Issue 18, October 16, 2009

Pyrethroid Insecticide Poisoning Study

There has been a recent study published on the incidence and severity of pyrethrin and pyrethroid insecticide illnesses. The study covers reported illnesses from 2001 through 2005 in the states of Oregon and Washington. Pyrethroid insecticides are perhaps the most commonly used insecticides inside and outside buildings by both professionals and homeowners. These results provide insight into pesticide illness throughout the nation.

Data were collected from the pesticide illness surveillance systems of the Washington Department of Health and the Oregon Public Health Division. Both systems are operated under the National Institute of Occupational Safety and Health (NIOSH) guidelines. Both systems utilize mandatory physician reports, reports from poison control centers and other state and local agencies, and reports from individuals.

Illness cases were assigned to four categories as follows:

  1. Low severity illness or injury resulting in three or fewer missed days from work or normal activity. These cases commonly do not require treatment.
  2. Moderate severity illness or injury that is not life-threatening, but that results in some time lost from work, usually less than five days.
  3. High severity illness or injury that usually requires hospitalization and loss from work or normal activities for more than five days.
  4. Death due to exposure to one or more pesticides.

There were 407 cases of pyrethrin and pyrethroid insecticide illness that were reported in the five year time period. Each state had a similar number of cases. The largest number of cases occurred during the summer. Sixteen percent of these cases were deemed as being definitely caused by pyrethrin or pyrethroid insecticides. 11% were deemed as probable, and 73% were classified as possible. Approximately one-fourth of these cases were work-related. Slightly more females (55%) were reported than males. Almost all (92%) were in the low severity illness or injury category, with 8% in the moderate and high severity illness or injury categories. There was one death reported due to pyrethrin or pyrethroid exposure.

There are three categories of these insecticides. The percentage of illness and injury cases in this study are given in parentheses.

  1. Pyrethrins (32%) are extracts of the flowers of chrysanthemum and are usually designated in the active ingredient statements on pesticide labels as pyrethrins. They have the lowest toxicity to mammals of the three categories. They are short-lived, typically lasting only a few days.
  2. Type I pyrethroids (41%) have a cyclopropane carboxylic acid structure. Their chemical name in the active ingredient statement on pesticide labels usually ends with the word, cyclopropane. In this study, type I pyrethroids reported in illness and injury cases included permethrin 16%), tetramethrin (9%), allethrin (6%), bifenthrin (4%), phenothrin (4%), imiprothrin (2%), prallethrin (<1%), and resmethrin (<1%).
  3. Type II pyrethroids (26%) have an alpha-cyano group attached to the benzylic carbon, which enhances the insecticidal properties. They are also more toxic to mammals, and are involved in most clinical cases of human poisoning due to pyrethrins and pyrethroid insecticides. Their chemical name in the active ingredient statement on pesticide labels includes alpha-cyano, commonly being the first part of the name. In this study, Type II pyrethroids reported in illness and injury cases included cypermethrin (7%), cyfluthrin (5%), deltamethrin (4%), esfenvalerate (4%), lambda-cyhalothrin (3%), tralomethrin (2%), fenvalerate (<1), and fenpropathrin (<1%).

All three groups accounted for approximately the same percentage of low severity cases with pyrethrins at 33%, Type I pyrethroids at 39%, and Type II pyrethroids at 27%. Type I pyrethroids were more involved with moderate severity cases at 63%, pyrethrins and Type II pyrethroids each were at 19%. There were only four high or fatal severity cases with two involving Type I pyrethroids (permethrin, bifenthrin), and one each involving pyrethrins and Type II pyrethroids (esfenvalerate).

The only death occurred in an elderly woman with a history of heart disease who entered her home three and one-half hours after an interior crack and crevice treatment of esfenvalerate and pyrethrins and an exterior application of permethrin. She experienced acute respiratory symptoms and cardiac arrhythmia. Resuscitation attempts were unsuccessful and she died at the scene.

Another severe case involved a 53-year-old man with a history of reactive airways disease and allergies to common therapeutic drugs. He sprayed his car with an aerosol home and garden insecticide containing phenothrin and allethrin. He left the car with the windows up until the following day, driving it without thoroughly ventilating it or cleaning interior surfaces. He reported that breathing difficulties developed within an hour which progressed to coughing and respiratory congestion. After out-patient treatment, he was admitted to the hospital ten days after exposure. He was treated in the hospital with intraveneous steroids, antibiotics, bronchodilators, and oxygen supplementation and discharged after three days in stable condition with a diagnosis of severe asthma exacerbation.

The entire study is available at: Walters, J.K., et al. 2009. Pyrethrin and Pyrethroid Illnesses in the Pacific Northwest: A Five-Year Review. Public Health Reports, Vol. 124: 149-159.--Phil Nixon

Phil Nixon

Return to table of contents