United States
Environmental Protection
Agency
Prevention, Pesticides
And Toxic Substances
(7508W)
EPA-738-F-98-003
July 1998
R.E.D. FACTS
Pesticide
Reregistration
Use Profile
Zinc Phosphide
All pesticides sold or distributed in the United States must be registered
by EPA, based on scientific studies showing that they can be used without
posing unreasonable risks to people or the environment. Because of advances
in scientific knowledge, the law requires that pesticides that were first
registered before November 1, 1984, be reregistered to ensure that they meet
today's more stringent standards.
In evaluating pesticides for reregi strati on, EPA obtains and reviews a
complete set of studies from pesticide producers, describing the human health
and environmental effects of each pesticide. To implement provisions of the
Food Quality Protection Act of 1996, EPA considers the special sensitivity of
infants and children to pesticides, as well as aggregate exposure of the public
to pesticide residues from all sources, and the cumulative effects of pesticides
and other compounds with common mechanisms of toxicity. The Agency
develops any mitigation measures or regulatory controls needed to effectively
reduce each pesticide's risks. EPA then reregisters pesticides that meet the
safety standard of the FQPA and can be used without posing unreasonable
risks to human health or the environment.
When a pesticide is eligible for reregi strati on, EPA explains the basis for
its decision in a Reregistration Eligibility Decision (RED) document. This fact
sheet summarizes the information in the RED document for reregi strati on case
0026, zinc phosphide.
Zinc phosphide is a rodenticide used to control gophers, mice, rats,
lagomorphs (e.g. jack rabbits), prairie dogs, and squirrels. Zinc phosphide has
indoor and outdoor uses, which are classified as food and non-food depending
upon the application method and label restrictions. Food uses of zinc
phosphide include: grapes, rangeland grasses, sugarcane and regional uses on
artichokes and sugar beets. Non-food uses include: indoor and outdoor
residential and agricultural areas (including in and around homes, on lawns,
around bulbs, indoor and outdoor commercial or institutional premises, golf
courses, alfalfa, barley, berries (dormant), oats, sugar maple, wheat, no-till
corn, orchards/groves (postharvest and dormant), timothy (hay).
Formulations include solid baits, tracking powders and dusts intended for
mixing into baits. Zinc phosphide is applied by hand, machine spreader,
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Regulatory
History
cyclone seeder, and aircraft. Use practice limitations are numerous and vary by
site, including a prohibition against livestock feeding in treated areas.
Zinc phosphide was first registered as a pesticide in the U.S. in 1947.
EPA issued a Registration Standard for zinc phosphide in June 1982 (PB85-
102499). A Data Call-In Notice (DCI) was issued in 1987 and another in 1991
requiring further data for reregi strati on. Following the issuance of the 1991
DCI, the Zinc Phosphide Consortium was formed. The consortium is made up
of technical, formulator, as well as end-use product registrants. The USDA
APHIS (Animal and Plant Health Inspection Service) is the consortium leader.
Currently, 59 zinc phosphide products are registered.
Human Health
Assessment
Benefits
Toxic rodenticides are the most efficient available means for controlling
existing infestations of large numbers of pest rodents. These agents also may
be the method of choice in controlling certain smaller rodent infestations and
often are needed to control rodents that cannot be removed by use of traps.
When buildings, including residences, are heavily infested, poisoning generally
is an integral component of successful abatement programs.
Rodents transmit various diseases either directly or indirectly, via
ectoparasites such as fleas, ticks or mites, or bodily waste products and
secretions. Approximately 14,000 humans are bitten by rats each year.
"Field" rodents such as ground squirrels, voles, and native mice and rats
cause significant damage to crops and rangelands. Certain crops, such as
sugarcane, are heavily damaged in the field by rats and mice. Zinc phosphide
plays an important role in the management of rodents associated with
agricultural crops.
Toxicity
In studies using laboratory animals, zinc phosphide is Toxicity Category
I (the highest of four categories) for acute effects via the oral or inhalation
route of exposure, Toxicity Category III (the second lowest of four categories)
for the dermal route, and Toxicity Category IV (the lowest of four categories)
for eye irritation.
Dietary Exposure
Although zinc phosphide is used in and around food crops, people are
expected to be exposed to minimal residues of zinc phosphide through the diet
because of its rapid degradation and restrictive application methods. Based on
application method and label use restrictions, artichokes (globe), grapes,
rangeland grasses, sugar beets and sugarcane are considered food uses.
Tolerances or maximum residue limits have been established for these
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commodities (please see 40 CFR 180.284). EPA has reassessed the zinc
phosphide tolerances and found that all are acceptable, and that a new
tolerance must be established for grasses, hay.
No Codex Maximum Residue Limits have been established for zinc
phosphide, therefore, issues of compatibility between Codex MRLs and U.S.
tolerances do not exist.
Because zinc phosphide residues on food are expected to be minimal to
non-existent, EPA has not assessed the dietary risk posed by zinc phosphide.
If additional uses be submitted for registration in the future that result in
residues on food, then a risk assessment will be conducted at that time.
Occupational and Residential Exposure
Based on current use patterns, handlers (mixers, loaders, and applicators)
may be exposed to zinc phosphide during and after normal use of bait and dust
formulations.
Human Risk Assessment
Although zinc phosphide is primarily used in agricultural and non-
residential settings, rodenticides, in general, that are used in and around the
home are responsible for a high number of accidental exposures each year.
EPA is concerned about the continued risk of exposure to humans, especially
children, from rodenticides used in residential settings as well as the cost and
trauma associated with treating those who might have been accidentally
exposed. Although few reported incidents are associated with zinc phosphide,
the Agency believes that the common use pattern should be the primary
determining factor shaping the regulatory decision regarding these rodenticides
used in and around the home. Additionally, a margin of exposure (MOE) of
0.5 was calculated for zinc phosphide based on an acute neurotoxicity study
and accidental ingestion of the bait formulation by a child. Generally, the
Agency seeks to ensure that exposures have an MOE of 100 or greater. The
Agency has also determined that a single swallow of zinc phosphide bait may
be fatal to a young child.
Zinc phosphide has not been classified for carcinogen!city. Since dietary
exposure to zinc phosphide residues in foods is negligible, no cancer risk is
expected for the general population.
Although the Agency has not identified any endpoints of concern from
which to perform a handler exposure and risk assessment, it is concerned for
inhalation exposure of occupational workers to the particulate fines or dust
that may be generated from the mixing and loading of the dust-concentrate or
wettable-powder formulations and from applying the pellet and bait
formulations. The Agency is confident that current labeling restrictions, when
combined with those required by this document, are adequate and will require
these formulation-specific protections for all appropriate products.
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Exposure to workers will be mitigated by the use of PPE required by this
RED, including: long-sleeve shirt and long pants, shoes plus socks, chemical-
resistant gloves made of any waterproof material, and a dust/mist filtering
respirator (for mixers and loaders, MSHA/NIOSH approval number prefix TC-
21C).
FQPA Considerations
No drinking water risk assessment was performed for zinc phosphide
because no residues are expected in either ground or surface water due to the
pesticide's rapid degradation and limited usage.
Although zinc phosphide may share a common mode of toxicity (the
generation of phosphine gas) with other chemicals, the Agency has determined
that any future cumulative risk determination involving these chemicals will not
include the uses of zinc phosphide discussed in this document. This
determination is based on the fact that exposures to phosphine from zinc
phosphide in food or water are negligible due to zinc phosphide's rapid
degradation and limited use patterns.
Tolerances with amendments and changes specified in the RED
document meet the FQPA safety standard for the general population and
infants and children.
Environmental
Assessment
Environmental Fate
The Agency has determined that a review of available literature is
sufficient to assess the environmental fate of zinc phosphide, therefore, few
guideline studies were required. The major route of degradation/dissipation of
zinc phosphide is hydrolysis, which results in the formation of volatile
phosphine and zinc ions. Zinc phosphide and its residues appear to be non-
persistent under most environmental conditions and relatively immobile (zinc
ions and dissolved phosphorus readily sorb onto soil) in laboratory and field
data. When applied to dry soil environments, zinc phosphide may be
moderately persistent (-40% of applied remaining at 30 days post-treatment).
The rates of hydrolysis and volatilization of phosphine appear to be pH and soil
moisture dependent with the hydrolysis rate increasing as the pH increases or
decreases from neutrality. Zinc phosphide and its degradation products appear
to have a low potential for ground water or surface water contamination.
Ecological Effects
The Agency has determined that zinc phosphide is highly toxic to avian
species (Bobwhite quail) on an acute oral and on a subacute dietary basis. The
results from studies also indicate that zinc phosphide is highly to very highly
toxic to small mammals on an acute oral basis. Due to the fatal nature of zinc
phosphide poisonings, chronic studies are not necessary.
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The Zinc Phosphide Consortium is currently conducting two terrestrial
field studies. One study is to determine the residues available on alfalfa
following broadcast applications of a 2% bait in flood irrigated and sprinkler
irrigated alfalfa fields. The other study is to determine nontarget hazards to
pheasants in alfalfa fields that have been treated with a broadcast application of
2% zinc phosphide. The testing is expected to be completed within a year.
Environmental Risk Characterization
The Agency has concluded from the studies reviewed, many of which are
not guideline studies, that the use of zinc phosphide in agricultural fields will
likely kill nontarget birds and mammals. Zinc phosphide is a very toxic
substance and will kill most animals to which it is administered. Rodents are
more sensitive than carnivores. Gallinaceous birds (pheasants, turkeys, other
large terrestrial birds) are more sensitive than other avian species, however,
some passerines (songbirds) are also sensitive.
The Agency also concludes that predators or scavengers who eat a target
animal that has been killed by zinc phosphide will not die, however, they may
become ill, listless, and regurgitate.
Risk Mitigation To mitigate the potential risk to children from accidental ingestion of
baits, the Agency is requiring several mitigation measures to be implemented in
two phases. During Phase I the Agency will require zinc phosphide products,
as well as those of several other rodenticides, to incorporate indicator dye (to
help identify whether a child or pet has actually consumed the pesticide) and
bittering agents into their formulations. These formulation changes are
required of all zinc phosphide products, except for those used exclusively in an
agricultural setting. In addition, registrants must update their product labels to
include the protective statements addressed in Section V of the RED. During
Phase II EPA will form a stakeholder group (including industry, states, various
poison control centers, rodent control experts, the medical community and
other interested parties) to develop additional means of significantly reducing
exposures to children and pets. It is the Agency's intent that within nine
months or less from the issuance of the RED, the stakeholder group will
conclude with recommendations to the Agency on how to mitigate risk to
children and pets. Possible outcomes of this group include: requiring all
rodenticide baits used in residential settings to be placed in disposable, child-
resistant bait stations or equivalently protective mechanisms; develop an
exhaustive educational and outreach program for consumers and enhanced
training for certified applicators; tamper-resistant bait stations; and additional
labeling improvements. To monitor the effectiveness of the mitigation
measures implemented during both phases, the Agency is requiring registrants
to submit annual National Poison Control Center Data for years 1999 through
2009. Registrants are encouraged to share the cost of generating data and new
technologies, whenever appropriate.
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To mitigate the potential exposure of the rodenticide to non-target
animals in an agricultural setting, the Agency is retaining the requirement that
all zinc phosphide products labeled for field use (except those limited to
underground baiting for pocket gophers and moles) must be restricted to use
by pesticide certified applicators, or persons under their direct supervision.
Because the use of zinc phosphide will still present a hazard to non-
target animals, the Agency is seeking ways to minimize exposure to these
animals. The Agency is especially concerned about the broadcast use of zinc
phosphide as it allows large tracts of land to be treated. However, the
available data do not show that hand-baiting will necessarily result in reduced
exposure to non-target animals. Rather than impose specific use restrictions at
this time, the Agency is continuing its evaluation of the risks associated with
hand baiting versus broadcast applications and may impose additional data
requirements or label amendments at a later date.
Additional Data EPA is requiring the following additional generic studies for zinc
Required phosphide to confirm its regulatory assessments and conclusions:
72-la Acute Fish Toxicity (bluegill sunfish)
72-1 c Acute Fish Toxicity (rainbow trout)
72-2 Acute Aquatic Invertebrate Toxicity
171-4e Storage Stability
171-4k Crop Field Trials
The Agency also is requiring product-specific data including product
chemistry and acute toxicity studies, revised Confidential Statements of
Formula (CSFs), and revised labeling for reregi strati on.
Product Labeling All zinc phosphide end-use products must comply with EPA's current
Changes Required pesticide product labeling requirements and with the comprehensive list of
labeling requirements required in Section V of the zinc phosphide RED
document.
End-Use Products 8-Month Submission
All registrants of zinc phosphide products must submit revised
Confidential Statement of Formula (CSF) and draft labeling to the Agency
reflecting all changes noted in Section V, except for changes in formula or
labeling related to indicator dye, bittering agent or special child risk warning.
The details of these requirements, which do not apply to products used
exclusively in agricultural settings, will be an outgrowth of a meeting held 30
days after the issuance of the RED document.
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Regulatory
Conclusion
For More
Information
Stakeholder Meetings
The Agency is planning to hold the initial stakeholders meeting within
120 days from the issuance of this RED in Washington, DC. As mentioned
earlier, these meetings will provide an open forum to develop workable
mitigation measures to adequately protect children from accidental rodenticide
exposures. For these meetings to be most efficient and successful, all
interested parties and viewpoints will be welcomed and considered. The
outcomes of these meetings will affect all rodenticide products with residential
uses, including those that were previously reregistered and those that have
been registered more recently and, hence, are not subject to reregi strati on.
The use of currently registered products containing zinc phosphide in
accordance with approved labeling will not pose unreasonable risks or adverse
effects to humans or the environment. Therefore, all uses of these products are
eligible for reregi strati on.
Zinc phosphide products will be reregistered once the required product-
specific data, revised Confidential Statements of Formula, and revised labeling
are received and accepted by EPA.
EPA is requesting public comments on the Reregistration Eligibility
Decision (RED) document for zinc phosphide during a 60-day time period, as
announced in a Notice of Availability published in the Federal Register. To
obtain a copy of the RED document or to submit written comments, please
contact the Pesticide Docket, Public Information and Records Integrity
Branch, Information Resources and Services Division (7502C), Office of
Pesticide Programs (OPP), US EPA, Washington, DC 20460, telephone 703-
305-5805.
Electronic copies of the RED and this fact sheet are available on the
Internet. See http://www.epa.gov/REDs.
Printed copies of the RED and fact sheet can be obtained from EPA's
National Center for Environmental Publications and Information
(EPA/NCEPI), PO Box 42419, Cincinnati, OH 45242-2419, telephone 1-800-
490-9198; fax 513-489-8695.
Following the comment period, the zinc phosphide RED document also
will be available from the National Technical Information Service (NTIS),
5285 Port Royal Road, Springfield, VA 22161, telephone 703-605-6000.
For more information about EPA's pesticide reregi strati on program, the
zinc phosphide RED, or reregi strati on of individual products containing zinc
phosphide, please contact the Special Review and Reregistration Division
(7508W), OPP, US EPA, Washington, DC 20460, telephone 703-308-8000.
For information about the health effects of pesticides, or for assistance in
recognizing and managing pesticide poisoning symptoms, please contact the
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National Pesticides Telecommunications Network (NPTN). Call toll-free 1-
800-858-7378, from 6:30 am to 4:30 pm Pacific Time, or 9:30 am to 7:30 pm
Eastern Standard Time, seven days a week.
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