Online
US EPA site as of October 2003: http://www.epa.gov/spdpublc/mbr/casestudies/volume2/sulfury2.html
Structural
Fumigation Using Sulfuryl Fluoride: DowElanco's Vikane Gas
Fumigant
Sulfuryl fluoride (SO2F2),
also known as Vikane (99.8 percent by weight sulfuryl fluoride
and 0.2 percent inerts), was developed by Dow Chemical in the
late 1950s as a structural fumigant. Vikane (currently manufactured
by DowElanco) possesses characteristics for the eradication of
structure-infesting insects (Derrick et al. 1990). It is non-flammable,
non-corrosive, and does not cause undesirable odors. It quickly
penetrates structural materials, is effective against a variety
of structural pests, and dissipates rapidly during aeration (Chambers
and Millard 1995, Stewart 1957 and 1966, Kenaga 1957). This material
is an established structural fumigant, and therefore is considered
an alternative to methyl bromide. Since first marketed in the
United States in 1961, it has been used to fumigate more than
one million buildings, including museums, historic landmarks,
rare book libraries, government archives, scientific and medical
research laboratories, and food-handling facilities (DowElanco
1994). Compared to methyl bromide, sulfuryl fluoride penetrates
structural materials more rapidly, and is effective against a
wide variety of pests, and leaves less residue in materials after
aeration. These characteristics make it a viable alternative to
methyl bromide in structural fumigation (Derrick et al. 1990).
Sulfuryl fluoride is
an excellent broad-spectrum fumigant, due to its toxicity to target
pests, good dispersion and penetrating qualities. It is commonly
used to control a wide variety of household pests, including drywood
and Formosan termites (Bess and Ota 1960, Stewart 1957), wood-boring
beetles (powder post beetles, death watch beetles, and old house
borers), fabric and museum pests (clothes moths and furniture
and carpet beetles), cockroaches, bed bugs, snails (Richardson
and Roth 1965), brown dog ticks, and rodents (rats and mice) infesting
buildings, furnishings, construction materials, and vehicles (DowElanco
1996a, Kenaga 1957, Bess and Ota 1960, Roth 1973).
Fumigant Qualities
Compared with Methyl Bromide
In many ways, Vikane
can be a preferred structural fumigant over the use of methyl
bromide. Unlike methyl bromide, sulfuryl fluoride does not react
with sulfur-containing materials to form off- or skunk-odors (DowElanco
1996a). Vikane passes through nylon and polyethylene sheeting
much more slowly than does methyl bromide, so that the gas is
easily confined by the plastic tarps commonly used in structural
fumigation. Furthermore, sulfuryl fluoride penetrates into and
aerates from wood much faster than methyl bromide (DowElanco 1996a,
Bond 1984, Grey 1960). Rapid penetration of substrates inhabited
by the pests allows for variable (shorter) exposure times compared
with standard exposure times for methyl bromide (DowElanco 1996a).
Lastly, because sulfuryl fluoride is about 20 times less soluble
in water than methyl bromide (i.e., 0.075 percent by weight at
77 F (Meikle and Stewart 1962)), water can be used to form a barrier
or bottom seal during the fumigation process (DowElanco 1996a).
Efficacy
Sulfuryl fluoride is
highly toxic to all post-embryonic life stages of insects (UNEP
1994), eggs of most species are less susceptible (DowElanco 1996a;
Bond 1984). The efficacy of sulfuryl fluoride depends on the concentration
reaching the target pest and the duration of exposure. As a result,
the dosage of sulfuryl fluoride required for a specific pest is
calculated in "ounce-hours," ounces of Vikane multiplied
by hours of exposure. In general, insect eggs require a higher
ounce-hour dosage of sulfuryl fluoride compared to later life
stages (i.e., a 10-fold increase in dosage for some insect species)
(UNEP 1992, UNEP 1994). However, the ability to control egg stages
of social insects (i.e., termites and ants) is not necessary because
these newly hatched larvae cannot survive without adult care.
Furthermore, the higher dosages required to control insect eggs
can be obtained by increasing the exposure time, concentration
of sulfuryl fluoride, or a combination of the two. Fumigators
use a "fumiguide calculation system" to determine the amount of
Vikane required for specific pest and fumigation conditions
(DowElanco 1994 and 1996a).
Sulfuryl fluoride prevents
insects from metabolizing the stored fats they need to maintain
a sufficient source of energy for survival by disrupting the glycolysis
cycle (Meikle et al. 1963). Mortality may be delayed for insects
for several days following fumigation (Osbrink et al. 1987), therefore
insects that have received a lethal exposure to sulfuryl fluoride
may still be alive immediately following fumigation (no longer
than 3 to 5 days for termites) (DowElanco 1994). Sulfuryl fluoride
has also been demonstrated to reduce oxygen uptake in insect eggs
(Outram 1970).
Usage
Vikane, a restricted-use
pesticide, is currently registered to control certain pests in
the following infested sites: structures, fumigation chambers,
construction materials and furnishings (including household effects),
and all vehicles except aircraft and subsurface water vessels
(Derrick et al. 1990, DowElanco 1996a). Vikane is odorless,
colorless, non-flammable, non-reactive, and non-corrosive at temperatures
normally encountered in structural and other fumigations. As a
result, it can be used to fumigate photographic supplies, metals,
paper, leather, rubbers, plastics, cloths, wallpapers, household
furnishings, and a variety of other articles (Trinkley 1996, Derrick
et al. 1990, Anonymous 1980). It has little or no effect on the
germination of weed and crop seeds; however, it is injurious to
green plants, vegetables, fruits, and tubers. Sulfuryl fluoride
does not form toxic surface residues on household items, and thus
dishes, cloths, and other items do not need to be removed or washed
following fumigation with Vikane (DowElanco 1994). It is
not registered for use where food and grain commodities are present
because food residue tolerances have not been established. Guidelines
for use of the fumigant specifically state that "under no conditions
should Vikane be used on raw agricultural food commodities,
foods, feeds, or medicinal products destined for human or animal
consumption, or on living plants" (UNEP 1994, Bond 1984).
Application
To control termites,
Vikane is applied to tarped or sealed structures for an
exposure period of 2 to 72, commonly 20-24 hours (the duration
depends on fumigant and labor cost considerations and time constraints),
followed by a 6 to 8 hour aeration period (UNEP 1994). It is packaged
in white cylinders as a liquid under pressure (99.8 percent Vikane
with no other pesticides, solvents, or additives); however, it
volatilizes rapidly upon release from the cylinder. Therefore,
the gas is released under its own vapor pressure through tubing
directly into the structure from pressurized cylinders. The released
sulfuryl fluoride is dense (3.5 times heavier than air), and will
extract heat from the air as it changes from a liquid to a gas.
Fans are used not only to distribute Vikane throughout the
fumigation area, but also to as heat exchangers to mix cool air
near the fumigation introduction site with surrounding warmer
air to prevent condensation of moisture from the air. Unlike methyl
bromide, no auxiliary heat source is required (Stewart 1957).
As with methyl bromide,
exposing sulfuryl fluoride to open flames can form acids which
may react with metals, glass, ceramic tiles, or china near the
heat source. Thus, prior to structural fumigation, all open flames
and glowing heat filaments are turned off or disconnected (i.e.,
pilot lights, electric heater elements, or automatic switches)
(DowElanco 1994, Derrick et al. 1990). Once the appropriate amount
of Vikane is introduced, the fumigator closes the cylinder
valve and removes the tubing from the cylinder. Concentrations
of Vikane can be monitored during fumigation using a fumiscope.
Because sulfuryl fluoride is odorless and does not irritate the
eyes or skin, trace amounts of a warning agent (e.g. chloropicrin,
which causes irritation to the eyes, tears, discomfort, and has
a noticeable disagreeable pungent odor) are typically introduced
into the structure prior to fumigation to act as a warning agent
(DowElanco 1994 and 1996a).
Because of a multitude
of structural, environmental, and fumigation variations, no two
fumigations are alike. The required dosages of Vikane are
influenced by the temperature at the site of the pest, the length
of the exposure period, containment or the rate the fumigant is
lost from the structure, and the susceptibility of the pest to
be controlled. As a result, dosages vary, however a typical dry
wood home fumigation uses 6-16 ounces (0.4 - 1.0 lbs.) per thousand
cubic feet. A specially designed Fumiguide calculator, which
takes into account varying fumigation conditions (e.g., wind speed,
relative humidity, tarp condition, volume in cubic feet being
treated, soil type around structure, target pest, fan capacity,
and exposure duration), is used to determine the required fumigant
dosage. Once the fumigation is complete, the fumigator will return
to the structure to conduct the aeration procedure (DowElanco
1994 and 1996a).
Aeration, the final
step in a fumigation, requires proper ventilation and clearance
of Vikane and the warning agent from a structure. According
to the Occupational Safety and Health Administration (OSHA) permissible
exposure limit (PEL) and the American Conference of Governmental
Industrial Hygienists (ACGIH) threshold limit value (TLV) for
Vikane, the fumigator must aerate the structure so that
the concentration of sulfuryl fluoride is 5 ppm or less prior
to reentry. Reentry must be approved by trained and state-licensed/certified
professionals (DowElanco 1996a). Because sulfuryl fluoride has
a very high vapor pressure (potential to escape from an area)
and a low boiling point (it is a gas at -67° F), it will quickly
diffuse from high concentrations within a structure to the outside
air where it rapidly dissipates to non-detectible levels (Ultraviolet
radiation and reactions with solid particles in the atmosphere
catalyze the breakdown of Vikane) (DowElanco 1994).
The relatively small
amounts of sulfuryl fluoride released are calculated to have virtually
no impact on the global atmosphere or environment. It is broken
down mainly through hydrolysis to release fluoride and sulfide
ions. Because it is fully oxidized it does not interact with or
contribute to local ozone formation. Furthermore, the relative
contribution of sulfuryl fluoride to acid rain is infinitely small
compared to massive amounts of sulfur released to the atmosphere
from industry. Lastly, sulfuryl fluoride contains no chlorine
or bromine and therefore does not contribute to stratospheric
ozone depletion (Chambers and Millard 1995, Baily 1992).
Toxicity
Sulfuryl fluoride is
a toxic gas that can be handled safely by trained professional
fumigators. This gas (as well as methyl bromide) is acutely toxic
to humans, although the severity of toxicological effects is dependent
on the exposure concentration and exposure duration. Short-term
inhalation exposure to high concentrations may cause respiratory
irritation followed by pulmonary edema (an accumulation of fluid
in the lungs, which can cause death), nausea, abdominal pain,
central nervous system depression, and numbness in the extremities.
Chronic longer-term inhalation exposure to concentrations significantly
above the threshold limit value (TLV) may result in fluorosis
(i.e., fluoride binding to the teeth and bones) because sulfuryl
fluoride is converted to fluoride ion in the body (DowElanco 1994).
Mammalian toxicity by inhalation is about equal to that of methyl
bromide (Bond 1984).
Market Trends
Currently, sulfuryl
fluoride is used in approximately 85 percent of all structural
fumigations, while methyl bromide is used for the remaining 15
percent (Sansone 1996). In California, fumigating dwellings with
sulfuryl fluoride has reduced the use of methyl bromide by more
than 80 percent. For example, about 2,300 tones of methyl bromide
were used in 1990 compared with 430 tones in 1992 (UNEP 1994).
DowElanco supplies
100 percent of the Vikane structural fumigation market.
The company is currently involved in efforts to increase the use
of Vikane gas fumigant in two selected markets: 1) quarantine
fumigation applications and 2) use in empty food processing facilities.
Under current quarantine procedures (USDA-APHIS PPQ Treatment
Manual and the AQIS Cargo Container Quarantine Aspects and Procedures
Manual), treatment rates for are provided for fumigation of non-food
cargo potentially infested with wood-infesting beetles. Efforts
are currently underway to develop treatment schedules for additional
target insect pests of non-food goods. An additional potential
quarantine fumigation opportunity for Vikane is the development
of treatment schedules to fumigate timber being imported into
the United States, Europe, and Japan to control wood-destroying
beetles and/or fungal pathogens.
Cost of Fumigating
with Vikane vs. Methyl Bromide
A general picture of
the kinds of fumigation costs associated with using Vikane and
Methyl Bromide is provided in the table below. Vikane application
rates, (and the associated fumigant costs) are derived from the
DowElanco Fumiguide calculator system (DowElanco 1996a). This
system uses a number of variables that can positively or negatively
affect the ability to achieve a lethal concentration. These factors
include: the target pest (insect); ground temperature at structure
site; structure size; the duration of the fumigation; the foundation
type of the structure (slab, crawl space, basement, etc.); and
whether or not the fumigation will be monitored. A fumigation
performed in warmer months, on larger structures, with a slab
foundation or a basement, can be more cost efficient than a comparable
fumigation using methyl bromide. As indicated by the figures in
the tables below, monitoring sulfuryl fluoride levels to confirm
lethal dose during the fumigation utilizes less chemical and is
less expensive than fumigation without monitoring.
The following cost
breakout examples are for a 35,000 cubic foot structure - a typical
home (Table 1), and for a 250,000 cubic foot structure - a commercial
structure (Table 2). Cost estimates are for non-monitored and
monitored (in parenthesis) fumigations. Label rates for methyl
bromide range from 1 to 3 lbs/1000 ft3. The examples listed below
use the 1 lb/1000 on a slab foundation and 2 lb/1000 for a structure
with a crawl space. The conditions for these examples were: Tarp
= good; Seal = Good; Wind = 4mph; Crawl space = sandy loam. The
temperature was 75o F and a 24 hour exposure period. The dosages
for these examples were calculated on the Fumiguide electronic
calculator.
These examples are
for fumigations to eliminate drywood termites. Fumigating for
other insects (like powder post beetles or wood borers) would
increase the amount of fumigant required for Vikane. There
are additional costs that are not considered in these examples.
These costs include: 1) Extended aeration times for methyl bromide
may require additional manpower and equipment costs for the fumigator.
There will also be costs absorbed by the structure owner because
the extended aeration period delayed re-occupancy . Methyl Bromide
costs for homes would include four nights hotel room rental (1
during fumigation + 3 for aeration) compared to 2 nights (1 during
fumigation + 1 aeration) for Vikane. 2) There also may be
potential replacement costs for material which may react with
methyl bromide to cause odor problems in fumigated structures.