Group Title: Circular
Title: Agricultural chemical drift and its control
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Title: Agricultural chemical drift and its control
Series Title: Circular
Physical Description: 7 p. : ; 28 cm.
Language: English
Creator: Cromwell, Richard P
Florida Cooperative Extension Service
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville
Publication Date: 1993
Subject: Pesticides -- Environmental aspects -- United States   ( lcsh )
Spraying and dusting residues in agriculture   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
Statement of Responsibility: Richard P. Cromwell.
General Note: Title from caption.
General Note: "June 1993."
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Bibliographic ID: UF00008572
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA6836
ltuf - AJQ6973
oclc - 28532397
alephbibnum - 001832867

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1 o



Florida Cooperative Extension Service

Agricultural Chemical Drift and Its Control'

Circular 1105
June 1993

Richard P. Cromwell2


Drift is the airborne movement of particles into
nontarget areas. It is both undesirable and, to a
degree, unavoidable. Drift problems can be minimized
by understanding what causes it and following
recommended procedures for limiting it.

The fear of chemical drift is out of proportion to
the threat that it poses to people and the environment.
This statement is supported by a video tape, "Big
Fears, Little Risks," presented by The American
Council on Science and Health. In this video tape,
Dr. Bruce Ames, a scientist at the University of
California, Berkeley and the developer of the Ames
test, a widely used test to determine the carcinogenic
nature of chemicals, states that the low levels of
chemicals in the environment are much less of a threat
to human health than life-style related factors such as
smoking, diet, sexual behavior, and others.

What about the threat of chemicals that drift from
treated fields and settle directly onto unsuspecting
people? Workers in adjacent fields have been
accidently sprayed by drift and received a dose
sufficient to cause serious illness. However, the safety
specialist at both the University of Florida in
Gainesville, FL, and the University of California in
Davis, CA, two of the more important agricultural
states, do not recall any documented cases of a person
's death being caused by spray drift. People have died
from accidental poisoning by agricultural chemicals,

but they were either mixing or loading the
concentrated chemical and were not using procedures
recommended on the product label.

Although hazardous pesticide residues on produce
grown in the United States is a rare occurrence, it is
still a major public concern. A 1984 consumer survey
showed that of all the possible harmful products found
in food, the public worried most about pesticide
residues. Because the public perceives that pesticides
in our food supply is a major problem, applicators of
pesticidal chemicals should apply them wisely to
minimize drift and to avoid drift problems. Some
recommended procedures for minimizing drift are
presented in this publication.

Note: The remainder of this publication will primarily
refer to pesticidal particles as droplets because the
majority of pesticides are applied as sprays. However,
small particles of dry material are also prone to drift.
Pesticidal dusts were once the most common
formulation used in agriculture in the late 1940's when
the use of synthetic pesticides became a common
practice. Dusts are seldom used in agriculture today
because they are so prone to drift from the target

1. This document is Circular 1105, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.
Publication date: June 1993.
2. Richard P. Cromwell is Associate Professor and Extension Specialist, Cooperative Extension Service, Institute of Food and Agricultural Sciences,
University of Florida, Gainesville FL 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national
origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean


10 5

Agricultural Chemical Drift and Its Control


SCIENCE Drift is comprised of two components, "swath
Lk$bgay displacement" and "long range drift". Swath
displacement is the lateral movement of relatively
large droplets 150-200 micrometers or larger (spray
droplets are measured in micrometers; 1 inch is equal
to 25400 micrometers) for a short distance. Swath
displacement can cause relatively high residues in
nearby fields.

Long range drift is the movement of small droplets
for great distances (miles from an application site) and
often over a wide area. The resulting level of residue
is usually very low, but can cause a problem when the
chemical settles on a crop that is highly sensitive to
that particular chemical.


The primary factors that influence drift are droplet
size, wind speed, humidity, formulation of the
pesticide, height of emission (primarily influences
swath displacement of large droplets), and the size of
the area treated with the pesticide. The factor that
has the greatest influence on the downwind movement
is droplet size.

Droplet Size

Droplet size is the single most important factor
that affects the distance that a droplet will drift from
the target area. The vertical velocity (settling velocity)
at which a droplet falls to the ground depends on the
size of the droplet. The settling velocity varies
approximately as the square of a droplet's diameter.
A 400 micrometer droplet would fall 4 times as fast as
a 200 micrometer droplet and would drift 1/4 as far
when transported at equivalent wind speeds.

Wind Speed

Wind speed influences the drift distance of
droplets, but does not have as great an influence as
droplet size. The distance that a droplet drifts is
approximately proportional to the wind speed. A
large droplet will travel twice as far in a wind twice as

Page 2


Droplets are reduced in size due to evaporation
of the volatile portion of the spray. Therefore,
droplets that are not particularly drift-prone during
humid conditions might become small enough to drift
from the target area when the humidity is low.

Sometimes applicators in the cotton growing
regions of the South apply pesticides in as little as 1/2
gallon per acre of crop oils (soybean or peanut oil).
Normally the pesticide might be applied in water at a
higher rate per acre. The reason that some
applicators give for using oil as a diluent is that it does
not evaporate; and, therefore, does not pose a drift
problem. This is not necessarily true because the
small amount of spray applied must be highly
atomized to achieve coverage and is drift-prone
without the effect of evaporation.


Aerial applicators are often called "crop dusters"
because most of the pesticides applied during the early
years of aerial application (late forties and fifties) were
formulated as dusts. Aerial applicators are "crop
dusters" in name only in today's agriculture. Most
applicators switched to sprays and granular
applications to reduce drift problems. Dusts are the
most drift-prone pesticide formulation because the
particles are very small.

Many pesticides are oil based formulations known
as emulsifiable concentrates (EC) because many
pesticidal chemicals are soluble in oil. These
formulations form a white, milky emulsion when
mixed with water. If a large droplet spectrum is used
to apply the emulsion, the resulting spray is not
particularly drift- prone. If the droplet spectrum
produced by the nozzle is small, the water phase of
the mixture can evaporate leaving nothing but a small
oil droplet that is prone to drift.

Some formulations of pesticides are more volatile
than others and the vapor phase of the chemical can
drift and cause problems outside of the target area.
The herbicide, 2,4-D, is formulated as an amine and
an ester. The ester formulation is often more
effective, but is more volatile. Ester formulations are
banned in some areas because some broadleaf crops
like tomatoes and melons are extremely sensitive to
2,4-D and even small concentrations of the chemical
vapor in a distant nontarget field can cause plant

Agricultural Chemical Drift and Its Control

The least drift-prone formulations of pesticides are
pellets and granules. The use of these formulations
is somewhat limited because they cannot be used to
apply chemicals to plant foliage. They are widely used
to apply chemicals to the soil or when treating aquatic

Height of Emission

The time that large droplets remain airborne and
consequently the time that the wind has to act on
them depends on the height of the nozzle above the

The emission height of small droplets is not
necessarily a major factor in determining distance that
it might drift. The weight of a small droplet is very
small and it can actually rise rather than fall because
upward components of the wind can generate friction
forces larger than the droplet's weight. A small
droplet can be emitted from a nozzle close to the
ground (18 inches or standard boom height) and
remain aloft much longer than a larger droplet emitted
at a much higher height.

Size of the Treated Area

This factor tends to be overlooked by many
applicators of agricultural chemicals. The amount of
residue that drifts onto a neighboring crop after
treating 10, 20, or 50 acres might not cause damage to
the crop, but at some number of acres the residue
level can cause damage. Because the amount of
residue in surrounding fields depends on the number
of acres treated in the target field, an applicator
should spread out the treatment of large fields over as
many days as feasible. This increases the chances of
the drift being spread out by various wind directions
at a residue level low enough to avoid crop damage.

The size of the area that can be treated by an
aircraft in a short period of time is what causes many
drift problems related to aerial application. It is often
reasoned that aircraft cause drift problems because of
the relatively high height of the spray boom. The
height of the spray boom can cause high residues in
an adjacent field due to swath displacement, but
probably is not a major factor in the level of residue
in more distant fields.

Airblast sprayers that are sometimes used to treat
low growing crops generally produce a very small,
drift-prone droplet spectrum. These sprayers are used
because they can cover a wide swath (40 to 80 feet)

without a cumbersome wide boom. If enough of these
sprayers were placed in a field so that approximately
1000 to 1500 acres could be treated in a day (an
acreage treatable by an aircraft), the resulting drift
problem in distant fields could be much greater than
one caused by an aircraft because there is a greater
percentage of small droplets emitted by the sprayers.


Drift would be much less of a problem, if nozzles
were available that could produce a narrow range of
droplet sizes with no droplets below approximately
150-200 micrometers in diameter. Commercially
available hydraulic nozzles produce a wide droplet
spectrum with droplets ranging from below 100 up to
500 micrometers and larger. The extremes on both
ends of the spectrum are not very effective in
controlling most pests. The small droplets are prone
to drift from the target field and the extremely large
ones contain a lot of pesticide that does not effectively
contribute to plant coverage.

Large droplets are not very effective in achieving
plant coverage because the volume of a droplet varies
as the cube of the droplet diameter. Neither a 250
nor a 500 micrometer droplet are very prone to drift
from the target field because both quickly settle to the
ground. A 500 micrometer droplet contains eight
times the pesticide as the 250 micrometer droplet and
this pesticide would be far more effective, in relation
to coverage, if it were in eight 250 micrometer
droplets. An ideal nozzle would produce droplets in
the 250 to 300 micrometer range. These droplets
would be large enough to avoid long range drift
problems, but small enough to yield acceptable spray
coverage. These droplets drift a very short distance
because the wind does not have much time to act on
them before they reach the ground.


Swath displacement is usually about 25 feet for
low pressure ground sprayers and up to 300 feet for
sprays applied by aircraft. Swath displacement is the
predictable component of chemical drift and is
dependent on the droplet size, the height from which
the droplet is released, and the wind speed. If swath
displacement was the only component of drift, drift
would not be as serious of a problem as it is. A
pesticide applicator would merely leave a buffer strip
between the last rows of the treated field and an

Page 3

Agricultural Chemical Drift and Its Control

adjacent field that would be wide enough to avoid
contaminating the nontarget field.


Long range drift of small droplets is not
predictable because a small droplet does not always
fall while suspended in air. Air can have a vertical
velocity component which is generally upward during
the middle of the day when the warm air at ground
level is displaced by the cooler, heavier air above it.
The vertical updraft can cause a friction force on a
small droplet that is greater than the droplet's weight.
When the upward force exceeds the droplet's weight,
the droplet will rise rather than fall. To make
matters worse, the weight of a spray droplet reduces
over time because water (most spray droplets are
predominantly water) in the droplet evaporates.

A small droplet will remain airborne until the air
mass transporting the droplet is calm long enough to
allow the small downward force of gravity to cause it
to settle to the ground. Because weather is
unpredictable, accurately predicting where a small
airborne droplet will eventually land is essentially
impossible. The location is largely a matter of chance.
There are computer models used to determine where
droplets will settle under given environmental
conditions. These models predict the paths taken by
the larger droplets, but are not very accurate when
predicting where the small droplets settle out. These
programs are useful for determining how wide a buffer
zone should be to keep swath displacement from
causing problems in nearby fields. Fortunately the
residue level that accumulates because of long range
drift is usually very low and often undetectable.

Whether the low level of chemical that results
from long range drift constitutes a problem depends
on who is asked. There are some people who feel
that chemicals settling on nontarget areas at any level
is a problem. If the only acceptable level of drift into
nontarget areas was zero, all spray operations would
have to be shut down along with many industrial
operations. There is no such thing as "zero drift" for
any operation where small particles are released into
the atmosphere. Even large objects are displaced
some small amount when falling in air that has an
horizontal wind velocity. However, it is possible to
keep drift to a such a low level that the benefits from
applying a chemical exceed the potential risks in the
minds of most rational people. If this were not so, the
application of the chemical would not be allowed by

the Environmental Protection Agency (EPA), the
agency that regulates agricultural pesticides.


Chemicals can drift from the target area and cause
a drift problem at the time of the application or at
some time after the application. Drift that occurs
after the application are caused by: (1) having the dry
residue of a wettable powder applied as a spray blown
into an adjacent area after the water carrier
evaporates, (2) having chemical vapors transported
downwind or (3) having high winds blow pesticide-
treated soil and plant particles from the target field
into a neighboring area some time after the chemical
was applied. Contamination resulting from a chemical
being transported into nontarget areas hours or even
days after application are an oddity. Very few of the
problems caused by drift have resulted from "post
application" drift. This publication is primarily
concerned with the drift of particles that begin their
flight into neighboring areas at the time of application.
This is the type of drift that is most prevalent and
deserves most of the attention.


There is a difference between drift and drift
problems. Virtually all spray applications result in
some small amount of spray drifting beyond the
immediate target area. This does not mean that the
drift has created a "problem".

The chemical may drift from the target area onto
an area totally within the holdings of the person
applying it. This person probably would not consider
the drift to be a problem. If the chemical drifts onto
a nontarget area not totally within the holdings of the
person applying it and the residue level is too small to
cause an immediate effect (offensive odor, illness of
residents, damage to plants, etc.), there probably will
not be a problem caused by the drift. Chemicals used
in today's agriculture are less likely to accumulate in
the soil or water because the pesticides that are used
are degraded by the effects of sunlight and soil
microorganisms. Rapid degradation without
accumulation of a chemical or its breakdown products
is a major consideration in determining whether the
chemical can be used as a pesticide.

For example, chemicals that remain as toxins in
the environment for a long time (years in some cases)
are known as persistent pesticides and the use of this

Page 4

Agricultural Chemical Drift and Its Control

type of chemical has been limited severely by the
Environmental Protection Agency. The chlorinated
hydrocarbons (DDT is the most renowned chemical
in this group) are an example of a pesticide group that
remained in the environment for years. These
chemicals were very effective pesticides with low
mammalian toxicity, but they accumulated over time
in the environment. DDT caused the egg shells of
birds of prey like the bald eagle to be very thin and
they often cracked before the young eaglets hatched.


In Florida, there is probably less drift of chemicals
into nontarget areas today than in the past because
growers are not relying solely on chemicals to control
pests. Growers are using "Integrated Pest
Management (IPM)" in their fight against pests. IPM
is an ecological approach to pest management that
often provides economical, long-term protection from
pest damage or competition. Concern about pesticides
in the environment and their potential harm to users
and the public spurred the interest in IPM. The
practitioners of IPM use a combination of pest control
methods to prevent their crops from suffering
economic losses. A few examples of nonchemical pest
control methods used by growers are: (1) planting
crop varieties that have natural resistance to pests, (2)
crop rotations that have proven to be helpful in
reducing pest problems, and (3) chopping and burying
residues from the previous year's crop.

IPM does not exclude the use of chemical control
methods, but chemical use is reserved as the last line
of defense in the growers' battle against pests.
Sometime data collected in the field show that the
pest numbers are high enough to warrant a chemical
application to avert serious economic losses in spite
of the grower using all of the best nonchemical
measures to avoid pest problems. When the decision
to apply a chemical is made, growers using IPM try to
choose a chemical that will kill the target pest while
sparing many of the beneficial insects that prey on
various pests in the field. Preserving beneficial insects
can prevent or at least delay having to apply chemicals
in the future.


Even though the application of agricultural
pesticides is not increasing in Florida because of the
use of IPM (pesticide use in citrus, one of Florida's

major crops, has decreased considerably), the number
of drift-related problems could increase. Many people
are moving to Florida and developments are being
built ever closer to long standing agricultural
operations. Many of these people are terrified at the
thought of an agricultural pesticide, regardless of the
level of residue, drifting into their residential area.

A large portion of Florida residents pay to have
pest control operators apply levels of pesticides that
have been scientifically tested for efficacy and safety
inside their house and in their yard. If the same level
of residue of the chemical occurred in the yard
because of spray drifting from an agricultural pest
control operation, it would probably be a matter of
great concern to residents in the area. This-is
somewhat perplexing, but understandable. The benefit
received from the pest control operator's application
is the obvious absence of roaches, fleas, chinch bugs
or whatever pest that plagues the homeowner. People
do not see the benefit of agricultural pest control "first
hand". Even though the public is told that agricultural
pesticide use makes for high quality, low cost food, the
benefit is not as obvious as the absence of household

Another reason for the strong potential for more
drift problems in the future, in spite of possibly less
drift, is the relatively new field of Environmental Law
coupled with equipment capable of detecting the
presence of extremely low levels of any substance.


The distance that a droplet moves laterally when
released in air depends on the time the droplet is
airborne and the average horizontal velocity of the
droplet during this time period. The distance can be
calculated with the equation below:

Drift Distance = suspension time
horizontal velocity

x average

This equation is simple enough, if you know the
time that the droplet is suspended in the air and its
average horizontal velocity. However, determining
either the suspension time or the average horizontal
velocity of small droplets (100-150 micrometers or
less) is virtually impossible because they are
dependent on weather.

Page 5

Agricultural Chemical Drift and Its Control

Large Droplets Drift Very Little

The time that a large droplet is suspended in the
air depends on the height from which it is released
and its average velocity in the vertical direction. A
large droplet released in air will fall at an increasing
velocity until the droplet's weight is equal to the
upward force on the droplet due to air friction. When
these two forces are equal, the object will fall at a
constant velocity known as the terminal or settling
velocity.The terminal velocity of a large droplet is
relatively high because it takes high velocities to
generate enough friction force to balance the object's
weight. Depending on the height from which a large
droplet is released, it could strike the ground before
reaching its terminal velocity. Large droplets drift
very little primarily because their suspension time is

Small Droplets Can Drift For Great Distances

The settling velocity of a small droplet is very low
because its weight is low. If the droplet is suspended
in air that has an updraft velocity greater than the
settling velocity of the droplet in still air, the droplet
will actually rise rather than fall.

The weight of most spray droplets diminishes with
time because the volatile portion of the spray droplet
evaporates. Evaporation reduces the weight of small
droplets more rapidly than larger ones because they
have more surface area relative to their mass.
Therefore, a small particle that consists primarily of
water is drift-prone initially and becomes even more
drift-prone with time.

Because the vertical velocity of small droplets can
be either down or up and the wind direction can
change during the extended time that a small droplet
is suspended, it is virtually impossible to predict the
distance that the droplet will drift. Neither the
suspension time nor the average horizontal velocity
can be predicted with any degree of certainty. A small
droplet will either completely vaporize or the
nonvolatile portion of the droplet (many spray
mixtures contain some small amount of nonvolatile
oil) will eventually settle to the ground due to the ever
present force of gravity. The air mass carrying the
droplet must remain calm for a relatively long time in
order for the droplet to settle out.


* Nozzles should be used that produce as large of
a droplet spectrum as possible while yielding
adequate plant coverage and pest control. Large
nozzle orifices and low spray pressure creates a
large droplet spectrum. It may be necessary to
apply higher than normal amounts of diluted spray
per acre when using large droplets to avoid drift
in order to get adequate coverage.

* Do not make applications during temperature
inversions. An inversion is a stable atmospheric
condition characterized by an increase in air
temperature with an increase in height above the
ground until at some height a barrier of cold air
is met. Use a column of smoke near the
application site to check for an inversion. The
smoke will rise to the level of the cold air barrier
and will then move laterally below it.

* Usually less material will drift from the target field
during an inversion, but the material that does
leave the target field remains in a more
concentrated cloud and the level of residue that
settles onto nontarget areas will be higher than
usual. Even though the amount of chemical that
drifts from the target area during an inversion is
often less, the potential for a drift problem can be
greater because the small droplets are not lofted
into the upper atmosphere, diluted and spread
over a large area.

* Make applications when the wind is blowing away
from any highly sensitive nontarget areas and the
wind velocity should range between 3 to 10 mph.
Extremely low winds are avoided because they
indicate inversion conditions and winds above 10
mph are avoided because relatively large droplets
can be transported into neighboring fields.

* Leave a buffer zone of approximately 300 feet
between the treated field and any particularly
sensitive areas. Buffer zones will avoid
contamination of neighboring areas by the
displacement of relatively large spray droplets.
However, buffer zones will not effectively
eliminate low level contamination of distant areas
by the small droplets formed by all commercially
available nozzles.

* The nozzles must provide adequate coverage and
pest control while minimizing small, drift prone

Page 6

Page 7

Agricultural Chemical Drift and Its Control

droplets. Request a droplet spectrum data sheet
from the manufacturer that gives information
about the percentage of the droplets that are
smaller than 150 micrometers. Use nozzles with
the smallest portion below this size.

Use the lowest pressure possible that will give
adequate coverage and control to limit the number
of drift-prone droplets.

Make the application at the minimum height that
provides a uniform spray pattern.

Shut off sprayers when turning at the ends of the

Spray early in the morning, late in the evening, or
at night, whenever possible, to avoid killing honey


Orient the nozzles straight back to minimize small
droplet formation due to wind shear.

Limit the boom length to 75% of the wingspan of
the aircraft in order to prevent small droplets
from becoming entrained in the wing tip vortices
and contributing to drift problems.

Limit aircraft speed when applying chemicals that
crops in neighboring nontarget fields are
particularly sensitive to. Modern turbine powered
aircraft capable of flying in the 150 to 160 mph
range will create more small drift- prone droplets
at these speeds.

Dress the ends of a field with a couple of passes,
so that the sprayer does not have to be turned on
before the aircraft is level at the beginning of a
pass or left on after pulling up at the end of a

If a customer wants a field treated "now" when
conditions indicate a high potential for a drift
problem, explain to the customer why it would be
unwise to make the application under the existing

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