How pesticides are controlled
The methods used to calculate exactly what are safe levels of chemicals, be they pesticides or food additives, in our food are yet again subjects of controversy. Some scientists argue that past screening of pesticides, concentrating on identifying animal carcinogens, has been expensive, time-consuming and of little value to the process of predicting the risks to humans from such chemicals. Others argue that our knowledge of human and animal toxicology is incomplete and more and fuller tests should be carried out before pesticides are cleared. Between these points lie a range of views.
The basic approach is to test new chemicals in a variety of ways, principally on laboratory animals. This immediately raises an issue which we should not ignore: namely the ethics of animal testing. It may be argued that pesticides need to be developed to control insects spreading diseases, to safeguard subsistence crop production or to take account of new climatic or ecological changes on crops. Significant parts of the agro-chemical market are devoted to cash crop production and not to subsistence farming. In these circumstances the ethics of animal testing should perhaps loom larger than it does in the debates.
At the moment the risk assessment of pesticides is still based on toxicology. Groups of animals are exposed to a pesticide usually in their diet, but sometimes through skin absorption or a combination of routes of entry. The animals are exposed to the chemical over a lifetime and the maximum level to which they can be exposed without adverse effects is called the ‘no observable effect level’ (NOEL) or sometimes the ‘no observable adverse effect level’ (NOAEL).
You may of course get observable effects which are not regarded as adverse: hence the NOEL and the NOAEL are not necessarily one and the same thing. Exactly what constitutes an adverse effect as opposed to a normal biological response is determined by groups of scientists.
Regulatory practices at the moment are based on a number of key concepts and use the following key terms which are defined below.
The Maximum Residue Level (MRL) is defined as the maximum concentration of a pesticide residue, usually measured in mg/kg but in some countries referred to in ppm, which is legally permitted in or on food commodities and animal feed. These MRLs are based on the adoption of good agricultural practice. In addition it is intended that foods derived from commodities which comply with the MRLs will be toxicologically acceptable.
The Acceptable Daily Intake (ADI) is the amount of a pesticide which can be consumed every day for an individual’s entire lifetime in the practical certainty, on the basis of all the known facts, that no harm will result. The ADI is expressed as mg of the chemical per kg of body weight of the consumer. The ADI is based on the toxicological ‘no effect’ level in the most sensitive animal species, or for people if appropriate data are available. It includes a safety factor to account for variations in responses between species and variations in human responses.
Account is also taken of any degradation or break-down products (metabolites) which may affect the toxicity of the residue reaching the consumer. The ADI is usually 100 times lower than the NOEL and is calculated on the basis that animal susceptibility may be 10 times greater than that observed in the animals in the test, and that human susceptibility may vary and be up to 10 times greater than that allowed for under normal circumstances. (Hence 10 x 10 = 100 and the ADI figure is established). In some instances, however, the ADI set is much lower than this. ADIs should include consideration of short-term and long-term tests, tests for carcinogenicity, mutagenicity, reproductive hazards, teratogenicity as well as general pharmacological and biochemical effects and modes of action.
Good Agricultural Practice (GAP) with regard to the use of chemicals relates to the nationally authorised safe uses of pesticides under the conditions necessary for effective and reliable pest control. It covers a range of pesticide applications up to the highest authorised use but applied in a manner which leaves a residue which is the smallest practicable. Authorized safe uses relate to national controls and registered recommended uses which take account of occupational, environmental and public safety considerations.
Different models have guided the regulatory approaches of different countries both in dealing with the approval of pesticides generally and in dealing with the specific problems presented by pesticide residues. Some models are based on the balance of probabilities approach and others are based on the beyond reasonable doubt approach.
The ‘balance of probabilities’ approach comes from the civil law concept and requires a lower level of proof, on the balance of probabilities- that a pesticide might present harm to humans or the environment. The approach is the most cautious, most conservative and builds in the biggest safety margins to chemical hazards.
The problem with the latter approach for a number of environmentalists and scientists is that sometimes by the time the proof is satisfactory and is beyond reasonable doubt a potentially toxic chemical could have been used among workers and affected the environment and the public for many years; the consequences could be irreversible. With hindsight it is possible to identify chemicals which do not meet current health and safety standards and for which there was balance of probability evidence in the past: asbestos, several bladder carcinogens and a number of pesticides would appear to fall into this category.
The World Health Organization criteria document on pesticide residue assessment and testing makes specific reference to the lack of standard tests on immunotoxicity and neurotoxicity throughout the 1980s. The criteria relating to existing tests have not been sufficiently developed to be of value in routine safety assessments.