Malathion:

Malathion

Trade and Other Names: Malathion is also known as carbophos, maldison and mercaptothion. Trade names for products containing malathion include Celthion, Cythion, Dielathion, El 4049, Emmaton, Exathios, Fyfanon and Hilthion, Karbofos and Maltox.

Regulatory Status: Malathion is a slightly toxic compound in EPA toxicity class III. Labels for products containing it must carry the Signal Word CAUTION. Malathion is a General Use Pesticide (GUP).

Chemical Class: organophosphate

Introduction: Malathion is a nonsystemic, wide-spectrum organophosphate insecticide. It was one of the earliest organophosphate insecticides developed (introduced in 1950). Malathion is suited for the control of sucking and chewing insects on fruits and vegetables, and is also used to control mosquitoes, flies, household insects, animal parasites (ectoparasites), and head and body lice. Malathion may also be found in formulations with many other pesticides.

Toxicological Effects:

• Acute toxicity: Effects of malathion are similar to those observed with other organophosphates, except that larger doses are required to produce them [2,8]. It has been reported that single doses of malathion may affect immune system response [2]. Symptoms of acute exposure to organophosphate or cholinesterase-inhibiting compounds may include the following: numbness, tingling sensations, incoordination, headache, dizziness, tremor, nausea, abdominal cramps, sweating, blurred vision, difficulty breathing or respiratory depression, and slow heartbeat. Very high doses may result in unconsciousness, incontinence, and convulsions or fatality. The acute effects of malathion depend on product purity and the route of exposure [33]. Other factors which may influence the observed toxicity of malathion include the amount of protein in the diet and gender. As protein intake decreased, malathion was increasingly toxic to the rats [78]. Malathion has been shown to have different toxicities in male and female rats and humans due to metabolism, storage, and excretion differences between the sexes, with females being much more susceptible than males [79]. Numerous malathion poisoning incidents have occurred among pesticide workers and small children through accidental exposure. In one reported case of malathion poisoning, an infant exhibited severe signs of cholinesterase inhibition after exposure to an aerosol bomb containing 0.5% malathion [44].
• Chronic toxicity:  Weanling male rats were twice as susceptible to malathion as adults.
• Reproductive effects: Several studies have documented developmental and reproductive effects due to high doses of malathion in test animals [2]. Rats fed high doses of 240 mg/kg/day during pregnancy showed an increased rate of newborn mortality. However, malathion fed to rats at low dosages caused no reproductive effects [8]. It is not likely that malathion will cause reproductive effects in humans under normal circumstances.
• Teratogenic effects: Rats fed high doses (240 mg/kg/day) showed no teratogenic effects. Malathion and its metabolites can cross the placenta of the goat and depress cholinesterase activity of the fetus [8]. Chickens fed diets at low doses for 2 years showed no adverse effects on egg hatching [8]. Current evidence indicates that malathion is not teratogenic.
• Mutagenic effects: Malathion produced detectable mutations in three different types of cultured human cells, including white blood cells and lymph cells [2,8]. It is not clear what the implications of these results are for humans.
• Carcinogenic effects: Adrenal tumors developed in the males at low doses, but not at the high doses [80], suggesting that malathion was not the cause. Three of five studies that have investigated the carcinogenicity of malathion have found that the compound does not produce tumors in the test animals. The two other studies have been determined to be unacceptible studies and the results discounted [2,8,80]. Available evidence suggests that malathion is not carcinogenic but the data are not conclusive.
• Organ toxicity: The pesticide has been shown in animal testing and from use experience to affect the central nervous system, immune system, adrenal glands, liver, and blood.
• Fate in humans and animals: Malathion is rapidly and effectively absorbed by practically all routes including the gastrointestinal tract, skin, mucous membranes, and lungs. Malathion undergoes similar detoxification mechanisms to other organophosphates, but it can also be rendered nontoxic via another simple mechanism, splitting of either of the carboxy ester linkages.  Most of the occupational evidence indicates a low chronic toxicity for malathion. One important exception to this was traced to impurities in the formulation of the pesticide [2].

Ecological Effects:

• Effects on birds: Malathion is moderately toxic to birds. The reported acute oral LD50 values are: in mallards, 1485 mg/kg; in pheasants, 167 mg/kg; in blackbirds and starlings, over 100 mg/kg; and in chickens, 525 mg/kg [2,6]. The reported 5- to 8-day dietary LC50 is over 3000 ppm in Japanese quail, mallard, and northern bobwhite, and is 2639 ppm in ring-neck pheasants [6]. Furthermore, 90% of the dose to birds was metabolized and excreted in 24 hours via urine [79].
• Effects on aquatic organisms: Malathion has a wide range of toxicities in fish, extending from very highly toxic in the walleye (96-hour LC50 of 0.06 mg/L) to highly toxic in brown trout (0.1 mg/L) and the cutthroat trout (0.28 mg/L), moderately toxic in fathead minnows (8.6 mg/L) and slightly toxic in goldfish (10.7 mg/L) [13,8,16]. Various aquatic invertebrates are extremely sensitive, with EC50 values from 1 ug/L to 1 mg/L [28]. Malathion is highly toxic to aquatic invertebrates and to the aquatic stages of amphibians. Because of its very short half-life, malathion is not expected to bioconcentrate in aquatic organisms. However, brown shrimp showed an average concentration of 869 and 959 times the ambient water concentration in two separate samples [12].
• Effects on other organisms: The compound is highly toxic to honeybees [13].

Environmental Fate:

• Breakdown in soil and groundwater: Malathion is of low persistence in soil with reported field half-lives of 1 to 25 days [19]. Degradation in soil is rapid and related to the degree of soil binding [12]. Breakdown occurs by a combination of biological degradation and nonbiological reaction with water [12]. If released to the atmosphere, malathion will break down rapidly in sunlight, with a reported half-life in air of about 1.5 days [12]. It is moderately bound to soils, and is soluble in water, so it may pose a risk of groundwater or surface water contamination in situations which may be less conducive to breakdown. The compound was detected in 12 of 3252 different groundwater sources in two different states, and in small concentrations in several wells in California, with a highest concentration of 6.17 ug/L [33].
• Breakdown in water: In raw river water, the half-life is less than 1 week, whereas malathion remained stable in distilled water for 3 weeks [12]. Applied at 1 to 6 lb/acre in log ponds for mosquito control, it was effective for 2.5 to 6 weeks [12]. In sterile seawater, the degradation increases with increased salinity. The breakdown products in water are mono- and dicarboxylic acids [12].
• Breakdown in vegetation: Residues were found mainly associated with areas of high lipid content in the plant. Increased moisture content increased degradation [33].

Exposure Guidelines:

• ADI: 0.02 mg/kg/day [38]
• MCL: Not Available
• RfD: 0.02 mg/kg/day [53]
• PEL: 15 mg/m3 (8-hour) (dust) [39]
• HA: 0.2 mg/L (lifetime) [53]
• TLV: Not Available

References:

References for the information in this PIP can be found in Reference List Number 5