Basic Knowlegde of Agricultural Chemicals
4. How are MRLs established?
(1) What are residual agricultural chemicals?
Agricultural chemicals, which are applied to crops to control diseases, pests, and weeds, to inhibit physiology of crops, or other purposes, are not eliminated immediately.
The agricultural chemicals attached to the crops may remain there and be ingested through the mouth, or the residue crops may serve as livestock feed, and finally be ingested by humans through milk or meat. The agricultural chemicals thus remaining in crops and others are called "residual agricultural chemicals." Strict inspections for safety are conducted upon the registration of agricultural chemicals, to ensure these residual agricultural chemicals will not harm human health.
(2) What is the safe range of the Maximum Residue Level?
First, the acceptable daily intake (ADI) per 1 kg body weight is calculated on the basis of the toxicity study results submitted at the time of the application for registration. This is an estimate of the amount of an agricultural chemical that can be ingested daily over a lifetime without appreciable health risk to the consumer.
Some of the sprayed agricultural chemicals adhere to the crops, some fail to adhere and enter the soil, or enter rivers via paddy water, and other rodents decompose, and thus humans take in these chemicals through crops and water. Therefore, it is necessary that agricultural chemicals taken in through various routes should be controlled and used not to exceed the ADI. The standards for withholding registration are established considering this point by the Minister of the Environment.
The maximum residue level was established for each active ingredient to determine the acceptable residual amount for food crops. Considering the intake from air and water, the maximum residue level is established so that the total of the residue level of each crop accounts for less than 80% of the ADI for that agricultural chemical.
Thus there is very little chance that the currently registered agricultural chemicals will exceed the standards and threaten the nation's health, if they are used following the directions on the label.
(3) How to establish the acceptable daily intake (ADI) per 1 kg body weight
This is an estimate of the amount of an agricultural chemical that can be ingested daily over a lifetime without appreciable health risk to the consumer. First, a study is selected in which the adverse effect was observed with the lowest concentration of all the long-term toxicity studies such as chronic toxicity studies in animals (e.g. rats or mice). Then the dose level at which no adverse effect was observed in that same study is calculated (NOAEL: no-observed adverse effect level) (mg/kg/day). Considering that this value is obtained from animal studies and there are variations among individuals, it is multiplied by an uncertainty factor (standard 1/100[1/ (10[species difference] × 10[personal difference])]) to obtain the level that is harmless to humans. The resultant value is the ADI (Fig. 6).
The obtained ADI is an acceptable daily intake per kilogram of body weight, and by multiplying the ADI by the Japanese mean weight of 53.3 kg, the acceptable daily intake per Japanese person is obtained (Fig. 7). This is the value on which the maximum residue level of an agricultural chemical is established.
Figure 6. The relationship between the incidence of adverse effects and doses of agricultural chemicals in long-term toxicity studies in animals
Figure 7. The flow chart of calculating the acceptable daily intake (ADI)
(4) How to establish the Maximum Residue Level
The agricultural chemicals sprayed on the surface of the crops decrease over time since the spray day, as they evaporate into the air, wash away due to weather, or decompose due to the interaction of light and water. However, some may remain at harvest.
The maximum residue level, which is established by the Minister of Health, Labour and Welfare in accordance with the Food Sanitation Law, shall serve as the standards withholding registration concerning agricultural chemical residue. The Food Safety Commission of the Cabinet Office was organized on July 1, 2003, and since then, the toxicity evaluation required to establish the maximum residue level has come to be conducted by the Pesticides Expert Committee of the Food Safety Commission.
The standards for withholding registration concerning water pollution shall be established by the Minister of the Environment, as before.
A Determination of the maximum residue level in crops
The maximum residue level in crops is established on the basis of the residual amount obtained from the residue studies in crops, which was submitted at the time of application for registration. Considering that the residue amount fluctuates due to various external factors such as weather conditions, the standards are established, building in a safety factor by making the values more generous than those obtained from the studies, and considering further the foreign standards and international standards (Codex MRLs).
Here we take an example of an agricultural chemical thatis used for crops such as soybeans, small red beans, sweet potatoes, and others (Table 4). Given the residue amount of 0.97 ppm for soybeans, 0.87 ppm for small red beans, and 0.47 ppm for sweet potatoes, which were obtained from studies with uniform application methods, each residue value is tentatively established as 2 ppm for soybeans, 2 ppm for small red beans, 1 ppm for sweet potatoes, and so on, all the way down to the last item (strawberries), making the values more generous for safety. Then the estimated intake of agricultural chemicals is calculated on the basis of the resultant values and the nation's mean food intake (food factors in the National Nutrition Survey by the Ministry of Health, Labor and Welfare). The total of the estimated intake for each crop is 0.2378 mg, which falls below 80% of the acceptable intake of 4.4184 mg. Therefore, the standards in this example are established as 2 ppm for soybeans, 2 ppm for small red beans, and 1 ppm for sweet potatoes.
| Crops | Application method | Maximum residue amount (ppm) |
MRLs (ppm) |
Food factor (g) | Estimated food intake (mg) |
Acceptable daily intake for Japanese (ADI×53.3) |
|---|---|---|---|---|---|---|
| Soybeans(dry) | Spraying | 0.97 | 2 | 56.1 | 0.1122 | |
| Beans | Spraying | 0.87 | 2 | 1.4 | 0.0028 | |
| Sweet potatoes | Spraying | 0.47 | 1 | 15.7 | 0.0157 | |
| Beat sugars | Spraying | 0.31 | 1 | 4.5 | 0.0045 | |
| Cabbages | Spraying | 0.82 | 2 | 22.8 | 0.0456 | |
| Onions | Spraying | 0.33 | 1 | 30.3 | 0.0303 | |
| Carrots | Spraying | 0.46 | 1 | 24.6 | 0.0246 | |
| Immature kidney beans | Spraying | 0.38 | 1 | 1.9 | 0.0019 | |
| Soybeans | Spraying | 0.16 | 0.5 | 0.1 | 0.00005 | |
| Strawberries | Spraying | 0.15 | 0.5 | 0.3 | 0.00015 | |
| Total | 0.2378 | 4.4184mg/person/day |
Estimated intake (mg: total of each applied crop [standard value (ppm) × food factor (kg)]) ≦ ADI(mg/kg) × 53.3(kg)
B Determination of the standards for withholding registration concerning water pollution
Some of the agricultural chemicals for paddy fields may be applied directly to paddy fields as well as drop onto the water surface. The applied agricultural chemicals may adhere to paddy soil, decompose in water, or even may run off into a ditch, and into the river, to be ingested finally in drinking water.
The standards for withholding registration concerning water pollution shall be established so that the acceptable daily intake of an agricultural chemical per Japanese person from drinking water should be below 10% of the ADI of that chemical, given that the mean daily drinking water intake per Japanese person is 2 liters.
The agricultural chemical is registered if the 150-day mean concentration of that chemical, which is calculated from the studies of water pollution, does not exceed the standards.
