Toxicity aspects of pesticides

The basic biochemical similarities between all groups of plants and animals means that any potential chemical chosen for its action against a weed, pest or disease may also be toxic to humans, pets, horticultural species and wildlife animals and plants. Prospective pesticides therefore have to go through a thorough examination over a period of several years to determine whether there are any dangers. This is carried out by the chemical companies and by contracted independent organizations. The evidence is scrutinized by government committees before there can be any possibility of the product’s commercial release. This ensures that no damage will occur to non-target species (particularly humans) if safety precautions are followed. During 2005, 360 pesticide commercial products were withdrawn from use in agriculture and horticulture. During the same period, 160 products were approved.

Acute toxicity to humans
An important indicator of the safety of an ingredient is the lethal dose figure (LD50) by ingestion of a chemical. It expresses the amount of active ingredient required to kill 50 per cent of a population of animals and is expressed as mg/kg of animal tissue. This oral LD50 is used as an indicator for establishing the precautions needed for a grower to safely mix and apply a product. The lower the LD50 figure for a chemical, the more toxic it is. To put toxicity levels into some perspective, five everyday substances are presented in Table 16.2 alongside a range of five pesticides/ growth regulators available to amateur and professional growers.

Other aspects of toxicity
Acute toxicity is not the only property of a potential pesticide to be assessed. Its chronic (long lasting) aspect must also be tested. For example, its survival time on the surface of the leaf may influence its suitability, particularly on leaf crops, such as lettuce, which have a large surface area of pesticide deposit and which are eaten fresh. Pesticides must also be checked against their ability to cause irritation and allergies in humans and their ability to cause cancer. An active ingredient may be particularly toxic to other mammals, fi sh, earthworms, bees and predatory animals. When testing active ingredients, research workers remember very well the havoc that chemicals such as DDT caused in killing animals at the top of food chain.

Product label
The ‘statutory area’ on the label present on each packet or bottle of pesticide must provide the following details:

• The ‘field’ of use, whether agriculture, horticulture, home garden or animal.
• The plant species, crop or situation where treatment is permitted.
• The maximum dose or concentration.
• The maximum number of treatments.
• The latest time of application or harvest interval (days between application and harvest).
• Any specific restrictions, such as clothing required, temperature at which application should be made. (The nature of the protective clothing stated on the label commonly reflects the LD50 status of the ingredient.)
• A reminder to read all other safety precautions on the label and directions for use.

The amateur gardener does not need to pass a proficiency test for pesticide usage. Active ingredients have been selected with care to ensure that no danger from toxicity is present. In 2008 there is a choice of 24 active ingredients (4 insecticides/acaricides, 4 fungicides, 1 animal repellent, 5 slug control chemicals, 9 herbicides, and 1 growth regulator for plant propagation). The professional horticulturist may need to use pesticides which have special requirements in terms of their storage, mixing and application. Three main items of legislation come into play for them.

The first item of legislation focuses on the skill and understanding of the operator as they approach a chosen pesticide application. This was seen in the Health and Safety Regulations 1975 which was summarized in the government ‘Poisonous Chemicals on the Farm’ leaflet. This document specifies the correct procedures for pesticide use. A detailed register must be kept of spraying operations and any dizziness or illness reported. Correct washing facilities must be provided. A lockable dry store is necessary to keep chemicals safe. Warnings of spraying operations should be prominently displayed. A suitable fabric coverall suit with a hood must be used to protect most of the body from diluted pesticide. Rubberized suits should be used in conditions of greater danger, such as in an enclosed greenhouse environment, when dealing with ultra-low volume spray or when applying upward-directed sprays into orchards. Rubber boots should be worn inside the legs of the suit. Thick gauge gauntlets are worn outside the suit when dealing with concentrates, but inside when spraying. Face shields should be worn when mixing toxic concentrates. A face mask covering the mouth and/or nose, and capable of filtering out less toxic active ingredients may need to be used for spraying, but a respirator with its large filter is required for toxic products, particularly when used in greenhouses where toxic fume levels build up.
With regard to wildlife, pesticides should not be sprayed near ponds and streams unless designed for aquatic weed control. Crops frequented by bees, such as apples and beans, should be sprayed with insecticides only in the evening when most of the insects ’ foraging has ceased. Beekeepers should be informed of spraying operations.

The second item of legislation was the important, wide ranging, Food and Environmental Protection Act 1985 (FEPA) which highlighted public and government concern about pesticide dangers, and the need for a UK-wide improvement in responsible pesticide usage. The Act further required that chemical manufacturers, distributors and professional horticulturists should be able to demonstrate skills in the choice and careful management of pesticides. The Act also sought to make information about pesticides more available to the public.

A very practical aspect of FEPA is the specific requirement that all professional personnel involved with pesticides should demonstrate a high level of competence. To this end, the Act requires anyone intending to apply a pesticide to have passed two tests. The first (PA 1) test assesses knowledge in the following subject areas; legislation, places of special environmental value, safe use of pesticides, keeping records of products used and applications performed, storage of pesticides, cleaning of equipment, protective clothing and appliances, disposal of unwanted pesticide, and dealing with contamination and poisoning incidents. The second test (PA 6) assesses practical ability in pesticide application (normally by means of a knapsack sprayer or tractor mounted field sprayer). This involves proficiency in choosing a product for a specified job, calculating the amount of product and volume of water needed for a given area of land, using the correct clothing and equipment for mixing a concentrate and for the application of diluted product, performing the spraying operation, disposing of excess spray liquid, and the cleaning of spray equipment.

A third item of legislation was the Control of Substance Hazardous to Health (COSHH) Regulations 2002 which formalized further the responsibility of the pesticide operator to assess whether each pesticide application was necessary. Once a decision to use a pesticide has been made, further investigation should lead to the choice of the most appropriate and safe active ingredients available, and the most appropriate clothing to wear. A fourth development relating to pesticides has been the Voluntary Initiative, set up in 2001 by the farming and crop protection industries in association with the UK government to minimize the impact of pesticides on the environment.

In the commercial horticultural sector, the number of active ingredients available for use is much greater than in the private garden area. In 2008 there are about 145 active ingredients (33 insecticides/acaricides, 2 nematicides, 39 fungicides, 6 sterilants/fumigants, 11 animal repellants, 2 slug control chemicals, 46 herbicides and 16 growth regulators). Many active ingredients with low LD50 values which were used in the recent past have been banned as too dangerous. The chemical sodium cyanide, which is used to control rabbits, has an oral LD50 of 5 mg/kg and must now be applied to rabbit burrows only by licensed operators. Once the approval by the UK government Pesticide Safety Directorate has been given, further details for the product are then formalized following the guidelines given in Regulations under the Food and Environmental Protection Act 1985 (FEPA), to ensure the pesticide’s transport, storage and application do not endanger humans and wildlife.