Introduction 

 The Antimicrobial Division (AD) Registration Review Team has evaluated the 

status of the human health assessments for terbuthylazine (PC Code 080814) and the 

terbuthylazine registration review case (Case 2645). Terbuthylazine is currently 

registered as an algaecide and microbiocide/microbiostat used to control algae and 

slime-forming algae, fungi, and bacteria in water systems such as recirculating cooling 

water towers, recirculating evaporative condenser water systems, recirculating heat 

exchanger water systems, and decorative and ornamental fountains. Mixing, loading and 

applying these products must be done through closed mixing/loading systems with PPE 

(long-sleeve shirt, long pants, and chemical resistant gloves). There are four registered 

products (one technical product (95% a.i.)., one manufacturing use product, and two end 

use product formulated as a flowable liquid at concentrations between 4 to 44.7% a.i..). 

All of the terbuthylazine registered uses are non-food uses. Currently, there are no 

tolerances or exemptions from the requirement of a tolerance, and there are also no 

indirect food uses. 

 Terbuthylazine used to be registered as an s-triazine herbicides. The s-triazines act 

as inhibitors of photosynthesis and disturb enzymatic processes in plants (J. Agric. Food 

Chem. 1999, 47, 2733-2737). Tolerances were originally established for terbuthylazine in 

or on corn fodder and forage, corn grain (including popcorn grain), and sorghum forage 

and grain (40 CFR 180.333). However, terbuthylazine is no longer used as a herbicide 

and these tolerances were revoked in 1992 (53 FR 4860, 56 FR 14471, and 57 FR 

30132). Products formulated for end-use were first registered as algaecide and 

microbicides/microbistats starting in 1986. 

 

 The primary source of information for this assessment are the risk assessments 

and supporting documents for the reregistration eligibility decision (RED) documents, 

which were finalized in March, 1995. The purpose of this screen is to determine whether 

sufficient data are available to support registration review, report on whether risk 

assessments have been conducted and to report whether new human health assessments 

are needed to support the registration review process. 

 The available data for terbuthylazine indicates that this compound is slightly 

soluble in water. Terbuthylazine has a low vapor pressure (e.g. <1E-6 mm Hg), is stable 

to hydrolysis and photolysis, and degrades slowly under aerobic conditions and anaerobic 

conditions in sediment. For these reasons, the terbuthylazine are likely to persist in water 

and soils. Table 1 presents the chemical identity of terbuthylazine. 

 

2
Section 1. Chemical Identity 

Table 1. Chemical Identity of Terbuthylazine 

Common 

Name 

Terbuthylazine 

CAS name 1,3,5-Triazine-2,4-diamine, 6-chloro-N-(1,1-

dimethylethyl)-N'-ethyl-; and, 

s-Triazine, 2-(tert-butylamino)-4-chloro-6-

(ethylamino)- 

PC Code 080814 

CAS registry 

number 

5915-41-3 

Registration 

Review Case 

No. 

2645 

Structure 

Section 2. Toxicology 

 The Agency reviewed the toxicity database for terbuthylazine. EPA confirmed that no 

additional toxicity data will be needed for registration review since the exposure patterns 

are non-food uses and the application is restricted to closed mixing/loading systems only. 

No toxicity studies have been submitted since the RED and no toxicity data was called in 

for the RED. 

Acute Toxicity 

According to the existing RED, there are acceptable acute toxicity data available 

for the generic active ingredient of terbuthylazine. A battery of tests (acute oral, dermal, 

inhalation, primary eye, primary skin, and dermal sensitization) was conducted using 

terbuthylazine and all were considered acceptable. A toxicity category of III was 

assigned based on a oral LD50 study in rats. The dose in rats was 1000 - 1590 mg/kg for 

males and 1503 mg/kg for females. A toxicity category of III was also assigned for acute 

dermal toxicity testing of terbuthylazine with a LD50>2,000 mg/kg for rats. Acute 

inhalation toxicity testing of terbuthylazine indicated a toxicity category of III with a 

LC50 greater than 5.3 mg/L over a 4 hour exposure period. Skin and eye irritation testing 

3
of terbuthylazine in rabbits showed the compound to be slightly irritating to skin and mild 

to moderate irritants to eyes with a toxicity category of III for skin and eye irritation 

effects. Terbuthylazine is not a dermal sensitizer as evidenced by a guinea pig study. 

Sub-chronic Toxicity 

According to the RED, the toxicity databases for terbuthylazine were considered 

complete for non-food uses. Acceptable data on reproductive toxicity is not available to 

the Agency at this time. However, a 2-generation reproduction study in the rat is not 

required to support reregistration of terbuthylazine since for its current non-food uses . In 

the future if food uses are proposed a 2-generation reproduction study will be required to 

support registration. The Agency does expect not expect that additional inhalation 

toxicity study will be needed for registration review since open pouring of the liquid is 

prohibited based on the RED and the existing label restrictions. 

 

Dermal Toxicity 

In a repeated dose dermal toxicity study (MRIDs 40514802 and 42059804), 

terbuthylazine (technical, 97.1% a.i.) was applied daily to the intact skin of 5 male and 5 

female New Zealand White rabbits for 29 consecutive days. The doses of 0.05, 0.5 or 500 

mg/kg/day were applied for 6 hrs/day under an occlusive wrap. At 500 mg/kg/day, 

reduced body weight gain compared to controls at day 28 was observed in males (-36%) 

and females (-39%). Food consumption was also decreased (-76% and -89% of controls 

during week 1 in males and females; between -11% to -54% of controls at other times). 

The LOAEL of 500 mg/kg/day is based on decreased body weight gain and food 

consumption in males and females and, in one female, hypothermia, cachexia and 

mortality. The NOAEL is 0.5 mg/kg/day. 

Another dermal toxicity study was considered in determination of appropriate 

toxicity endpoints for short- and intermediate-term occupational and residential 

risk assessment for the RED. In this study male and female New Zealand White rabbits 

were dermally exposed to terbuthylazine (technical, 99.8% a.i.) at 0, 5, 50 or 500 

mg/kg/day. Effects were seen at the 5.0 mg/kg/day dose. Several clinical signs exhibited 

in treated animials included dyspnea, piloerection, sedation and curved body posture. 

Dermal rritation was seen for all animals. The LOAEL of 5.0 mg/kg/day is based on 

clinical signs in males and females. No NOAEL was reported. 

Inhalation Toxicity 

There was no inhalation study available. The existing RED did not called in an 

inhalation toxicity study (90-day) (870.3465) and mitigated risk by requiring closed 

mixing/loading systems For registration review, EPA reviewed the labels and notice that 

all labels are restricted to closed mixing systems that reduce inhalation exposure 

concerns. 

Developmental Toxicity 

4
 According to the RED, a rabbit developmental toxicity study (MRID 00130744), 

female New Zealand White rabbits were dosed by gavage from days 7 through 19 of 

gestation with terbuthylazine (technical, 98.5% a.i.) at 0, 0.5, 1.5 or 4.5 mg/kg/day. No 

signs of maternal toxicity were observed at any dose tested (in a preliminary study, body 

weight loss was observed at 12.5 mg/kg/day but not at 5 mg/kg/day in the rabbit). The 

maternal toxicity LOAEL is greater than 4.5 mg/kg/day. The maternal toxicity NOAEL is 

equal to or greater than 4.5 mg/kg/day. No signs of developmental toxicity were observed 

in the rabbit at any dose tested. The NOAEL for developmental toxicity is equal to or 

greater than 4.5 mg/kg/day. The LOAEL for developmental toxicity is greater than 4.5 

mg/kg/day. Although an LOAEL for developmental toxicity was not established, this 

study is considered adequate for regulatory purposes since (1) the data indicate that the 

rabbit is not more sensitive than the rat for developmental toxicity since the NOAEL is 

not significantly lower in the rat (4.5 mg/kg/day vs. 5.0 mg/kg/day, respectively), as 

described below, and (2) the rabbit preliminary study indicated that maternal toxicity was 

observed at 12.5 mg/kg/day. 

In a rat developmental toxicity study (MRID 41962701), female rats were 

administered 0, 1, 5 or 30, mg/kg/day terbuthylazine (technical, 96.4% a.i.) by gavage on 

days 6 through 15 of gestation, inclusive. Maternal toxicity was observed at 30 

mg/kg/day as significantly reduced body weight gain (60% less than controls) during the 

treatment period compared to controls and food intake was also reduced (18%). The 

maternal toxicity LOAEL is 30 mg/kg/day based on decreased body weight gain and food 

intake. The maternal toxicity NOAEL is 5 mg/kg/day. Developmental toxicity was also 

observed at 30 mg/kg/day based on a dose-related increased incidence of absent 

ossification of the posterior phalanx of anterior digit 2 (30% litter incidence vs. 10% of 

the controls). The developmental toxicity LOAEL is 30 mg/kg/day based on absent 

ossification in anterior digit 2. The developmental toxicity NOAEL is 5 mg/kg/day. 

Mutagenicity 

The Agency identified several mutagenicity studies in the RED. These studies 

included two bacterial gene mutation assays submitted to satisfy guideline 870.5100, one 

in vitro cell gene mutation study (870.5300), two chromosome aberration assays 

submitted to satisfy guideline 870.5385, and two unscheduled DNA synthesis (UDS) 

studies. The RED DCI did not specify the need for any other mutagenicity studies and no 

new mutagenicity studies were submitted since the RED. The Agency does not anticipate 

the need for the additional mutagenicity studies for registration review and the results 

indicate that terbuthylazine does not appear to not exhibit negative mutagenic effects in 

the studies submitted. 

 

870.5100 Bacterial reverse mutation test 

There were two gene mutation assays. In the first study, Terbuthylazine was 

negative for reverse gene mutation in Salmonella typhimurium strains in assays when 

tested with or without metabolic activation up to limits of solubility (5 mg/ml) in two 

independently conducted studies (MRIDs 00108817 and 00140816;MRID 41634001). 

870.5300 In vitro cell gene mutation test 

5
In a mouse L5178T/TK+/- assay, terbuthylazine did not cause increased mutation 

frequency with or without metabolic activation when tested up to 1 mg/ml (MRID 

00151618). 

870.5385 Mammalian bone marrow chromosomal aberration test 

Terbuthylazine did not induce unscheduled DNA repair in cultured rat 

hepatocytes at test concentrations up to 125 Fg/ml or 1000 Fg/ml in two independently 

performed studies (MRIDs 41391801 and 42059806; MRID 00151619). Terbuthylazine 

was also negative when tested for unscheduled DNA repair at concentrations of up to 125 

Fg/ml in cultured human fibroblasts (MRID 00151620). 

870.5550 Unscheduled DNA synthesis in mammalian cells in culture 

Terbuthylazine did not induce unscheduled DNA repair in cultured rat 

hepatocytes at test concentrations up to 125 Fg/ml or 1000 Fg/ml in two independently 

performed studies (MRIDs 41391801 and 42059806; MRID 00151619). Terbuthylazine 

was also negative when tested for unscheduled DNA repair at concentrations of up to 125 

Fg/ml in cultured human fibroblasts (MRID 00151620). 

Chronic Toxicity and Carcinogenicity 

Mouse: 

In a 2-year chronic feeding/carcinogenicity study (MRID 00156487), 

terbuthylazine (technical, 98% a.i.) was administered in the diet to mice at dose levels of 

0, 30, 150 or 750 ppm (males: 0, 3.28, 16.99 or 86.76 mg/kg/day; females - 0, 3.22, 16.66 

or 88.54 mg/kg/day). Percent body weight gain of males in the 750 ppm group was 

decreased by approximately 10%, while in females it decreased by approximately 23% 

throughout most of the study. Food consumption in males at 750 ppm was decreased by 

approximately 20% throughout most of the study. The LEL for systemic toxicity is 750 

ppm based on decreased body weight in females and a possible decrease in food 

consumption in males. The NOAEL for systemic toxicity is 150 ppm. There was no 

evidence that administration of terbuthylazine was associated with an increase in tumors. 

Rat: 

In a 2-year chronic feeding/carcinogenicity study (MRID 00156486), 

terbuthylazine (technical, 96.8% a.i.) was administered for 24 months to rats at dose 

levels of 0, 30, 150 or 750 ppm (males: 0, 1.24, 6.97 or 41.47 mg/kg/day; females: 0, 

1.37, 7.81 or 52.80 mg/kg/day). At 30 ppm and above, decreased body weight gain was 

observed in males (10%, 28% and 49% less than controls at week 54, low to high dose) 

and females (12%, 32% and 47% at week 54, low to high dose). At 30 ppm and above, 

food consumption was decreased in males (9%, 14% and 25% at 54 weeks) while in 

females only at 150 ppm and above (10% at 54 weeks). The NOAEL is less than 30 ppm. 

Terbuthylazine was associated with increased incidence of testicular interstitial 

cell tumors in males and mammary gland carcinomas in females, but only at a dose at 

6
which excessive systemic toxicity was also observed. The Office of Pesticide Program's 

Health Effects Division (HED) Carcinogenicity Peer Review committee considered the 

systemic toxicity observed at 750 ppm to be excessive (exceeding the maximum tolerated 

dose or MTD) and the slight increases in tumor incidence to be of uncertain relevance to 

human cancer risk assessment (see "Carcinogenicity", below). 

In a second 2-year chronic feeding/carcinogenicity study designed to determine a 

NOEL for chronic systemic toxicity (MRID 00157342), terbuthylazine (technical, 98% 

a.i.) was administered to rats at dose levels of 0, 6 or 30 ppm (males - 0, 0.35 or 1.6 

mg/kg/day; females - 0, 0.36 or 1.6 mg/kg/day). At 30 ppm there were decreases in 

percent body weight gain in males (7% less) and females (12% less) as well as decreases 

in food consumption in males (6% less than controls) and females (11% less) as 

compared to controls. The LOAEL for systemic toxicity is 30 ppm based on transient 

decreases in body weight and food consumption consistent with another study. The 

NOAEL for systemic toxicity is 6 ppm. Terbuthylazine administration was not associated 

with an increase in tumors at the doses tested. 

On May 25, 1994 (Peer Review Document dated August 24, 1994), the Agency's 

Carcinogenicity Peer Review Committee classified terbuthylazine as a Group D 

Carcinogen (inadequate evidence to determine carcinogenicity in humans). The incidence 

of benign interstitial tumors in testes of male rats and of mammary gland carcinoma in 

female rats was increased, but the increase was only observed at a dose at which 

excessive toxicity was observed (750 ppm). The classification was assigned because 

although terbuthylazine is structurally related to other s-triazines that induce similar types 

of tumors, tumors were only observed at a dose that exceeded the MTD and were only 

seen in one species.

Section 3. Dietary Assessment 

Dietary 

Dietary (food) exposures are not anticipated as a concern. Terbuthylazine is not 

expected to be introduced into the food. Terbuthylazine is no longer used as a herbicide 

and these tolerances were revoked in 1992 (53 FR 4860, 56 FR 14471, and 57 FR 

30132).There are no existing food use tolerances for the active ingredient, no registered 

food uses and no indirect food uses. 

Drinking Water 

 The registered uses of tebuthylazine (e.g., recirculating cooling water and 

decorative and ornamental fountains) are not expected to result in contamination of 

surface water and ground water. However, the need for a dietary drinking water exposure 

assessment for terbuthylazine will be determined upon review of environmental fate data 

the Agency anticipates requiring. The environmental data will be useful to determine if 

terbuthylazine can pass through wastewater treatment facilities into drinking water or 

migrate into residential groundwater wells as a result of its current use patterns. 

Therefore, the need for a drinking water assessment will be revised upon submission and 

review of the anticipated environmental fate data. 

7
 

Section 4. Aggregate Exposure 

 In examining aggregate exposure, EPA takes into account the available and 

reliable information concerning exposures to pesticide residues in food and drinking 

water, and non-occupational pesticide exposures. 

Food exposures of concern are not anticipated as a concern based on the fact that 

terbuthylazine is not expected to be introduced into the food, there are no a food use 

tolerances, no registered food uses and no indirect food uses. Residential exposure is not 

expected. The need for a drinking water assessment will be revised upon submission and 

review of the anticipated environmental fate data; therefore, the need for an aggregate 

will be evaluated once a decision is made on the drinking water assessment. 

 Section 408(b)(2)(D)(v) of FFDCA requires that, when considering whether to 

establish, modify, or revoke a tolerance, the Agency consider "available information” 

concerning the cumulative effects of a particular pesticide's residues and "other 

substances that have a common mechanism of toxicity.” 

 

Section 5. Cumulative Exposure 

 During the risk assessment stage for registration review, the Agency will address the 

possibility that terbuthylazine shares a common mechanism of toxicity with other 

members of the Triazine Common Mechanism Group (CMG) and whether terbuthylazine 

should be included in a cumulate exposure group with the rest of the triazines. 

 The Agency examined the triazine class of pesticides with respect to a potential 

common mechanism of toxicity. This examination may be found at 

http://www.epa.gov/oppsrrd1/cumulative/triazines/triazinescommonmech.pdf. Several 

members of this class have been identified as causing toxicity through a similar 

mechanism of action. Specifically, atrazine, simazine, propazine, and certain degradates 

are considered to act through disruption of the hypothalamic-pituitary-gonadal axis to 

attenuate the luteinizing hormone surge. As a result of this attenuation, exposure of the 

animal to estrogen is continuous, providing an environment conducive to the 

development of mammary gland tumors. Mammary gland tumors have been observed in 

female rats after administration of these chemicals, and the postulated mode of action of 

the triazine pesticide atrazine has been reviewed by the FIFRA Scientific Advisory Panel 

and has been found to be plausible (U.S. EPA, 2000). 

 Although there are no tolerances for terbuthylazine, the Agency has not been able to 

determine if there is a need for a drinking water assessment for registration review, thus 

the Agency cannot at this time say definitively that terbuthylazine does not involve 

exposure to the general public. The Agency will reevaluate if a drinking water assessment 

is needed once it receives the applicable environmental fate data during the risk 

assessment process. The Agency will then be able to make a determination of the need 

8
for aggregate and cumulative assessment based on the Agency’s determination of 

drinking water since this is the only applicable potential exposure to the general public. 

Section 6. Endocrine Disruption 

As required under FFDCA section 408(p), EPA has developed the Endocrine 

Disruptor Screening Program (EDSP) to determine whether certain substances (including 

pesticide active and other ingredients) may have an effect in humans or wildlife similar to 

an effect produced by a “naturally occurring estrogen, or other such endocrine effects as 

the Administrator may designate.” The EDSP employs a two-tiered approach to making 

the statutorily required determinations. Tier 1 consists of a battery of 11 screening assays 

to identify the potential of a chemical substance to interact with the estrogen, androgen, 

or thyroid (E, A, or T) hormonal systems. Chemicals that go through Tier 1 screening 

and are found to have the potential to interact with E, A, or T hormonal systems will 

proceed to the next stage of the EDSP where EPA will determine which, if any, of the 

Tier 2 tests are necessary based on the available data. Tier 2 testing is designed to 

identify any adverse endocrine related effects caused by the substance, and establish a 

dose-response relationship between the dose and the E, A, or T effect. 

Between October 2009 and February 2010, EPA issued test orders/data call-ins 

for the first group of 67 chemicals, which contains 58 pesticide active ingredients and 9 

inert ingredients. This list of chemicals was selected based on the potential for human 

exposure through pathways such as food and water, residential activity, and certain postapplication agricultural scenarios. This list should not be construed as a list of known or 

likely endocrine disruptors. 

For further information on the status of the EDSP, the policies and procedures, the 

list of 67 chemicals, the test guidelines and the Tier 1 screening battery, please visit our 

website: http://www.epa.gov/endo/. 

Terbuthylazine is not among the group of 58 pesticide active ingredients on the 

initial list to be screened under the EDSP. Under FFDCA sec. 408(p) the Agency must 

screen all pesticide chemicals. Accordingly, EPA anticipates issuing future EDSP test 

orders/data call-ins for all pesticide active ingredients. 

Section 7. Occupational/Residential Exposure 

 An occupational and/or residential exposure assessment is required for an active 

ingredient if (1) certain toxicological criteria are triggered and (2) there is potential 

exposure to handlers (mixers, loaders, applicators, etc.) during use or to persons entering 

treated sites after application is complete. For the terbuthylazine, the Agency does not 

expect occupational and or residential handler exposure concerns since the application 

occurs in closed liquid pump systems for occupational settings only. In addition, the 

registered uses also recommend personal protective equipment (PPE) when handling 

terbuthylazine. As a result the Agency does not anticipate the need to conduct 

occupational and residential handler dermal and inhalation risk assessments since 

exposure is minimized by engineering controls as well as PPE. 

9
10

Postapplication Exposure 

 The Agency did not identify potential occupational postapplication exposures for 

terbuthylazine in the previous RED. Thus, the Agency does not expect any 

postapplication exposure concerns for registration review. 

Section 8. Incidents 

 In the OPP database, there were two incidents recorded for terbuthylazine. However, 

these incidents were likely a result of misuse of these products. 

Section 9. Anticipated Data Needs 

 The Agency does not anticipate the need for any hazard or exposure data for 

terbuthylazine for registration review based on its current registered uses which prevent 

exposure based on engineering controls (e.g. closed mixing/loading). 

 

Section 10. Tolerances 

 There are no tolerances or exemptions from tolerances for the terbuthylazine active 

ingredients and no registered food uses. Since the only registered uses are for non-food 

uses and no dietary assessment is needed, no tolerances or exemptions from tolerances 

will be needed to support the existing active ingredient registered uses of terbuthylazine. 

 

Section 11. Overall Conclusions 

 The Agency reviewed the hazard and exposure databases for tebuthylazine and 

anticipates that additional human health data will not be needed at this time. The Agency 

has not been able to determine if there is a need for a drinking water assessment for 

registration review, thus the Agency cannot at this time say definitively that 

terbuthylazine does not involve exposure to the general public. The Agency will 

reevaluate if a drinking water assessment is needed once it receives the applicable 

environmental fate data during the risk assessment process. The Agency will then be able 

to make a determination of the need for aggregate and cumulative assessment based on 

the Agency’s determination of drinking water since this is the only applicable potential 

exposure to the general public. The Agency did not identify any specific need for 

additional label changes at this time. 

Section 12. Reference 

• Reregistration Eligibility Decision (RED) Terbuthylazine, March, 1995. 

http://www.epa.gov/pesticides/reregistration/status.htm