Pesticide residues

Pesticides are chemicals developed and produced for use in the control of agricultural and public health pests. The main groups of pesticides are insecticides, herbicides, and fungicides. Pesticides are of vital importance in the fight against diseases, e.g., malaria, and for the production and storage of food. In spite of their extensive use, an average of 35% of the produce is lost worldwide.

Common classes of pesticides include organochlorine compounds, organophosphates, and carbamates.

Many members of the various classes are highly toxic. A common misconception is that pesticides have the same mode of action. The ways in which they act are as diverse as their chemistry. Chlorinated cyclodiene insecticides (e.g., aldrin) are neurotoxicants that interfere with y-aminobutyric acid transmitters in the brain. In humans and experimental animals, seizures have been reported, in addition to symptoms such as nausea, vomiting, and headache. The toxicity mechanism of the chlorophenoxy herbicides 2,4-dichlorophenoxyacetic acid and 2,4,5- trichlorophenoxyacetic acid is poorly understood. They induce their herbicidal effects by acting as growth hormones in plants. However, they do not act as hormones in experimental animals. In animals, effects such as stiffness of the extremities, inability to coordinate muscular movements, paralysis, and eventually coma have been observed.

The organophosphorous insecticides (e.g., parathion) inhibit acetylcholinesterase, resulting in symptoms (that mimic the action of acetylcholine) such as lachrymation, pupillary constriction, convulsions, respiratory failure, and coma.

Carbamate herbicides such as propham (isopropyl-N-carbanilate) have relatively low acute toxicities. The oral LD50 of propham in rats is 5 g per kg. Herbicidal carbamates are not inhibitors of cholinesterase.

The toxicological risks from residues of s ynthetic pesticides in foods are minimal because of careful food safety legislation and regulation. Contamination of vegetables may result from treatment as well as from conditions such as improper use of pesticides, residues from preceding treatments in the soil and cross-contamination (particularly during harvesting). Sources of residues in products of animal origin include contaminated water or feed, pesticide-treated housing, and contaminated milk (during weaning).

Table 4.5 lists the pesticide residue levels in food in the US.

Organochlorine insecticides deserve particular attention, as they are very stable and can accumulate in food chains. Products of animal origin as well as mother's milk almost always contain residues of organochlorine compounds. The residue content of mother's milk is 10 to 30 times higher than that of cow's milk.

From May 1990 through July 1991, 806 milk samples from 63 metropolitan areas in the US were collected and analyzed for pesticide residues by the FDA. In the samples from eight of the metropolitan areas, no residues could be detected. Pesticide residues appeared to contaminate 398 milk samples though. The most frequently occurring residues were p,p -DDE (4,4'-dichlorodiphenyltrichloroethane) (in 212 samples) and dieldrin (in 172 samples). The highest residue level measured was 0.02 ppm p,p'-DDE (whole milk basis). These chlorinated pesticides have not been registered for agricultural use for about 20 years.

Table 4.5 Pesticide residues in food in the US in 1991

Samples with residues

below

above

Number

permissible

permissible

Food

Origin

of samples

level in %

level in %

Grains/grain products

Domestic

495

40.8

0.8

Import

396

25.5

2.3

Milk/dairy products/eggs

Domestic

809

12.5

0

Milk/dairy products

Import

216

10.2

0

Fish/shellfish/other meats

Domestic

536

41.6

0.2

Fish/shellfish

Import

611

23.2

0.2

Fruits

Domestic

2168

50.9

0.5

Import

3481

34.1

1.3

Vegetables

Domestic

3811

30.6

1.3

Import

4311

28.3

3.3

Other

Domestic

462

19.5

0

Import

918

17.9

3.5

Source: Food and Drug Administration Pesticide Program, Residue Monitoring 1991 (5th annual report).

Source: Food and Drug Administration Pesticide Program, Residue Monitoring 1991 (5th annual report).

The use of organochlorine compounds is decreasing in favor of that of organophos-phates and carbamates. Both latter classes of pesticidal chemicals are much more readily degraded, in the environment as well as during processing.

Many of the techniques presently used in food processing give a considerable reduction of pesticide residue levels. Many types of residues are degraded to harmless products during processing due to heat, steam, light, and acid or alkaline conditions. In addition, major reductions of residue levels result from their physical removal by peeling, cleaning or trimming of foods such as vegetables, fruits, meat, fish and poultry.

Table 4.6 lists the results of the Total Diet Study 1991 on the occurrence of pesticides in food. In general, residues present at or above 1 ppb could be measured. Malathion continues to be the residue most frequently found; it is used on a wide variety of crops, including many post-harvest uses on grains. From 1987 to 1991, the occurrence of malathion has decreased from 23 to 18% (see Table 4.6), and that of DDT from 22 to 10%.

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