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products were withdrawn from use in agriculture and horticulture.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
|Table 16.2 A comparison of LD50s in households, private
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
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.
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
- 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
- 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’
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)
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
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.