesticide through different exposures will allow an environmental risk assessment for immediate and long term. Then, according to the risks, different measures should be taken to address this problem.

I - Permethrin

1 - Physico-chemical properties

For a better understanding of the fate of the pollutant, the knowledge of its properties is essential.

Permethrin (figure 1), or 3-Phenoxybenzyl (1RS)-cis,trans-3-(2,2-dichlorovinyl) -2,2-dimethylcyclopropanecarboxylate (IUPAC name) belongs to the family of synthetic chemicals called pyrethroids. Its molecular formula is C12H20O3 and its molecular mass is 391.29 g/mol.


Figure : Permethrin molecule

Permethrin is a colourless crystalline solid under 34,5°C (melting point) and a brown viscous liquid under 200°C (boiling point) at a normal atmospheric pressure of 1013,25 hPa. Its vapour pressure at 25°C is 0,002 mPa, so it isn't volatile.

It is denser than seawater with a density of 1,29 g/mL while seawater density doesn't exceed 1,050 g/mL in basic conditions i.e. a salinity lower than 60 g/L and a temperature higher than 0°C (Sharqawy et al, 2010). So it sinks to the bottom of seawater.

Moreover, this insecticide is hydrophobic (insoluble in water) with a solubility of 6*10-3 mg/L. In fact, the octanol-water partition coefficient (Kow) equal to 1,26*106 shows that permethrin is more soluble in octanol (fat alcohol) than in water, so it is lipophilic and could be absorbed by the skin and bioacculmulated by animals.

Permethrin is also very adsorbable to the soil. In fact, its Koc = 1,00*105, this means that its concentration adsorbed by the soil is higher than its concentration in water.

2 - Permethrin uses

Permethrin is a pesticide that paralyses the nervous system of insects. It drives to their death (by contact or ingestion) or to their repulsion (Rusiecki et al, 2009). It is widely used in agriculture and public health with about 2 million pounds and more than 100 million applications made every year in and around U.S. homes. In agriculture, it's used on a variety of food/feed crops such as cotton, wheat, com, alfalfa..., and also on livestock. In the public health, it's used to control insects like mosquito (in order to avoid mosquito-borne disease such as dengue fever and malaria) in buildings, residential dwellings, aircrafts, but also on pets. For humans, it's used on clothing for repellent effects (considered to be the most efficiency), but also in lice shampoos, body lotions for scabies, dusts, emulsifiable concentrates, smokes, ultra-low-volume concentrates and wettable-powder formulations (Rusiecki et al, 2009). According to Moore et al (2009), 900,000 kg of permethrin is used annually with only 30% for agriculture settings.

II - Toxicology

In order to better understand the toxic effect of permethrin on environment, some useful toxicology data need to be known.

1 - Acute toxicity

According to IUPAC, acute toxicity is adverse effects of finite duration occurring within a short time (up to 14 days) after administration of a single dose (or exposure to a given concentration) of a test substance or after multiple doses (exposures), usually within 24 hours of a starting point.

1 - Toxicity category

Firstly, the toxicology category of permethrin will be determined to allow knowing the acute hazard of pesticide products in the environment. It is found with the concentration of a substance that is lethal to 50% of the organisms exposed after 24 hours (LC50 (24h)). Permethrin can have two different structures: cis and trans. For the rainbow trout:

LC50 (24h) = 0,025 mg/L (cis)

LC50 (24h) = 0,014 mg/L (trans)

So the cis version of permethrin is more toxic than the trans version. According to the table below, permethrin is 'very highly toxic' for aquatic medium because the LC50 (24h) found are between 0,01 and 0,1 mg/L.

Table : Toxicity category

Toxicity category

LC50 - 24hr (mg/L, mg/kg, ppm)

Super toxic


Very highly toxic (Extreme)

0.01- 0.1

Highly toxic


Moderately toxic


Slightly toxic


Not acutely toxic


2 - Score

The score allows knowing if the risk that the pollutant is toxic is serious or not by different absorption routes (oral, dermal, inhalation, water for aquatic animals). The table below shows the score in terms of concentrations of oral LD50, Dermal LD50 and aquatic LC50 (96h).

Table : Toxicity score


Oral LD50 (mg/kg, ppm)

Dermal LD50 (mg/kg, ppm)

Aquatic LC50-96hr (mg/l, ppm)


< 5

< 5

< 1









1 (low=2000)




0 (Not serious)




1 - Oral exposure (oral LD50)

The oral LD50 is the dose of a substance that is lethal for 50% of the organisms exposed after ingestion. The oral LD50 is determined for rats, mouse and birds.

This experiment shows that the permethrin is more toxic by oral routes if it is in corn oil than in water. In fact, for mouse males, oral LD50 = 490 mg/kg in corn oil while oral LD50 > 4000 mg/kg in water. This confirms that permethrin is lipophile because it is more soluble in corn oil and so can be in higher concentrations.

For mouse (males and females), oral LD50 = 96 mg/kg for cis-permethrin and 3150 mg/kg for trans-permethrin. The cis-permethrin is again more toxic than trans-permethrin and it is shows too with comparison of the percentage of cis/trans permethrin. If racemic cis/trans-permethrin is 75/25, oral LD50 = 310 mg/kg; 50/50, oral LD50 = 470 mg/kg and 25/75 = 1620 mg/kg. Even if the oral LD50 is higher when trans-permethrin predominantly (1620 mg/kg if racemic cis/trans-permethrin = 25/75 against 310 mg/kg if racemic cis/trans-permethrin = 75/25), the less toxic substance is the trans-permethrin (oral LD50 = 3150 mg/kg) that has low severity. However, both of the racemic cis/trans-permetrin 25/75 and trans-permethrin have the same score of 1.

For birds, oral LD50 are very high with >11275 mg/kg for mallard duck and >32000 for starlings.

The toxicity score of permethrin depends especially of isomer ratio and carrer. For rats and mouse, the toxicity risk with oral absorption of permethrin is medium to low. For birds, the toxicity of permethrin is not serious (score of 0). So permethrin hasn't the same toxicity for mammals and birds and depends also of the class. According to MDDEFP, permethrin is quickly metabolised and its residues are almost completly eliminated in few days in urine.

2 - Dermal exposure (dermal LD50)

The dermal LD50 is the dose of a substance that is lethal for 50% of the organisms exposed after contact with the skin. The dermal LD50 is determined for rats, mouse and rabbits.

Dermal LD50 for racemic permethrin without solvent is higher than 2500 mg/kg for rats and mouse and higher than 4000 mg/kg for rabbits. Bigger animals have a higher resistance to the permethrin without take into consideration the bioaccumulation throughout the food chain. With a solvent, racemic permethrin has a dermal LD50 of 750 mg/kg for rats.

Permethrin is more dangerous by contact with the skin with a solvent, so in aquatic medium, but still at the same score of 1 so isn't very toxic by contact. Indeed, Permethrin has a low toxicity for eyes and skin. Some tests on rabbits show that irritation disappear within 48 hours for dermal exposures and 72 hours for ocular exposures. However, it is very toxic for cats and can cause depression, vomiting, convulsion, and lead to death.

3 - Aquatic toxicity

A chemical is unlikely to be toxic to aquatic life if it meets the following criteria:

Table : Criteria for unlikely toxicity


EC50 Daphnia (48hr)>10mg/l (ppm)

LC50 fish (96hr)>10mg


EC50 algae (96hr)>10mg/l


EC50 Daphnia (21-day or 14-day)

- Aquatic LC50 (96h)

Aquatic LC50 (96h) is the concentration of a substance that is lethal to 50% of the organisms exposed after 96 hours in aquatic medium. Aquatic LC50 (96h) is determined for several fish species, aquatic crustaceans and sediment dwelling organisms.

Table : Organisms aquatic LC50 (96h)

Organisms exposed

Aquatic LC50 (96h) (mg/L)

- Fishes

Rainbow trout

0,0001 - 0,0005

Channel catfish


Large-mouth bass (fingerlings)


Mosquito fish




- Aquatic crustaceans

Opossum shrimps


- Sediment dwelling organisms

Chironomus riparius


For all these organisms, the aquatic LC50 (96h)is under 1 mg/L so the toxicity score is the higher one, that means that permethrin is seriously toxic for the organisms of aquatic medium. A tiny concentration of permethrin in water lead to the death of the half of a studied population in only 96h.

- EC50 Daphnia (48h)

EC50 is the concentration of a substance that affects a designated end point by 50%. For Daphnia magna, EC50 (48h) = 0,0006 mg/L, so < 10 mg/L. According to table 3, this result confirms that permethrin isn't unlikely to be toxic to aquatic life.

- EC50 algae (72h)

The results for EC50 algae (96h) haven't been found. EC50 (72h) = 0,0125 mg/L, so the EC50 (96h) would have a lower concentration and that would be lower than 10 mg/L and won't be untoxic to aquatic life (table 3).

For acute toxicity, the more dangerous exposure is the aquatic exposure that is highly toxic whereas oral and dermal exposures have a medium to low toxicity. Indeed, according to table 3, the acute toxicity isn't unlikely to be toxic to aquatic life.

For humans, dermal exposure to permethrin can cause irritations but they don't last more than 24 hours. Ocular exposures may result in redness, pain, or a burning sensation. Usually, ingestion and inhablation leads to pains, nausea and vomiting. So permethrin hasn't a high acute toxicity for humans.

2 - Chronic toxicity

Chronic toxicity is adverse effects following continuously (14 days or more) or repeatedly exposure.

According to MDDEFP, permethrin isn't toxic chronically and isn't carcinogen. Moreover, it isn't toxic for reproduction, doesn't have teratogen effects and isn't genotoxic.

3 - Environment fate

1 - Bioaccumulation potential

The bioaccumulation potential is the ratio between the concentration in organism and the concentration in food. It is determined with the Kow (octanol-water partition coefficient). Higher the Kow, more lipophilic the pesticide is.

For fishes, Kow = 1,26*106, so permethrin is more soluble in octanol (fat alcohol) than in water and could be easily absorbed by the skin and bioacculmulated by animals in their fatty tissues.

2 - Bioconcentration factor (BCF)

The bioconcentration factor indicates the amount of chemical likely to be uptaken by aquatic organisms. BCF = 300, that means that the concentration of permethrin in organism's fatty tissue can be 300 times higher.

4000 tonnes of permethrin was spilled and diluted in 120*109 L of water. The concentration of permethrin in water is 4*1012/120*109 = 33,3 mg/L. So the concentration in organism's fatty tissue can reach 9990 mg/kg. It means that if an aquatic organism eats another, the concentration that it will absorb through oral routes will be very high and will kill it. Only birds can eat contaminated fish and won't dead because oral LD50 of birds is higher than 11275 mg/kg.

3 - Biomagnification

Biomagnification is the progressive accumulation of a substance which has been acquired via the diet. Biomagnification is high because permethrin is persistent in organisms.

4 - Persistence

The persistence of a pesticide is determined with its half-life. Half-life is the time required for half of a compound to degrade or be eliminated from the environment of from the body of an organism.

The half-life of permethrin in the water column is about 19-27 hours, but in the sediment it can persist more than a year.

According to WWF (2000), a substance can be considered as a Persistent Organic Pollutant (POP) if its log Kow is greater than 4. For the permethrin, log Kow = 6,1, so it is a POP.

III - Discussion

The spill of permethrin is an immediate serious environmental problem because it takes place in an aquatic area. Indeed, its acute toxicity is very high for aquatic exposure (LC50 (24h)<0,1mg/L) whereas the toxicity of oral and dermal exposures is considered from medium to low. The concentration of permethrin in seawater is 33,3 mg/L so most of the organisms staying in this area will died quickly (about 24 hours) such as algae, invertebrates... Fishes can make longer displacement so don't stay for long in the contaminated area and still alive longer. The problem is that they will go in other areas and can be eaten by non-contaminated organisms that will become contaminated because of the high bioaccumulation potential of permethrin or die and be adsorbed in sediment. The contamination will be toxic only if preys stayed enough time in the contaminated area to bioconcentrate permethrin because the medium concentration is too low to be toxic (33 mg/L and need exceed 50 mg/L) and permethrin concentration in organism can reach 9990 mg/L (BCF = 300). In fact, permethrin isn't chronically toxic so there isn't a high risk if predator eat several fishes with low concentration (<50 mg/L) of permethrin (within the limit of the time of elimination). Nevertheless, to contaminate their predator, fishes have to be eaten rapidly because even if they stayed a short time in the contaminated area, the concentration of permethrin in seawater is so high that they probably will die.

Permethrin won't stay for a long time in water column. Indeed, it isn't persistent in water, with a half-life of about 24 hours and sinks to the bottom of water because of its density higher than seawater and be adsorbed to sediments because the Koc of permethrin is high (1,00*105). More easily adsorbable to sediments than soluble in water, it can persist in sediments more than a year.

The toxicity of permethrin will be very toxic to aquatic life in the first 24 hours because of its high concentration in water. In fact, after 24 hours, most of species will die and the the pollutant will be degraded in water. Even if the pollutant spread in food chain through because of aquatic organisms displacement, the concentration in organisms won't be toxic to them in most of case. Moreover, the aquatic exposure is higher than the oral exposure so organisms will die if the concentration is too high or contribute to bioaccumulation of others through food but at a level untoxic for uptaken. Nevertheless, the sediment will be very contaminated because of the spill that fall and of the dead organisms. Permethrin can persist more than a year in sediment so can continue to contaminate organisms. So the better things to do will be forbid fishing and in a zone larger than this contaminated area for 2 years to be sure that almost all the permethrin have disappeared. Even if it takes a long time, it is better than remove the sediment because it will release pollutant into water column. Water activities and baths should be forbidden as well because even if the concentration is too low to be toxic, dermal contact will lead to at least irritation.

To have a better idea of the toxicity of the pollutant, it should be useful to know the isomer ratio of the spill of permethrin, the exact place where the spill took place to know its distance with coast, the approximate temperature of water (for solubility) and local species. Moreover, winds and currents would be useful to know the future dispersion of the pollutant. More organisms in this area, more death there is and so more contaminated organic matter on sediment and more time to eliminate the pollutant.

IV - Conclusion

Permethrin is a pesticide that has a high acute toxicity for aquatic life. Fortunately, its half life in water is very short (about 24 hours) and death occurs mainly in the first days. However, it can persist in sediment more than a year and continue to be bioconcentrated and bioaccumulated by organisms. So, to avoid risk for human health, it would be better to forbid fishing, aquaculture and water activities for a least a year (2 would be better).

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