Exposure Routes and Environmental Fate of Pesticides

Transcription

1 Exposure Routes and Environmental Fate of Pesticides INTRODUCTION An integrated approach to vector-borne disease prevention includes the use of non-insecticide practices such as source reduction and mechanical barriers, as well as insecticides. Source reduction, such as the elimination of breeding sites and harborages, is an environmentally sound approach with long-term benefits. The use of mechanical barriers, such as screens, air curtains, and doors further enhances protection against vectors of human disease. Insecticides used in an integrated program provide further protection against vectors of human disease and when properly mixed and applied by trained personnel, pose minimal risk to camp personnel, the environment and non-target organisms. An integrated approach to vector-borne disease is dependent upon a robust surveillance program. The surveillance program identifies breeding sites and harborages, identifies population trends, and documents the effectiveness of the control program. Routine surveillance will identify areas where vectors breed or seek harborage. These sites can then be selectively targeted for long-term source 1

2 reduction or in the case of mosquitoes, the application of highly selective biorational insecticides. Harborages that cannot be eliminated can be selectively sprayed when the vector is present. By placing a premium on surveillance and subsequent removal or treatment of breeding sites and harborages, fogging operations with non-selective insecticides can be reduced to the absolute minimum necessary to prevent transmission of vector-borne disease. Safely mixing and applying insecticides is of paramount importance in protecting the health of the applicator as well as individuals residing within the camp and work sites. Personal protective equipment as well as strict adherence to best practices for pesticide application, reduces the impact to non-target organisms, as well as minimizing negative environmental consequences. Taken together, integrated vector-borne disease prevention management, surveillance and applicator training will enhance personal protection, environmental protection and minimize pesticide effects on non-target organisms. Key Activities are Surveillance and the Safe Handling of Chemicals Surveillance Safety Training for Handling Chemicals 2

3 Personal Precautions Pesticides may enter the human body one of three ways; dermal exposure, inhalation and ingestion. Pesticide mixers and applicators are particularly exposed and personal protective equipment must be used, based on the insecticide being used, to minimize exposure. The required protective equipment for mixing and application are specified on the pesticide label. Examples of routine protective equipment includes pesticide resistant gloves, goggles/face shields, respirators, waterproof foot wear and coveralls. Additional safety equipment may be needed as detailed on the pesticide label and Material Safety Data Sheet (MSDS). Safety of camp personnel is dependent upon adherence to correct labeling, application procedures, spill prevention, and controlling access to pesticide storage. Training and supervision are critical components to safely mixing and applying pesticides. The procedures to safe guard against unintentional camp personnel exposure are specific to the insecticide, formulation and method of application, but as a general rule, entry into any building should be 15 minutes after fogging operations and until the pesticide dries for wall spraying. Similar rules apply for outdoor applications, 15 minutes after fogging and wait until the insecticide is dry before walking on or clearing vegetation that has been sprayed. A specific example using a common insecticide for mosquito control is shown below. Training and supervision are critical components to safely mixing and applying pesticides. Minimum Safety Precautions as described in the label and by the MSSDS for Permanone 30:60, a specific formulation of permethrin used in mosquito control, is listed below. Pesticide mixers: Pesticide applicators: Camp Personnel: Impervious gloves and safety goggles or face shield. Avoid breathing vapors. Impervious gloves and safety goggles or face shield. Avoid breathing vapors. Wait 15 minutes from time of fogging to re-enter an area. If permethrin is applied to walls or ceilings, wait until the spray has dried before re-entry. Permethrin belongs to the pyrethroid class of insecticides. This is broad class of general use insecticides. Pyrethroids are manufactured, but are based on pyrethrums, which are naturally occurring insecticides that are extracted from chrysanthemum flowers. Pyrethroids affect the voltage-gated sodium channel causing uncontrolled firing of the nerve membranes. Insects are unable to metabolize these insecticides and are rapidly affected by exposure. Mammals rapidly metabolize pyrethroids and are far less susceptible to poisoning. Pyrethroids are highly toxic to aquatic life. Pyrethroids are widely used in agriculture, public health, and households. Permethrin is used throughout the world and may be applied as a 3

4 residual spray to walls and ceilings and as a space spray. The United States Environmental Protection Agency (EPA) places permethrin in category III for dermal and oral toxicity. There are four categories of toxicity with IV being the least toxic and I being the most toxic. Acute Oral LD50 High Toxicity Category I Up to and including 50 mg/kg ( 50 mg/kg) TOXICITY CATEGORY - PERMETHRIN Moderate Toxicity Category II Greater than 50 through 500 mg/kg (> mg/kg) Low Toxicity Category III Greater than 500 through 5000 mg/kg (> mg/kg) Very Low Toxicity Category IV Greater than 5000 mg/kg (> 5000 mg/kg) Inhalation LC50 Up to and including 0.05 mg/l ( 0.05 mg/l) Greater than 0.05 through 0.5 mg/l (> mg/l) Greater than 0.5 through 2.0 mg/l (> mg/l) Greater than 2.0 mg/l (> 2.0 mg/l) Dermal LD50 Up to and including 200 mg/kg ( 200 mg/kg) Greater than 200 through 2000 mg/kg (> mg/kg) Greater than 2000 through 5000 mg/kg (> mg/kg) Greater than 5000 mg/kg (> 5000 mg/kg) Primary Eye Irritation Corrosive (irreversible destruction of ocular tissue) or corneal involvement or irritation persisting for more than 21 days Corneal involvement or other eye irritation clearing in 8 21 days Corneal involvement or other eye irritation clearing in 7 days or less Minimal effects clearing in less than 24 hours Primary Skin Irritation Corrosive (tissue destruction into the dermis and/or scarring) Severe irritation at 72 hours (severe erythema or edema) Moderate irritation at 72 hours (moderate erythema) Mild or slight irritation at 72 hours (no irritation or erythema Modeled after the U.S. Environmental Protection Agency, Office of Pesticide Programs, Label Review Manual, Chapter 7: Precautionary Labeling. 4

5 Environmental Fate When a pesticide is released into the environment many things happen to it. Some of the pesticide reaches the target. The remainder will be broken down in the air, deposited on plants or soil in the target area and some will drift or runoff to non-target areas. Many processes affect what happens to pesticides in the environment. These processes include adsorption, transfer, breakdown and degradation. Transfer includes processes that move the pesticide away from the target site. These include volatilization, spray drift, runoff, leaching, and absorption. Each of these processes is explained in the following sections. Transfer Processes Adsorption is the binding of pesticides to soil particles. The amount a pesticide is adsorbed to the soil varies with the type of pesticide, soil, moisture, soil ph, and soil texture. Pesticides are strongly adsorbed to soils that are high in clay or organic matter. They are not as strongly adsorbed to sandy soils. Volatilization is the process of solids or liquids converting into a gas, which can move away from the initial application site. This movement is called vapor drift. Vapor drift from insecticides commonly used in mosquito control is rare. Spray Drift is the airborne movement of spray droplets away from a treatment site during application. Spray drift is affected by: spray droplet size - the smaller the droplets, the more likely they will drift wind speed - the stronger the wind, the more pesticide spray will drift Mosquito fogs are designed to drift. In that way they can reach their target, the flying mosquito. Drift can contaminate water in ponds, streams, and ditches and 5

6 harm fish or other aquatic plants and animals. Insecticides applied to vegetation or walls uses a much greater size droplet and pesticide drift is minimal. Runoff is the movement of pesticides in water over a sloping surface. The pesticides are either mixed in the water or bound to eroding soil. Runoff can also occur when water is added to a field faster than it can be adsorbed into the soil. Pesticides may move with runoff as compounds dissolve in the water or attache to soil particles. The amount of pesticide runoff depends on: the slope the texture of the soil the soil moisture content the amount and timing of a rain-event (irrigation or rainfall) the type of pesticide used Public health use of insecticides is generally targeted to a relatively small area and is applied as a fog or barrier spray to vegetation with small volumes of water or other diluents. Runoff from areas treated with pesticides can pollute streams, ponds, lakes, and wells. Pesticide residues in surface water can harm plants and animals and contaminate groundwater. Water contamination can affect livestock and crops downstream. Runoff is generally associated with agriculture use of pesticides, not public health use. Public health use of insecticides is generally targeted to a relatively small area and is applied as a fog or barrier spray to vegetation with small volumes of water or other diluents. Leaching is the movement of pesticides in water through the soil. Leaching occurs downward, upward, or sideways. The factors influencing whether pesticides will be leached into groundwater include characteristics of the soil and pesticide, and their interaction with water from a rain-event such as irrigation or rainfall. These factors are summarized in the table below. Leaching can be increased when: the pesticide is water soluble the soil is sandy a rain-event occurs shortly after spraying the pesticide is not strongly adsorbed to the soil Groundwater may be contaminated if pesticides leach from treated areas, mixing sites, washing sites, or waste disposal areas. The potential mobility of a chemical is on the high side when all of the following factors apply: solubility in water > 30 ppm, soil halflife 3 months, and low adsorption (Ko c < ); it decreases with every factor that does not apply. In general, pyrethroids, the most commonly used insecticides for mosquitoes and other vectors have very low solubility in 6

7 water, binds tightly with soil and has a soil half-life of less than 3 months. Halflife is the amount of time for one half of the parent compound to be eliminated. If the half life of a substance is one week, then after one week one-half of the parent compound will be left, after two weeks, one-fourth of the parent compound will be left and so forth. Insecticides are broken down in the environment in a variety of ways. They may be broken down by photo degradation, microbial degradation, hydrolysis, and plant metabolism. Generally, degradation reduces the toxicity of the parent compound, but in some cases, such as malathion to malaoxon the metabolite is more toxic. The transfer process, as well as how quickly the pesticide is degraded to a nontoxic form determines its environmental fate and likelihood of damage to the environment. Using permethrin again, as an example, the environmental impact can be more accurately judged. Insecticides are broken down in the environment in a variety of ways. They may be broken down by photo degradation, microbial degradation, hydrolysis, and plant metabolism Permethrin Breakdown of Permethrin in Soil and Groundwater As described above, insecticides that are insoluble in water and bind tightly to soil are unlikely to contaminate ground water. Permethrin binds very strongly to soil particles and is nearly insoluble in water. The importance of microbial activity on the degradation of permethrin was studied by Chapman et al. (1981) using sterilized and natural soils. Permethrin was applied at 1 ppm to dried natural mineral soil (ph = ), natural organic soil (ph = ), sterilized organic soil (ph = ), and sterilized mineral soil (ph = ). After 8 weeks the percentages of permethrin remaining in soil were 6%, 16%, 100%, and 101%, respectively. Chapman et al. (1981) concluded that microorganisms are more important than purely physical and chemical processes in the degradation of permethrin. Breakdown of Permethrin in Water The results of one study (Exotoxnet - Permethrin) indicate that synthetic pyrethroids can present a significant threat if they are used near estuarine areas, because they tend to bio-concentrate in these environments. In this study, permethrin had a halflife of less than 2.5 days. When exposed to sunlight, the half-life was 4.6 days. Permethrin degrades rapidly in water, although it can persist in sediments. There is a gradual loss of toxicity after permethrin ages for 48 hours in sunlight at 50 parts per billion (ppb) in water. Because it binds tightly with soil, even when there is permethrin in the sediment, it is not toxic in the water column. Breakdown of Permethrin in Vegetation Permethrin is naturally broken down by vegetation through metabolic pathways. The most common method is ester cleavage, which is also seen in mammals. 7

8 In general, insecticides, when used according to label instructions, will have minimal impact on nontarget organisms. An extensive literature exists on the effects of using permethrin in agriculture, forestry, and in vector control in many parts of the world. Effect in non-target organisms in the environment. The effect of pesticides on non-target organisms is dependent upon the class of insecticide, specific insecticide, formulation and application method. In general, insecticides, when used according to label instructions, will have minimal impact on nontarget organisms. Permethrin is one of the most widely used insecticides for mosquito control. Other pyrethroid insecticides that may be used will have similar characteristics differing mostly in quantitative aspects rather than qualitative aspects. The effect of permethrin in the environment is summarized below. (Taken from International Programme On Chemical Safety - Environmental Health Criteria 94 - Permethrin In laboratory tests, permethrin has been shown to be highly toxic for aquatic arthropods, LC50 values ranging from μg/litre for larval stone crabs to 1.26 μg/litre for a cladoceran. It is also highly toxic for fish, with 96-h LC50 values ranging from 0.62 μg/litre for larval rainbow trout to 314μg/litre for adult rainbow trout. The noobserved- effect level for early life stages of the sheepshead minnow over 28 days is 10 μg/litre and the chronic no-effect level for fathead minnow is μg/litre. Permethrin is less toxic to aquatic molluscs and amphibia, 96-h LC50 values being 1000μg/litre and 7000 μg/litre, respectively. In field tests and in the use of the compound under practical conditions, this high potential toxicity is not manifested. An extensive literature exists on the effects of using permethrin in agriculture, forestry, and in vector control in many parts of the world. Some aquatic arthropods are killed, particularly when water is over-sprayed but the effects on populations of organisms is temporary. There have been no reports of fish killed in the field. This reduced toxicity in the field is related to the strong adsorption of the compound to sediments and its rapid degradation. Sediment-bound permethrin is toxic to burrowing organisms but this effect also is temporary. Permethrin is highly toxic for honeybees. The topical LD50 is 0.11 μg/bee, but there is a strong repellent effect of permethrin to bees, which reduces the toxic effect in practice. There is no evidence of significant kills of honeybees under normal use. Permethrin is more toxic to predator mites than to the target pest species. Permethrin has very low toxicity to birds when given orally or fed in the diet. The LD50 is >3000 mg/kg body weight for acute single oral dosage and for dietary exposure it is >5000 mg/kg diet. It has no effect on reproduction in the hen at a dose of 40 mg/kg diet. Permethrin is readily taken up by aquatic organisms, bio-concentration factors ranging from 43 to 750 for various organisms. In all the aquatic organisms studied, absorbed permethrin is rapidly lost on transfer to clean water. There is no bioaccumulation in birds. The compound can, therefore, be regarded as having no tendency to bio-accumulate in practice. 8

9 Summary The safety and health of the workers is of paramount importance and should be the primary factor in deciding the use of vector-borne disease prevention methods. These decisions should in no way, minimize the need to carefully consider an insecticide s effect on the environment and non-target organisms. Fortunately, as detailed above, an integrated vector-borne disease prevention program that incorporates the use of insecticides, can protect the health of the workers while minimizing any effect on the environment and non-target organisms. Sources: Chapman, R.A., C.M. Tu, C.R. Harris and C. Cole Persistence of five pyrethroid insecticides in sterile and natural, mineral and organic soil. Bulletin of Environmental Contamination and Toxicology 26: Exotoxnet Permethrin. 9

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