Pyrethrins: Uses, Health Effects, Safety and Regulation

Pyrethrins are natural insecticides, a mixture of six chemicals from some chrysanthemum flowers. People use them against mosquitoes, fleas, flies, moths and ants. They are of low acute toxicity to people and low chronic toxicity overall. Still, the health effects depend on the route of contact. The Environmental Protection Agency classifies them as not likely to cause cancer. How long they persist in the environment depends on light and on the medium. They are approved in the European Union and regulated in the United States through EPA classification and registration review. Residue limits in food apply to total pyrethrins, and a few simple steps help you use them more safely.

What Are Pyrethrins?

Pyrethrins are natural insecticides, a mixture of six chemicals toxic to insects that occur in some chrysanthemum flowers. Pyrethrins contain pyrethrin-I and pyrethrin-II as their two most prominent active ingredients. Cinerin I and II and jasmolin I and II are the other four components. Jasmolin is one of the six active components, together with cinerin.

Pyrethrins are generally separated from the flowers, but they typically contain impurities from the flower. Whole, crushed flowers are known as pyrethrum powder. Pyrethrins have been registered for use in pesticides since the 1950s. Anita Tewari wrote in Toxicology and industrial health about pyrethrin and pyrethroid toxicity.

How Do Pyrethrins Kill Insects?

Pyrethrins kill insects by exciting the nervous system of any insect that touches or eats them. This excitation quickly leads to paralysis and ultimately death. Pyrethrins and pyrethroids act mainly through prolonged activation and opening of voltage-dependent sodium channels.

Pyrethrins and pyrethroids are approximately 2000 times more toxic to insects than to humans. Insect sodium channels are more sensitive to them. Toxicological reviews carried a 2005 paper reporting low acute toxicity in people. Enzymes in the insect swiftly detoxify the natural pyrethrins. Piperonyl butoxide is a synergist added to pyrethrin formulations that enhances absorption. A knockdown dose does not mean a killing dose, so an insect can recover from it.

How Do Pyrethrins Differ From Pyrethroids?

Pyrethrins differ from pyrethroids in origin, potency and stability, and toxicity to mammals. Pyrethrins come from chrysanthemum flowers, while pyrethroids are semisynthetic derivatives of the chrysanthemumic acids. Allethrin is a common synthetic pyrethroid insecticide, and it was the first commercial pyrethroid, synthesized in Japan in 1953.

Attribute	Pyrethrins	Pyrethroids
Origin	Natural, from chrysanthemum flowers	Semisynthetic derivatives of chrysanthemumic acids
Potency and stability	Inactivated by light and air	Greater potency and more resistant to degradation
Toxicity to mammals	Higher than most pyrethroids	Less toxic to mammals

Pyrethroids are more resistant to degradation than natural pyrethrins. They display greater potency and can be produced cost-effectively on a large scale. Type II pyrethroids are the exception, since they are generally more potent, environmentally persistent and toxic to humans. Tomoya Yamada reported in Toxicology research on liver tumours that pyrethroids and pyrethrins induce in rodents.

What Are Pyrethrins Used For?

Pyrethrins are used against mosquitoes, fleas, flies, moths and ants. Pyrethrins come in foggers, sprays, dusts and pet shampoos. Buildings, crops and ornamental plants receive them as well. Certain pets and livestock receive them too. Head lice products regulated by the Food and Drug Administration contain them. The Food and Drug Administration is the US agency that regulates pyrethrin head lice products.

About 2,000 registered pesticide products contain pyrethrins, according to an NPIC fact sheet reviewed in November 2014. Pyrethrin compounds have been used primarily to control human lice, mosquitoes, cockroaches, beetles and flies. Some pyrethrin products can be used in organic agriculture. Anita Tewari of the Department of Veterinary Public Health & Epidemiology lists inhalation, ingestion and dermal routes.

How Toxic Are Pyrethrins to People?

Pyrethrins are of low acute toxicity to people, and they are of low chronic toxicity overall. A 2005 study by Alex T Proudfoot in Toxicological reviews found that convulsions may occur if substantial amounts are ingested. Dose and route decide how toxic pyrethrins are to people.

Oral LD50 values of pyrethrins in rats range from 200 mg/kg to greater than 2,600 mg/kg. The lowest lethal oral dose of pyrethrum is 750 mg/kg for children and 1,000 mg/kg for adults. The oral LD50 of pyrethrum in mice is 370 mg/kg.

The most severe poisonings have been reported in infants, who are not able to efficiently break down pyrethrum. There are currently no data to suggest that children have increased sensitivity specifically to pyrethrins. On broken skin, pyrethrum produces irritation and sensitization, which sun exposure further aggravates. Anita Tewari reported in 2024 in Toxicology and industrial health that exposure routes matter. Sustained skin or inhalation exposure, or direct contact with open wounds, results in higher toxicity to mammals.

What Is the Acceptable Daily Intake of Pyrethrins?

The acceptable daily intake for pyrethrins is 0 to 0.04 mg/kg body weight. The Joint FAO/WHO Meeting on Pesticide Residues is the joint expert panel that sets intake values for pesticide residues. It established this ADI for the tested blend of refined pyrethrum extract.

The ADI rests on a NOAEL of 100 ppm, equal to 4 mg/kg bw per day. That NOAEL comes from a long-term rat study with a safety factor of 100. Thomas G Osimitz reported in Critical reviews in toxicology that high doses produce liver tumors in female rats.

An acute reference dose of 0.2 mg/kg bw was allocated. It rests on a NOAEL of 20 mg/kg bw for acute neurotoxicity in rats. That value used a safety factor of 100. The acute and long-term toxicity of the pyrethrins differ significantly, so the two values are set separately. The acute reference dose is five times the upper ADI value.

What Are the Health Effects of Pyrethrins?

The health effects of pyrethrins depend on the route of contact. Pyrethrins are of low chronic toxicity to humans overall. A 2024 study titled Respiratory system: Highly exposed yet under-reported organ in pyrethrin and pyrethroid toxicity lists three exposure routes. Key effects include:

• Skin contact: Pyrethrins on the skin can be irritating and can cause tingling or numbness at the site of contact.
• Eye exposure: Children who got lice shampoo containing pyrethrins in their eyes experienced irritation, tearing, burns, scratches to the eye and blurred vision.
• Inhalation: Inhaled pyrethrins cause irritation of the respiratory passages, runny nose, coughing, difficulty breathing, vomiting and diarrhea.
• High exposure: Inhaling high levels can cause asthmatic breathing, sneezing, headache, nausea, incoordination, tremors and convulsions.

Dogs fed extremely large doses of pyrethrins have experienced drooling, tremors, uncoordinated movement and difficulty breathing.

Can Pyrethrins Cause Asthma or Allergies?

It depends. Research has not found a link between pyrethrins and the development of asthma or allergies, according to NPIC. People exposed to pyrethrins have reported some of the same symptoms associated with asthma. Pyrethrum has caused allergenic problems, which are the most common problems in humans.

Alex T Proudfoot of City Hospital notes that the possibility of hypersensitivity has been debated for many years. A few clinical reports support this suggestion, but the limited epidemiological evidence available is against it. Anita Tewari of the College of Veterinary Science & Animal Husbandry found a paucity of data on overall pulmonary health. The evidence on respiratory effects is therefore weak. Inhalation is the route that produces asthma-like breathing.

How Should Pyrethrins Poisoning Be Treated?

To get pyrethrins poisoning treated, start with decontamination, which should be among the first management measures in any pesticide exposure. Give pyrethrins exposure close monitoring, observation and supportive care. Wash local skin irritation with soap and water. Follow the First Aid instructions on the product label carefully.

Contact the Poison Control Center at 800-222-1222 if an exposure occurs. The National Pesticide Information Center is the US pesticide information service run by Oregon State University Extension. Consult a medical toxicologist or poison center on management of pyrethrin or pyrethroid toxicity.

Expect localized skin irritation to resolve spontaneously, typically within 24 hours. Watch for convulsions after ingestion. Alex T Proudfoot linked them to swallowing substantial amounts in 2005. Remember that pyrethroid poisoning can mimic organophosphate toxicity. Fatality is exceedingly rare and often associated with other toxic concomitant ingestions.

Is Pyrethrins Likely to Cause Cancer?

No. Pyrethrins are classified by the Environmental Protection Agency as not likely to cause cancer. The Environmental Protection Agency is the US agency that registers pesticides and assesses their risks. The liver changes that lead to tumors occurred only above a certain threshold. The rating is therefore limited to doses below it.

A 2009 study by Thomas G Osimitz showed that high doses of pyrethrins produce liver tumors in female rats. Pyrethrins produce these tumors through induction of hepatic xenobiotic metabolising enzymes, hypertrophy and increased cell proliferation. Long-term studies reported increased tumour incidences in liver, thyroid, skin (rats) and lungs (mice). In two studies, mice and rats were fed low to high doses daily for 1.5 to 2 years.

A 2018 study by Tomoya Yamada ties the rat tumours to constitutive androstane receptor (CAR) activation. No carcinogenic status has been established for pyrethrins or pyrethroids.

How Long Do Pyrethrins Persist in the Environment?

How long pyrethrins persist in the environment depends on light and on the medium. Pyrethrin 1 is a major component of pyrethrins. Its half-life in water in sunlight is 11.8 hours, and on soil surfaces it is 12.9 hours.

Without light, pyrethrin 1 breaks down more slowly in water, with half-lives of 14 to 17 days. Half-lives of pyrethrin 1 in sediment are 10.5 to 86 days. Soil half-lives of 2.2 to 9.5 days have been reported. Light and air inactivate and decompose pyrethrins. When water was more acidic, pyrethrin 1 did not readily break down. Alex T Proudfoot of the National Poisons Information Service (Birmingham Centre) reviewed poisoning due to pyrethrins.

How Toxic Are Pyrethrins to Honey Bees and Fish?

Pyrethrins are highly toxic to honey bees, and pyrethrins are highly to very highly toxic to fish. Their slight repellent activity and rapid breakdown limit some of the risk to pollinators. Pyrethrins are very highly toxic to lobster, shrimp, oysters and aquatic insects, yet practically non-toxic to birds.

Pyrethrins can enter water through soil erosion or drift. Toxicity to aquatic life increases with higher water temperatures and acidity. The National Poisons Information Service (Birmingham Centre) published its review of pyrethrins poisoning in Toxicological reviews.

Is Pyrethrins Approved in the European Union?

Yes. Pyrethrins is approved in the European Union, and it is deemed approved under Regulation (EC) No 1107/2009. Regulation (EC) No 1107/2009 is the EU law on placing plant protection products on the market. The active substance is listed in Part A of the Annex to Implementing Regulation (EU) No 540/2011. Only uses as insecticide may be authorised.

Commission Implementing Regulation (EU) No 798/2013 of 21 August 2013 amended the conditions of approval of pyrethrins. It confirms that the active substance is to be deemed approved. Critical reviews in toxicology published a 2009 study on pyrethrins and human cancer risk.

What Conditions Apply to Pyrethrins Approval in the European Union?

The conditions that apply to pyrethrins approval in the European Union require the applicant to submit confirmatory information. It is appropriate to require further confirmatory information on the substance. The applicant shall submit data on the specification of the technical material, as commercially manufactured, including its relevant impurities.

Pyrethrin 1 must be shown to represent the behaviour of the substance in soil and water. The applicant shall submit confirmatory information on the risk from inhalation and on the residue definition. Toxicology research published a 2018 evaluation of the human relevance of liver tumours from pyrethroids and pyrethrins.

How Is Pyrethrins Regulated in the United States?

Pyrethrins are regulated in the United States through EPA classification and registration review. The EPA classifies pyrethrin-I as a Restricted Use Pesticide. Restricted Use Pesticides may be purchased and used only by certified applicators.

EPA assessed pyrethrins and pyrethroids as a class in the registration review, with a special docket for both. EPA concluded that ecological risk mitigation is necessary for the entire class. The mitigation reduces spray drift, reduces runoff and promotes soil conservation practices. Alex T Proudfoot of City Hospital wrote the review titled Poisoning due to pyrethrins.

What Is the Status of the EPA Registration Review for Pyrethrins?

The EPA registration review for pyrethrins stands at Proposed Interim Decision. A registration review decides whether a registered pesticide still meets the current safety standard. Thomas G Osimitz studied pyrethrins and human cancer risk in 2009.

Pyrethrins has docket EPA-HQ-OPP-2011-0885. The review lists a draft ecological risk assessment rationale. It lists a draft preliminary comparative ecological risk assessment and a draft human health risk assessment. These three assessments are still drafts.

What Residue Limits Apply to Pyrethrins in Food?

The residue limits for pyrethrins in food apply to total pyrethrins, the sum of six biologically active pyrethrin esters. This residue definition is the one used for compliance with the maximum residue limits. The Meeting concluded that residue intake from the uses it considered is unlikely to present a public health concern.

The international estimated daily intakes from five regional diets represented 0 percent of the ADI for pyrethrins. Pyrethrin 1 is degraded extensively by photolysis within 10 days after application to plants. No predominant metabolite is formed. In stored grain, 50 percent or more of the applied pyrethrins disappear in the first three or four months. Tomoya Yamada examined in 2018 the human relevance of rodent liver tumours from pyrethrins.

How Can Pyrethrins Be Used More Safely?

To have pyrethrins used more safely, follow the 4 steps below. Pyrethrins use indoors matters because indoor use of an insecticide poses a serious risk to human health. In 2024, Anita Tewari of the College of Veterinary Science & Animal Husbandry named those most at risk. They are women, children and stay-at-home people.

1. Follow the label instructions and take steps to minimize exposure, because the label sets the safeguards for each product.
2. Avoid applying sprays or dusts in windy conditions, since wind can expose the person applying them.
3. Ventilate rooms properly and stay out of a fogged room until it is safe, because people using foggers are exposed when they come back too early or fail to ventilate.
4. Wear gloves when using a pet shampoo that contains pyrethrins, since exposure occurs when the shampoo is used without gloves.

What Is the Workplace Air Limit for Pyrethrins?

The workplace air limit for pyrethrins is 5 mg/m3. The exposure guidelines for pyrethrins include a PEL of 5 mg/m3 and a TLV of 5 mg/m3. They come from the ExToxNet profile revised in 1996.

Rats and dogs inhaled high doses of pyrethrum for 30 minutes each day for 31 days. This caused slight lung irritation. Inhaling high levels of pyrethrum may bring about asthmatic breathing, sneezing, nasal stuffiness and headache. Anita Tewari, writing in 2024, called the respiratory system a highly exposed organ.