United States
Environmental Protection
Agency
Prevention, Pesticides
And Toxic Substances
(7508C)
EPA-738-F-04-012
September 2005
R.E.D. FACTS
Thidiazuron
Pesticide
Reregistration
All pesticides sold or distributed in the United States must be registered by
EPA (the Agency), 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, which were first registered
before November 1, 1984, be reregistered to ensure that they meet today's more
stringent standards.
In evaluating pesticides for reregistration, 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 (FQPA) 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 registers pesticides that meet current health and safety
standards and can be used without posing unreasonable risks to human health or the
environment.
When a pesticide is eligible for reregistration, EPA explains the basis for its
decision with the Reregistration Eligibility Decision (RED) document. This fact
sheet summarizes the information in the RED document.
Use Profile
Thidiazuron [l-phenyl-3-(l,2,3-thidiazol-5-yl) urea] is used as a pre-harvest
cotton defoliant or growth regulator. It removes green leaves and immature fruiting
structures, which contribute to cotton staining. There are no registered residential
uses. Thidiazuron can be applied with aerial or ground equipment, such as
groundboom sprayers. There are eighteen active products containing thidiazuron
(one technical and seventeen end-use products). End-use product formulations
include: wettable powders, soluble concentrates, and emulsifiable concentrates.
Regulatory
History
Thidiazuron has been registered in the United States for use as a cotton
defoliant since 1982.
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EPA completed the reassessment for the 22 thidiazuron tolerances on
September 28, 2005. Since there are no residential uses, an aggregate exposure
assessment was conducted. The Agency concluded that there is a reasonable
certainty of no harm to any population subgroup from aggregate exposure to
thidiazuron from dietary (food and water) exposure. Some tolerances will be
proposed for revocation and one new tolerance will be proposed for establishment.
Human Health Toxicity
Thidiazuron is considered to have low acute toxicity. Thidiazuron is placed in
the following acute Toxicity Categories: oral III; dermal IV; inhalation IV; eye
irritation IV; and, dermal irritation IV. Thidiazuron is classified as "not likely to be
carcinogenic to humans."
Dietary Risks
EPA determined that there is reasonable certainty that no harm to any
population subgroup will result from aggregate exposure to thidiazuron when
considering dietary (food and water) exposure. An acute dietary assessment was not
performed on thidiazuron, because there were no effects observed in the available
toxicology studies that could be attributable to a single exposure (dose). However, a
chronic dietary assessment was conducted for thidiazuron. Chronic dietary exposure
is expected to be less than 8% of the chronic Population Adjusted Dose (cPAD) for
the general U.S. population and all population subgroups, and is therefore below the
Agency's level of concern. A cancer dietary risk assessment was not conducted for
thidiazuron. Carcinogenicity studies in both rats and mice produced no treatment-
related increase in tumor incidence and the standard battery of genotoxicity and
neurotoxicity tests were negative. Therefore, thidiazuron has been classified as "not
likely to be carcinogenic to humans."
The estimated annual average (chronic) surface water concentration for the
parent and both photoproducts, photo-thidiazuron and l-cyano-3-phenylurea, was 1.0
ppb. This conservative estimate of 1.0 ppb was used for the chronic groundwater
concentration to assess chronic risk, which included thidiazuron and both
photoproducts, photo-thidiazuron and l-cyano-3-phenylurea. The dietary exposure
analyses for thidiazuron resulted in dietary risk estimates for food and water that are
below the Agency's level of concern for chronic dietary exposure.
Residential and Other Non-Occupational Risks
There are no registered residential uses for thidiazuron. Therefore, residential
exposure is not expected.
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FQPA Considerations
The FQPA Safety Factor (as required by the Food Quality Protection Act of
1996) is intended to provide up to an additional 10-fold safety factor (10X), to protect
for special sensitivity in infants and children to specific pesticide residues in food,
drinking water, or residential exposures. EPA chose to waive the FQPA safety factor
(i.e., reduce it to IX) based on a conclusion of no increased susceptibility to children
and no residual uncertainty.
EPA did not perform a cumulative risk assessment for thidiazuron because
unlike other pesticides for which EPA has followed a cumulative risk approach based
on a common mechanism of toxicity, EPA has not made a common mechanism of
toxicity finding for thidiazuron and any other substances. Therefore, EPA has
assumed that thidiazuron does not share a common mechanism of toxicity with other
compounds.
Worker Risks
Occupational workers can be exposed to a pesticide through mixing, loading,
and/or applying a pesticide, or re-entering treated sites. Occupational handlers of
thidiazuron include the following: (i) individuals who mix/load liquids for
groundboom and aerial application; (ii) individuals who mix/load wettable powders
for groundboom and aerial application; (iii) applicators who apply liquids via aerial
application; (iv) applicators who apply via groundboom equipment; and (v) flaggers
for liquid sprays. Non-cancer risks for all of these potentially exposed populations is
measured by a Margin of Exposure (MOE), which determines how close the
occupational exposure comes to a No Observed Adverse Effect Level (NOAEL),
taken from an animal study. For thidiazuron, MOEs greater than 100 do not exceed
the Agency's level of concern. All short-term and intermediate-term inhalation
MOEs are not of concern to the Agency when all handlers wear baseline level of
protection (i.e., long-sleeved shirt, long pants, shoes plus socks, and no respirator).
An occupational post-application assessment for agricultural workers (following
application to cotton) was not conducted, because there were no dermal toxicological
endpoints of concern identified, and because post-application inhalation exposure is
expected to be negligible once sprays have dried.
Environmental Ecological Fate
In soil, thidiazuron is persistent, as evidenced by laboratory and field half-
lives of approximately one year. It has intermediate soil adsorption coefficients.
Such persistence and intermediate mobility would allow some year-to-year
accumulation and the potential for runoff from application sites to occur. Based on
its solubility, vapor pressure, and other laboratory evidence, thidiazuron is non-
volatile. In addition, based on its relatively low octanol/water partitioning coefficient,
thidiazuron is not expected to bioconcentrate. When thidiazuron reaches surface
water, photolysis is expected to be the major route of transformation. Aqueous
photolysis rapidly yields two photoproducts. One of the photodegradates
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(photothidiazuron) is a structural isomer of the parent, while the other has a
substantially altered chemical structure (l-cyano-3-phenylurea).
Ecological Risks
The environmental risks for thidiazuron were based on a screening-level
assessment for both terrestrial and aquatic environments. The assessment was
performed for geographic areas where the highest use rates and expected exposures
are likely to occur. Results show that the use of thidiazuron on cotton indicates some
potential ecological risks (exceedence of LOG) above the Agency's level of concern
for: (1) 15g mammals foraging on short grass and broadleaf forage and small insects;
and (2) terrestrial and semi-aquatic plants. However, in an effort to help mitigate
environmental risks, the Agency is requiring specific label language clarifying
allowable use rates and conditions for all thidiazuron products.
Risk Mitigation
Dietary Risk
For all supported commodities, the acute and chronic dietary exposure
estimates (food and drinking water) are below the Agency's level of concern.
Therefore, no risk mitigation measures are required to address exposure from food
and drinking water.
Ecological Risk
The Agency has concluded that there are potential chronic risks of concern for
terrestrial and semi-aquatic plants as well as some chronic risks to mammals when
maximum EECs are assumed. In order to address the risks identified in the
screening-level assessment for thidiazuron, registrants will be required to decrease the
maximum labeled use rate from 0.2 Ibs ai/acre to 0.125 Ibs ai/acre under normal
conditions of use, with an annual maximum application of 0.3 Ibs ai/acre. However,
under certain circumstances such as during periods of rank growth/high fertilizer
conditions, extreme weather conditions (such as extended periods of rain and/or low
temperatures - 60° to 65° degree Fahrenheit), as well as on full-season cotton
varieties, the higher rate (0.2 Ibs ai/acre) may be used but total seasonal use may not
exceed 0.3 Ibs ai/acre.
Additional Data
Required
The generic database supporting the reregistration of thidiazuron has been
reviewed and determined to be substantially complete. However, the following
additional data requirements have been identified by the Agency as confirmatory and
are listed below.
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OPPTS GLN 850.2300
OPPTS GLN 850.4225
OPPTS GLN 860.1340
OPPTS GLN 860.1380
OPPTS GLN 860.1480
Avian Chronic Tests on the Mallard Duck and
Bobwhite Quail
Tier 2 Terrestrial Plant Toxicity (seedling
emergence) on Onion and Oat
Residue Analytical Method - Livestock
Storage Stability Data - Plant and Livestock
Meat, Milk, Poultry, and Eggs - (Additional data
for the ruminant feeding study is required).
Regulatory
Conclusion
The use of currently registered products containing thidiazuron 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
reregistration, in compliance with FIFRA, provided that: the risk mitigation outlined
in the RED document is adopted and label amendments are made to reflect these
measures. These products will be reregistered once the required product specific
data, confidential statements of formula (CSFs), and revised labeling are received and
accepted by EPA.
For More
Information
To obtain a copy of the thidiazuron RED document, please contact the OPP
Public Docket (7502C), US EPA, Ariel Rios Building, 1200 Pennsylvania Avenue,
NW, Washington, DC 20460-0001, telephone: (703) 305-5805. Electronic copies of
the thidiazuron RED and all supporting documents are also available on the Agency's
electronic docket at http://www.epa.gov/edocket.
For more information about EPA's pesticide reregistration program or the
thidiazuron RED, please contact the U.S. EPA, OPP, Special Review and
Reregistration Division (7508C), 1200 Pennsylvania Avenue, NW, Washington, DC
20460, telephone (703) 308-8000.
For more information about the health effects of pesticides, or for assistance
in recognizing and managing pesticide poisoning symptoms, please contact the
National Pesticide Information Center (NPIC). Call toll-free (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. The NPIC internet address is http://www/npic.orst.edu.
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