3.1 Unintended spreading by air
a) Wind drift after pesticide treatment: The drift or spreading
of pesticides by air will vary significantly depending on the type of equipment
and method used, the wind force, the form of pesticide used (liquid, emulsion or
dust), the height of the spray nozles, the size of drops or particles, the
evaporation of the pesticide, temperature and humidity.
The equipment and method used: When spraying a field by hand
sprayers, knapsack sprayers and tractor sprayers, there are generally few
problems with unintended spraying. When such equipment is used correctly, the
liquid spray can be controlled very well - the deviation is normally only about
one metre. However, when the pesticide is sprayed from an aeroplane or a
helicopter, the area of spraying is considerably less precise.
Wind force: Low wind force is of great importance to minimise
the risk of wind drift. Strong wind can result in significant spreading of the
pesticide spray. It can, however, be difficult to find a precise definition of
"low wind force". It is recommended that the wind is within the limits of calm
to light air (0-1 knots) or slight breeze (4-6 knots) on Beaufort's wind scale.
A more precise definition of acceptable wind force must rely on the other
conditions mentioned in this chapter.
The form of pesticide used: The extent of wind drift will also
depend on the form of pesticide used (liquid, emulsion or dust). Spraying 1-2
metres above ground will result in a wind drift of a few hundred metres for
particles, and up to one kilometre for drops, and several kilometres for gas.
The size of drops or particles: Small drops or particles are
carried by wind more easily than large drops. The size of drops is significant
for at least two different reasons: First of all, large drops fall to the ground
more quickly than small drops. From a height of 80 cm, which is the most common
boom height for spraying, a drop of 10 nanometres takes 4.5 minutes to hit the
ground. If the size of the drops is multiplied by ten, the drops take only 2.5
seconds to hit the ground. Second, the size of drops affects the evaporation of
the pesticide liquid. The smaller the drops, the faster the evaporation. In
certain conditions (a temperature of 20° Celsius and a relative humidity of
80% ) drops of 100 nanometres will evaporate completely in 57 seconds. If the
drop size is halved and other conditions remain the same, it will only take 14
seconds for a drop to evaporate.
Height of fall: The spraying quality depends to a great extent
on the height of the nozzle above ground. The higher above ground, the larger
quantities may be spread by wind to surrounding areas. The height of fall, and
consequently, the wind drift, is greatest by aeroplane or helicopter spraying.
The evaporation of the pesticide: Different types of pesticides
evaporate from liquid to gas form at different points. Pesticides in gas form
are transported by the wind much more easily than pesticides in liquid form.
Some kinds of pesticides evaporate quickly, while others take longer.
Temperature and humidity: Temperature and humidity conditions
affect the evaporation speed of pesticide drops. The higher the temperature and
the lower the humidity, the more will evaporate. Drops of 50 nanometres will
evaporate in 14 seconds if the temperature is 20° Celsius and if the
relative humidity is 80 %. However, if the temperature changes to 30° and
the relative humidity to 50%, drops of the same size will evaporate in just 4
It is important to note that the above factors must be viewed
collectively before estimating the unintended spreading of the pesticide to
surrounding areas. The effect from unintended spreading of chemical pesticides
on humans, animals and vegetation depends on the toxicity of the pesticide, and
on the extent to which it comes into direct contact with the organism.
b) Spreading by evaporation: Pesticides can evaporate after
reaching the ground. Consequently, they can rise to high air strata and be
transported across enormous distances. This has happened with DDT, linden,
dieldrin and other stabile components, remnants of which can be found in lake
sediments, ocean sediments and aquatic organisms. The extent of evaporation
depends not only on the already mentioned conditions for evaporation from
spraying, but also on the pesticide binding to soil particles, and on its water
solubility. Evaporation will be reduced if the pesticide is subjected to soil
c) Spreading by soil particles: Pesticides are often bound to
soil particles, which in turn can be transported by the wind. This is
particularly true in areas where the soil is dry and light. Research has show
that particles larger than a diameter of 0.06 mm are only transported a few
metres by air. Particles larger than 0.02 mm fall relatively quickly to the
ground after having been transported by the wind. Smaller particles can be
transported across very great distances.
d) Mitigative measures: The following mitigative measures can be
useful to reduce the risk of unintended spreading of pesticides by air:
· There should be no pesticide
spraying in strong winds.
· When possible, spraying from
aeroplane and helicopter should be avoided, and should as a rule never be done
in the proximity of housing areas.
· Spraying upwind of housing
areas should be avoided.
· The spray nozzle should be as
close to the ground as possible.
· Pesticide drops should be as
large as possible, and mist blowing should be used as seldom as possible.
· Spraying small drops should be
avoided in periods with low relative humidity and high temperatures.
· Preference should be given to
pesticides which do not evaporate