Journal of Pesticide Reform, Volume 12, Number 4, Winter 1992
Northwest Coalition for Alternatives to Pesticides, Eugene, OR
Malathion is one of the most widely used organophosphate insecticides in the United States and throughout the world. It is used to control pests of agricultural crops, ornamentals, greenhouses, livestock, stored grain, forests, buildings, households, and gardens. Industrial, commercial, and government applications constitute most of the annual U.S. usage. These uses include schools, hospitals, warehouses, eating establishments, food processing plants, and wide scale pest control or eradication programs.(1)
Contributing to its popularity is malathion’s relatively low acute mammalian toxicity. But like DDT and other pesticides that have been found to cause irreparable damage to human and environmental health, malathion may pose a greater risk than the product label would lead one to believe.
Shown to be mutagenic, a possible carcinogen, implicated in vision loss, causing myriad negative health effects in human and animal studies, damaging to nontarget organisms, and containing highly toxic impurities, malathion has a legacy of serious problems.
Malathion was first registered for use in the United States in 1956, and 10 to 15 million pounds of active ingredient are used annually based on 1990 and 1991 data.(2) American Cyanamid Co. was the first manufacturer of malathion; now there are at least 14 primary producers worldwide. The U.S. Environmental Protection Agency (EPA) lists 342 registrants (manufacturers and formulators), and 1218 products were registered as of 1987.(1) Malathion is used in a wide variety of situations to provide broad spectrum control of many insects and mites, especially household pests, aphids, spider mites, and scales. Malathion has also been used extensively in eradication programs for mosquitoes, grasshoppers, gypsy moths, Mediterranean fruit flies, and other pests. Malathion is sold under many trade names both alone and in mixtures with other pesticides. Malathion is applied in ground and aerial sprays, aerosols, foggers, baits, paints, petcollars, animal dips, animal dust bags, and cattle feed blocks.(1,3)
Malathion’s widespread use makes potential for human exposure high. The National Institute for Occupational Safety and Health estimates that between 20,000 and 100,000 workers are occupationally exposed to malathion in the U. S.(4) For example, grain elevator workers in Louisiana were exposed to grain dust contaminated with 0.17 to 32 parts per million (ppm) of malathion.(5) In another incident, office workers complained of headaches and nausea after working in a room adjacent to a pesticide storage shed for a mosquito control program. Investigation showed that malathion was still evaporating from a wall where a pesticide spill had occurred five years earlier.(6) In California, where physicians make mandatory reports of pesticide related illnesses, malathion was the third most frequently reported pesticide. Malathion caused five times more occupational illnesses, per pound sold, than did the average pesticide.(1) (These data are based on reports collected between 1981 and 1985.)
Exposure due to drift and overspray can also be problematic, as is well illustrated by several examples. A homeowner adjacent to a school in Arizona sprayed his garden with malathion. The spray drifted into the school ventilation system and caused nearly 300 elementary school children to be hospitalized with headaches, nausea, and breathing difficulties.(7)
During the 1990 medfly eradication spray program in California, two baseball diamonds were sprayed while children’s games were in progress.
Most of those present reported symptoms, including headaches, sore throats, irritated eyes, hives, rashes, and nausea.(8) Exposure due to household use is also common. In EPA’s recent home and garden pesticide use survey, malathion was the 22nd most commonly used home and garden pesticide (out of 78 chemicals reported in the survey).(9) Nearly 60 percent of the uses are for roses, other ornamentals, and lawns, and 20 percent for food crops.
Malathion was still detectable in hand rinsings from a home gardener seven days after spraying.(4)
*MODE OF ACTION*
Malathion is an organophosphate insecticide, one of a class of pesticides that are highly toxic to vertebrates and are chemically related to nerve gases used during World War II.(10) Like all organophosphate pesticides, malathion kills insects and other animals, including humans, through its effect on the nervous system. It inhibits an enzyme, acetylcholinesterase (AChE), that breaks down acetylcholine, a chemical essential in transmitting nerve impulses across junctions between nerves. Without functioning AChE, acetylcholine accumulates, producing rapid twitching of voluntary muscles, incoordination, convulsions, paralysis, and ultimately death. (11)
Effects of AChE inhibition on nerve cells in the brain appear to be particularly important.(12) Malathion can also inhibit liver enzymes that affect biological membrane function.(13)
The toxicity of malathion is compounded by its metabolites and contaminants. Malaoxon, a Metabolite produced by the oxidation of malathion in mammals, insects, plants, and in sunlight, is the primary source of malathion’s toxicity and is 40 times more acutely toxic than malathion.(14,15)
Over 11 chemical contaminants and analogues created in the production process have been found in technical malathion.(16,17) These chemicals can act synergistically with malathion to potentiate (increase) its toxicity. Some of these compounds inhibit not only AChE,(18) but other enzymatic systems in the liver that would typically detoxify the contaminants.(15)
Malathion and malaoxon are more toxic to immature than to adult rats, due to a slower rate of inactivation of the insecticide in the livers of immature animals.(14) Acute Toxicity Symptoms of acute organophosphate poisoning in humans include headaches, nausea, dizziness, salivation, tearing, urination, diarrhea, convulsions, muscle weakness, incoordination, abdominal cramps, blurred vision and pupil constriction, slowed heart beat, respiratory depression, paralysis, and coma.(19) Other acute effects of malathion exposure include skeletal muscle damage (after inhalation)(20) and abnormal eye movement.(21) Inhalation of malathion may be particularly hazardous. In animal studies (rabbit and quail), inhalation of malathion caused inhibition of AChE equivalent to that caused by an oral dose 15 to 20 times larger.(22) The acute oral LD50 of malathion (the dose that kills 50 percent of a population of test animals) varies between 1522 to 1945 milligrams per kilogram of body weight (mg/kg) in rats.(1) If humans are as sensitive, less than 5 ounces would be fatal to a 70 kilogram human.
Effects on skin and eyes: Repeated exposure to malathion has caused allergic responses in humans, guinea pigs, and mice.(23,24) A single exposure to the skin of a 10 percent malathion solution induced contact sensitization in almost half of human volunteer subjects, and once sensitized, very weak dilutions of malathion (1 ppm) would trigger skin reactions.(23) Technical malathion is mildly irritating to the eyes,(1) can cause temporary visual disturbances,(25) and questions remain regarding its ability to produce external eye irritation.(25)
*SUBCHRONIC AND CHRONIC TOXICITY*
Physicians have described longer term effects of malathion exposure in humans. For example, asphyxia of a pesticide applicator under anesthesia during an operation was associated with his exposure to malathion.(26)
An older man suffered acute kidney failure after malathion exposure.(27)
In laboratory animals, malathion exposure has caused stomach ulcers, testicular atrophy, chronic kidney disease,(28) increased liver and kidney weights, adverse gastrointestinal tract affects,(29) and changes in the adrenal glands, liver, and blood sugar levels.(30,31)
Use of malathion by farmers in Iowa and Minnesota has recently been linked to an increased risk of one type of cancer, non Hodgkin’s lymphoma.(32)
Risks of the disease were elevated for those farmers who had ever handled malathion and significantly elevated for farmers who used malathion prior to 1965. Risks were greater for those treating livestock than for those using malathion on crops. In another study, similar increased risks were found for Nebraska farmers using malathion.(33)
A nationwide study of flour mill employees found a significant excess risk of developing non-Hodgkin’s lymphoma among employees of flour mills.(34) The risk rose with increased duration of work. Malathion is commonly used in flour mills for insect control.The National Cancer Institute (NCI) has studied the carcinogenicity of malathion and malaoxon in rats and mice.
An independent review of this study found benign and malignant tumors of the endocrine glands, brain, liver, lung, and blood.(28) Liver neoplasms were found in mice.(28) An EPA toxicologist also found evidence of thyroid tumors in rats and liver tumors in mice.(35) Dr. Brian Dementi, another toxicologist with EPA, concurred and ìfound that the NCI study indicates a positive oncogenic response.î(36) In reexamining its original study, NCI found dose dependent increases in thyroid tumors in malaoxon treated rats, but no other evidence of carcinogenicity.(37) Tumors of the adrenal glands and leukemia have also been associated with malathion exposure and rats exposed to malaoxon developed benign mammary gland tumors.(1)
The cancer causing potential of malathion and its metabolites has been the subject of controversy since the early 1980s. In light of this, EPA has requested three new cancer bioassays from American Cyanamid.(25)
Malathion is mutagenic (causing genetic damage) in human, animal, and bacteria cells. Frequencies of chromosomal aberrations were significantly higher in cotton field workers exposed to malathion and other pesticides.(38,39) (The design of the study did not permit conclusions about a specific chemical.) Increased chromosome breaks and aberrations occurred following acute malathion human poisonings(40) and in human blood cells exposed to malathion.(41,42) Malathion caused sister chromatid exchanges (exchanges of genetic material within a pair of chromosomes) in human blood cells(41 44) and fetal cells.(45) Malathion has also caused mutations in laboratory animals, including mice and hamsters,(46 49) and induced DNA breakage in the bacteria Escherichia coli.(50)
In some cases malathion induced genetic damage at doses far below acutely toxic levels (45,51) and effects can be cumulative.(45) Birth DefectsIn humans, maternal exposure to malathion during early pregnancy possibly caused an almost total absence of skeletal muscle in a developing fetus.(52) The mother had repeatedly used a hair lotion containing malathion for treatment of lice. In the San Francisco Bay area, a two year study examined the relationship between aerial sprays of malathion for medfly and the occurrence of congenital anomalies and low birth weights.The researchers found no definitive associations, but they also admit to limitations in the data and analysis. However, they did find positive associations between malathion exposure in individual years and increases in ear anomalies, bowing of leg bones, clubfoot, and other deformities.(53)
Malathion has also been associated with birth defects in domestic and laboratory animals. In rabbits, malathion crosses the placenta and acts on the central nervous system.(54) Injection of malathion into the yolk sac of chicken eggs caused reduced growth and weakening of a leg bone,(55) increased production of insulin,(56) reduced chick weights, reduced hatch, short legs, bleached down, nerve damage two to six weeks after hatching,(57) sparse plumage, limb shortening, growth reduction, and beak defects.(58)
Juvenile male rats exposed to daily doses of malathion had decreased numbers of sperm forming cells.(59,60) In two rat teratology studies, maternal exposure to malathion reduced pup weights, increased the incidence of hemorrhagic spots on the backs of pups, and decreased weight gain of the mothers.(25) Doses of 50 and 100 mg/kg/day of malathion caused pregnant rabbits to have reduced maternal weight gain and greater increases of fetal resorptions (dead fetuses absorbed into the mother, not aborted); statistically significant increases in maternal deaths occurred at all doses.(25) A two generation study of male and female rats exposed to malathion yielded offspring that weighed less than the controls, and had increased susceptibility to ring tail disease.(61) In sheep, malathion exposure of pregnant ewes resulted in an increase in aborted fetuses, still births, low birth weight babies. Longer duration and earlier initiation of malathion exposure resulted in more severe problems.(62)
*EFFECTS ON VISION*
Between 1957 and 1971 Japanese school children experienced a tremendous increase in cases of myopia (nearsightedness), that correlated with the increased use of organophosphate insecticides, including malathion.(63)
In 1969, 98 percent of the children examined from Saku, an agricultural area where malathion was regularly applied, had reduced visual keenness. Other examples of what is now called ‘Saku disease’ in both children and adults were reported throughout Japan where organophosphate pesticides were applied. In California, a lawsuit is pending on behalf of a 15 year old boy who was declared legally blind after being outside while helicopters were spraying malathion. An ophthalmologist and a pesticide expert both agree that the boy may have Saku disease.(64)
Repeated exposures to malathion produce toxic effects in the liver regardless of an animal’s nutritional status, but malnourished rats, especially those on low protein diets, are more susceptible to the negative effects of malathion.(13,65 69) Malathion was two to three times more acutely toxic to rats on a low protein diet.(70) This is due, at least in part, to the malnourished liver’s decreased ability to detoxify malathion.
According to the researchers, ‘…people sustaining on nutritionally inadequate diets may be more prone to the toxic effects of these pesticides as compared to those having nutritional adequacy,'(69) and they advise assessing the nutritional status of a community prior to exposure.(69)
Behavioral EffectsMalathion blocked the ability of rats to learn to climb a pole when a buzzer sounded.(71) Learning to avoid a cage that gave rats electrical shocks, and the ability to remember this behavior, was also impaired by malathion.(72,73) In one study, this impairment occurred within one hour after exposure without significant inhibition of AChE activity.(73)
*IMMUNE SYSTEM EFFECTS*
Eradication programs for pests such as mosquitoes and fruit flies expose thousands of people to malathion applied in aerial applications. These type of pesticide applications often provoke complaints of allergic reactions and flu like symptoms.(8,25,74) In laboratory animals, oral doses of purified malathion disrupted immune system function in mice at levels far below the dose required to cause cholinesterase inhibition.(75) This work suggests that malathion can cause sensitization and allergic reactions in humans and animals.
Impurities present in technical malathion can further disrupt immune system function.(76 78) These immune system effects may have serious human health implications. Stimulation of immune responses may increase allergic reactions and also cause tissue damage.(77,78) Immunosuppression may enhance susceptibility of mammalian systems to bacterial, viral, or parasitic infection or possible increased tumor formation.(77) Changes in immune system functions in animals exposed to impurities in malathion may also trigger lung damage.(79) Both malathion and the impurities in malathion can directly affect one immune system function that creates risks for individuals with liver damage.(80)
*CONTAMINANTS AND ANALOGUES*
Technical formulations of malathion contain over 11 contaminants and analogues that increase the toxicity of malathion.(16,17,81) These occur as a result of the pesticide production process, and their formation and concentration is affected by secret ‘inert’ ingredients and pesticide storage conditions.(16) These contaminants act to increase the toxicity of technical malathion up to ten fold.(15,16) Although the concentration of contaminants in commercial formulations is usually less than five percent, environmental conditions can increase the concentration during storage.(16,17) In 1976, an epidemic of malathion poisoning occurred among 7500 Pakistani spray applicators. After hand spraying malathion to control mosquitoes, five workers died and 2800 became ill due to exposure to the analogue isomalathion in the pesticide.(82) Storage of malathion at high temperatures increased its toxicity by increasing the percentage of isomalathion and other contaminants in the product.(15,16,17,83 87)
Other studies have shown similar effects during storage. Malathion stored at 40C for six months was 33 percent more toxic to mice than unstored malathion.(82) Exposure of malathion to sunlight,(88) high relative humidity during storage,(84) and formulation with certain clays and surfactants, can increase contaminant formation in malathion.(85,88)
Some contaminants directly inhibit AChE activity,(18) and exposure to large doses causes symptoms similar to organophosphate poisonings.(81) At low doses (down to 15 mg/kg) some impurities cause a characteristic ‘delayed toxic’ effect in laboratory animals, with death occurring slowly days or weeks after exposure.(87 89) In several studies, rats appear normal except for weight loss from the time of pesticide exposure up until the time of death.(77,88 91) Other reported complications from mammalian exposure to contaminants or analogues include lung damage and bacterial pneumonia,(80,91,92) liver damage,(92,93) kidney damage,(91) and prolonged blood clotting time.(93)
The use of malathion in combination with other pesticides can increase its toxicity to mammals. These synergistic effects are important because pesticides are often applied in combination rather than alone. In rats single low doses of malathion and carbaryl (a carbamate insecticide) increased certain enzymes that are indicative of cellular damage in the liver.(94) The same combination of pesticides reduced the number of livefetuses in pregnant rats and also reduced maternal weight gain.(95)
Combinations of malathion and endosulfan (an organochlorine insecticide) are more toxic than malathion alone because the combination interferes with malathion detoxification.(96) Diazinon (another organophosphate insecticide) was also shown to potentiate the toxicity of malathion causing greater inhibition of cholinesterase in rats than malathion alone.(97)
Based on U.S. Food and Drug Administration residue analyses, malathion is the most commonly detected pesticide in food products.(98) Malathion residues were in 18 percent of the 936 food items tested, indicative of its widespread use in many crops. It is also commonly found in animal feeds. In 1988, EPA estimated that children could be consuming malathion residues 1133 percent in excess, and adults 507 percent in excess, of the amount currently determined not to cause adverse health affects.1In produce, malathion tends to concentrate in the peel,(99) and may not be readily removed by washing in water alone.(100) Peeling, cooking, and heat processing reduce residues.(99 101) In one greenhouse study, malathion applied at recommended rates was easily detected on plant surfaces up to 9 weeks after spraying.(102)
Malathion residues increased with storage time in treated wheat, barley,(101,103) and rice.(104) Apparently, this occurred because the malathion dust used to treat the grain adheres to the grains’ surface and was absorbed over time.
*SECRET ‘INERT’ INGREDIENTS*
Formulated products of malathion, like all pesticides, contain many compounds that are classified as ‘inert’ ingredients and do not have to be identified on the label. There is little publicly available information about ‘inerts’ in malathion. At least two formulations contain xylenes(105,106) and xylene was present in a formulation of malathion that resulted in a human fatality.(106) Xylenes cause skin, eye, nose and throat irritation; impaired memory; liver and kidney damage; incoordination; dizziness; hearing loss; and fetal death and decreased fetal weight gain.(107)
Water: Hydrolysis (break up of the malathion molecule by a chemical reaction with water) is a major pathway of degradation of malathion in aquatic systems, and increases with increasing alkalinity(108) and temperatures.(109)
Half lives in water range from 1.5 days(109) to 21 weeks.(4) In one river, 30 percent of the initial malathion was present after 30 days(110) Degradation by light may compete with hydrolysis in certain waters.(4)
Microbial activity and interaction with sediments may increase degradation and are important in estuarine environments,(4,111) but high concentrations of malathion are toxic to microbes.(112) Soil: Malathion is degraded faster in soil than in water.(113) Reported half lives in soil range from 24 hours to an average of 6 days,4 and rates increase with increased moisture and decreasing acidity.(113) Hydrolysis and microbial activity are both important degradation mechanisms, with microbial activity predominating.(114) Malaoxon has a half life of five days in neutral soils, but persists several days longer under slightly acidic conditions.(115)
*GROUND AND SURFACE WATER CONTAMINATION*
Malathion does not have the chemical characteristics that make it likely to contaminate ground water, but it has been detected in wells in Ontario (Canada),(116) California,(117) Mississippi, and Idaho.(118). Unfortunately few comprehensive ground water studies exist, so information on the extent of malathion contamination of ground water is sketchy. NCAP has no reports indicating that testing for malathion’s metabolites and contaminants has been done. Malathion has also been detected in surface waters. In California, five of 28 county water systems tested were contaminated with malathion(4) and storm drains in Santa Clara County (where aerial sprays of malathion had been used for eradication programs) concentrated malathion and malaoxon, eventually draining into San Francisco Bay.(119)
*CONTAMINATION OF AIR AND FOG*
Malathion was detected in 27 percent of the air samples from 14 states in 1970, and in 21 percent of air samples from 16 states in 1972.(4) In 1987, researchers found that fog droplets in California’s Central Valley and in Maryland were concentrating, moving, and depositing pesticides away from their original source. Malathion and other organophosphates were the most numerous contaminants.(120,121) Malaoxon concentrations were 10 times higher than malathion in nonagricultural areas and increased with increasing distance from pesticide application sites.(120)
Symptoms of acute malathion poisoning in humans include dizziness, muscle twitching, excessive salivation, and urination. Malaoxon,a metabolite, has much greater acute toxicity than malathion itself. Other contaminants and secret ‘inert’ ingredients can increaseits toxicity. At least one ‘inert,’ xylene, is both acutely and chronically toxic. Exposure of farmers and flour mill workers to malathion is associated with increased risks of one type of cancer, and both malathion and malaoxon have caused benign and malignant tumors in laboratory animals. Birth defects, reproductive problems, and genetic damage have been associated with malathion exposure in humans and animals. Visual disorders, behavioral changes, learning impairment, and skin sensitization may also be triggered by malathion exposure. Low protein diets increased malathion’s toxicity in laboratory animals, and may be a concern where malnourished human populations are subjected to malathion exposure. Immune system disruptions due to exposure to malathion or contaminants may increase susceptibility of mammalian systems to bacterial, viral, or parasitic infection,and increased tumor formation. Combinations of malathion with other pesticides can increase toxic effects.
Malathion is the most commonly detected pesticide in food products in the U.S. Over seven million malathion products are used in homes, gardens, and yards in the U.S. annually. Drift and aerial spray eradication programs can expose children to levels of malathion that can cause illness. Malathion has been detected in ground and surface water, air, and fog. Malathion is lethal to beneficial insects, snails, microcrustaceans, fish, birds, amphibians, and soil microorganisms. Sublethal exposure of these species can cause a variety of behavioral and physiological abnormalities.
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