UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

OFFICE OF 

CHEMICAL SAFETY AND 

POLLUTION PREVENTION 

PC Code: 109509 

DP Barcode: D399850 

April 24, 2013 

MEMORANDUM 

SUBJECT: 

FROM: 

Preliminary Ecological Risk and Endangered Species Assessment in Support of 

the Registration Review of Acetaminophen 

F. Nicholas Mastrota, Ph.D., Wildlife Biologist ~ ~ 'f/J.'-1/13 

James K. Wolf, Ph.D., Environmental Scientist ~( W~ 

Rosanna Louie-Juzwiak, Risk Assessment Process LVa"der/1) ] , ___ ,_ • lfl ,J 

Environmental Risk Branch 3 V~ Z'1J B 

Environmental Fate and Effect Division 

Office of Pesticide Programs 

THROUGH: Dana Spatz, Branch Chief ~~--....~~'- Melissa Panger, Ph.D., Senior Biologist 

James Hetrick, Ph.D., Senior Advisor 

Environmental Risk Branch 3 

Environmental Fate and Effects Division 

TO: Bonnie Adler, Chemical Review Manager 

Linda Arrington, Acting Chief 

Risk Management and Implementation Branch 5 

Pesticide Re-Evaluation Division (7508P) 

This memorandum transmits the preliminary ecological risk and endangered species assessment 

in support of the registration review of the use of acetaminophen for control of the brown tree 

snake in Guam. Based on currently registered uses and labeled use information, this assessment 

presents the draft conclusions of potential risks to non-target organisms, including listed species 

in Guam and the Commonwealth of the Northern Mariana Islands (CNMI). Currently, the brown 

tree snake is only found in Guam, and is the action area that is the focus of this assessment when 

considering potential exposure to non-target organisms, including endangered species. 

With the limited and transient exposure expected from acetaminophen uses (i.e., in baited mice), 

EPA believes that a "no effect" determination can be made for all aquatic organisms. 

1 
2 

Based on available data, there is a potential for smaller mammals to be exposed to an amount of 

acetaminophen that could exceed the acute level of concern for both nonlisted and listed 

mammals. Concerning listed mammals, the two endangered mammals found in Guam are bats 

that only feed on fruit and other plant matter. Therefore, EPA believes that adverse effects to the 

listed fruit bats are not reasonably expected to occur because exposure is anticipated to be 

limited. 

While there is a potential for birds that feed on carrion (and swallow an entire acetaminophen 

bait) to be exposed to acetaminophen, all known listed birds have since either been extirpated 

from Guam, or are in captivity; therefore, any inadvertent exposure of acetaminophen to listed 

birds is not expected (i.e., will not be exposed to mouse pup carrion) that are used in the baiting 

programs to control the brown tree snake. 

Exposure of other nontarget terrestrial organisms (i.e., insects and plants) to acetaminophen 

residues is expected to be low as a result of its consumption by the brown tree snake and other 

carrion feeders, as well as its decomposition in the environment. Thus, adverse effects are not 

reasonably expected to occur. 

The conclusions of this assessment are consistent with the findings in the November 2, 2011 U.S. 

Fish and Wildlife Service Biological Opinion (USFWS BiOp) issued as a result of the formal 

consultation between the USDA APHIS Wildlife Service (USDA WS) and the USFWS on the 

pesticidal use of acetaminophen to control the brown tree snake in Guam. 
3 

 

The Preliminary Ecological Risk 

and Endangered Species Assessment 

in Support of the Registration Review of 

 Acetaminophen (Paracetamol)

PC Code 109509

(CAS No. 103-90-2) 

Prepared by:

Nicholas Mastrota, Ph.D., Biologist

James K. Wolf, Ph.D., Environmental Scientist

U. S. Environmental Protection Agency

Office of Pesticide Programs

Environmental Fate and Effects Division

Environmental Risk Branch 3

1200 Pennsylvania Ave., NW

Mail Code 7507P

Washington, DC 20460

Rosanna Louie-Juzwiak, RAPL

Reviewed by:

Melissa Panger, Ph.D., Senior Biologist

James A. Hetrick, Ph.D., Senior Advisor

Approved by:

Dana S. Spatz, Branch Chief

April 26, 2013 

Office of Chemical Safety 

and Pollution Prevention 
4 

Table of Contents

1.0 Executive Summary ............................................................................................................. 5

1.1 Nature of Chemical Stressor ............................................................................................ 5

1.2 Summary of Effects and Risk Conclusions ...................................................................... 5

1.3 Potential Risk to Federally Listed Threatened or Endangered Species ........................... 6

2.0 Problem Formulation ........................................................................................................... 7

2.1 Nature of Regulatory Action ............................................................................................ 7

2.2 Nature of Chemical Stressor ............................................................................................ 8

2.3 Overview of Pesticide Usage ........................................................................................... 8

2.4 Ecosystems Potentially at Risk ........................................................................................ 9

2.5 Receptors .......................................................................................................................... 9

3.0 Exposure Assessment......................................................................................................... 10

3.1 Environmental Fate and Transport Characterization ..................................................... 10

4.0 Effects Characterization ..................................................................................................... 14

5.0 Risk Characterization ......................................................................................................... 17

Risk Estimation and Risk Description ............................................................................... 17

5.1 Risk to Terrestrial Animals ............................................................................................ 17

Mammals .............................................................................................................................. 17

Birds, Reptiles and Terrestrial-Phase Amphibians .......................................................... 17

Invertebrates ....................................................................................................................... 19

5.2 Risk to Aquatic Organisms ............................................................................................ 19

5.3 Risk to Terrestrial Plants ................................................................................................ 20

6.0 Endangered Species Assessment ....................................................................................... 20

6.1 Formal Consultation on the Aerial Application of Acetaminophen for Brown Treesnake 

Control on Guam....................................................................................................................... 24

7.0 References .......................................................................................................................... 26
5 

1.0 Executive Summary

1.1 Nature of Chemical Stressor

Acetaminophen, N-(4-Hydroxyphenyl) acetamide, is derived from the interaction of paminophenol and an aqueous solution of acetic anhydride. Acetaminophen, also known as 

paracetamol, is commonly used as a nonprescription analgesic and antipyretic drug similar to 

aspirin. It is the active ingredient (a.i.) of several over-the-counter pain-relief medications (e.g.,

Tylenol®). The only registered pesticidal use of acetaminophen is for brown tree snake (Boiga 

irregularis) control on Guam and the Commonwealth of the Northern Mariana Islands (CNMI) 

(EPA Reg. No. 56228-34). The use involves inserting acetaminophen tablets in a dead mouse 

pup to form a bait, which is then consumed by the brown tree snake. The baits can be placed by 

ground or air. 

The mode of action of acetaminophen in the tree snake is not completely understood, but is 

believed to be the result of induced hepatic (liver) necrosis, as in mammals (MRID 45716714, 

Savarie et al., 2002). In mammals, acetaminophen is biotransformed into a potentially toxic 

metabolite which is then detoxified by conjugation with glutathione. When the dose is large 

enough to deplete hepatic supplies of glutathione, the toxic metabolite becomes available for 

binding with cell membranes and hepatic necrosis ensues (Rumack and Lovejoy, 1986). 

1.2 Summary of Effects and Risk Conclusions

Aquatic Organisms

The registered use of acetaminophen in baits for control of the brown tree snake is not predicted 

to result in significant exposure to aquatic organisms from any mechanism of transport. 

Available submitted data on acetaminophen toxicity to aquatic species are limited to acute 

toxicity tests with aquatic invertebrates published in the open literature (MRID 45716711). The 

aquatic risk assessment is based on the most sensitive aquatic invertebrate species, the Sudanese 

fairy shrimp (Streptocephoreum proboscideus). A test with the waterflea (Daphnia magna), 

which is considered a preferred test species, has an only slightly higher EC50 value of 40.7 mg/L 

(Calleja et al., 1993). Results from both of these tests categorize acetaminophen as slightly toxic 

to aquatic invertebrates. No LC50 estimate is available for acute toxicity to fish or 

marine/estuarine invertebrates.

Terrestrial Organisms

The use of acetaminophen for brown tree snake control is expected to result in minimal exposure 

to terrestrial plants and animals, with the possible exception of exposure to animals that may feed 

on the baited mouse pups. There is a potential for terrestrial invertebrates to be exposed, 

including insects and land crabs, which are commonly found on Guam. However, given the 

limited and targeted application area, and the relatively short period of time that the 

acetaminophen-containing baits are expected to persist in the environment (approximately one 

week) due to scavengers eating mouse pups, picking around the acetaminophen tablet, such as 

land crabs, or degradation in the environment (USFWS BiOp, 2011), any direct exposure is 
6 

expected to be very limited and negligible. Albeit an atypical behavior, available avian studies 

show that there may be rare behavior that could occasionally lead to an exposure that reaches a 

toxic dose (i.e., swallow the entire baited mouse, rather than pick at the carcass around the 

acetaminophen tablet). 

The available acute toxicity data on acetaminophen indicate that it is categorized as practically 

nontoxic to birds, and moderately toxic to mammals. Due to potential concerns for exposure to 

listed birds (i.e., the Mariana crow, Corvus kubaryl), additional studies conducted on surrogate 

birds (i.e., fish crow), to examine the potential acute toxicity from acetaminophen. Although 

regurgitation was observed in the majority of birds dosed (at 80 mg a.i./kg and 160 mg a.i./kg 

doses), one bird did die at the higher dose after repeated regurgitation, which also retained the 

highest percentage of the administered dose (75%). One published paper studied the toxicity of 

acetaminophen to terrestrial plants in the Indian mustard (Brassica juncea), showing that 

exposure to a 1mM solution of acetaminophen in soil may cause phytotoxicity to dicots; 

however, it is unknown how soil exposure relates to environmentally relevant exposure levels 

resulting from acetaminophen baits, and the tablets became exposed to the environment. 

1.3 Potential Risk to Federally Listed Threatened or Endangered Species

The results of this preliminary risk assessment on the use of acetaminophen to control brown tree 

snakes indicate that, with the exception of potential risk to certain-sized mammals and birds, all 

other exposures to terrestrial organisms are below the EPA’s levels of concern. Currently, the 

two endangered mammals found on Guam are the Mariana fruit bat (Pteropus mariannus) and 

the Little Mariana fruit bat (Pteropus tokudae). These two bats are not expected to be directly 

exposed to acetaminophen, as they only feed on plants (primarily fruit). In the 2011 BiOp, the 

USFWS determined that, while aerial applications may adversely impact the Mariana fruit bat 

from a potential collision with a helicopter, this exposure is discountable. EPA expects that the 

same potential exposure and conclusions would apply to the Little Mariana fruit bat. The only 

listed plant identified in Guam is the Serianthes nelsonii tree; however, as determined in the 

2011 USFWS BiOp, because of the proximity of the acetaminophen applications relative to the 

location of the tree, no adverse effects are expected. 

The November 2, 2011 USFWS BiOp concluded that while there is a potential to impact the 

Mariana crow, this use will not jeopardize their survival or recovery in the wild, nor will their 

critical habitat be affected. 

Any potential acetaminophen exposure to aquatic organisms is expected to be very limited and 

below levels of concern; therefore, EFED concludes that there is a “no effect” for all listed 

aquatic species and their designated critical habitat. The potential for effects to listed species 

from the assessed use are summarized in the table below (Table 1). Aside from the conclusions 

described for the Mariana fruit bat, the Little Mariana fruit bat, and the Mariana crow, the 

proposed use of acetominophen for brown tree snake control is not expected to result in adverse 

direct or indirect effects to any other federally listed threatened or endangered species (Table 1).
7 

Table 1. Potential Effects to Federally Listed Taxa Associated with Direct or Indirect Effects from the Use of 

Acetaminophen for Brown Tree snake Control

Listed Taxon Direct Effects Indirect Effects

Aquatic nonvascular plants No No

Aquatic vascular plants No No

Freshwater fish No No

Amphibians, Aquatic-phase No No

Freshwater Invertebrates No No

Marine/estuarine fish No No

Marine/estuarine invertebrates No No

Birds Yes* No

Mammals No No

Reptiles No No

Amphibians, Terrestrial-phase No No

Terrestrial Invertebrates No No

Terrestrial Plants No No

*As discussed in the November 2, 2011 USFWS Biological Opinion, while there is a potential to impact the 

Mariana crow, the registered use of acetaminophen to control the Brown tree snake in Guam will not jeopardize 

their survival or recovery in the wild, nor will their critical habitat be affected. Additionally, as the Mariana crow 

was not observed during multiple recent surveys of known habitat in Guam, USFWS believes that there are no 

Mariana crows remaining in the wild, and only exists in captivity (Earl Campbell, USFWS, personal 

communication).

2.0 Problem Formulation

Problem formulation provides a strategic framework for the risk assessment. By identifying the 

important components of the problem, it focuses the assessment on the most relevant chemical 

properties, exposure routes, and endpoints. 

2.1 Nature of Regulatory Action 

As part of the implementation of the Registration Review program pursuant to Section 3(g) of 

FIFRA, the Agency is evaluating all current and active pesticide registrations for acetaminophen

to determine whether it continues to meet the FIFRA standard for registration. The structure of 

this risk assessment is based on guidance contained in the Overview Document (USEPA, 2004) 

and the U.S. EPA’s Guidance for Ecological Risk Assessment (USEPA, 1998a).

The registration review ecological risk assessment considers only the pesticidal use of 

acetaminophen as an active ingredient for the control of the brown tree snake. However, it 

should be stressed that the non-pesticidal use of acetaminophen is expected to dominate both 

human and wildlife exposure to this chemical. The only registered pesticidal use of 

acetaminophen is for brown tree snake control on Guam and the Commonwealth of the Northern 

Mariana Islands (CNMI). Guam is currently the only place where acetaminophen is used for 

brown tree snake control, and is the focus of the regulatory action being assessed. 
8 

Although the use of acetaminophen is currently registered for use in Guam and the CNMI, EPA 

considers the action area for the registered use of acetaminophen for control of the brown tree 

snake to be Guam. Presently, this is the only place in the United States or its territories where 

the brown tree snake is established. Should a need arise in the future to use the product in other 

locations, an ecological risk assessment and endangered species effects determination may need 

to be conducted for the additional locations. For example, a risk exists for possible invasion of 

the brown tree snake in the state of Hawaii, where active prevention programs are currently in 

place (Colvin et al., 2005). 

2.2 Nature of Chemical Stressor 

Acetaminophen is a white, highly water soluble, odorless crystalline powder. A saturated 

aqueous solution has a pH of about 6 and is stable, but stability decreases in acid or alkaline 

conditions, slowly degrading to acetic acid and p-aminophenol (Paracetamol Information Centre, 

2011). The label (EPA Reg. No.56228-34) states the acetaminophen is comprised of 72.73 % 

active ingredient (a.i.) and 27.27% inert ingredients. The label also indicates that acetaminophen 

is formulated into 40- and 80-mg acetaminophen tablets. Because “acetaminophen” is the name 

of the product and also the a.i., it is unclear as to whether the 40- and 80-mg doses stated on the 

label reflects the mass of the a.i. in a tablet or the total mass of the tablet (a.i. plus inerts). In this 

assessment, we assume the dose is expressed in terms of active ingredient (i.e., 40- and 80-mg 

a.i./tablet).

2.3 Overview of Pesticide Usage

Acetaminophen is an active ingredient in a product used only by wildlife managers to control the 

brown tree snake in Guam and the CNMI. The directions for use on the product label states that 

the acetaminophen tablets be placed in a dead mouse pup to form a bait, which is then consumed 

by the brown tree snake. The baits can be placed by ground or air. In the 2011 USDA-WS 

Environmental Assessment, broadcast application by air (via helicopter) was said to be necessary 

for inaccessible areas. Generally the intent is to suspend the baits in the tree canopy in order to 

maximize exposure to the target pest, i.e., the brown tree snake. 

SINGLE BAIT PREPARATION – Manually insert one 80-mg or two 40-mg acetaminophen 

tablets into the throat of a dead mouse pup (approximate age: 10-15 days). 

BAIT STATIONS – Construct bait stations from lengths of PVC pipe that are 2 to 4 inches (5.1 

to 10.2 cm) in diameter and 12 to 18 inches (30.5 to 45.8 cm) in length. Alternatively, use wire 

mesh funnel traps. Hang bait stations in trees and on fences at heights of approximately 4.9 feet 

(1.5 meters) above ground. Place single mouse baits inside individual bait stations. Place bait 

stations at intervals of approximately 22 yards (20 meters) or greater in and around forested areas 

and along fence lines. Bait density must not exceed 15 baits per acre (36 baits per hectare). Bait 

stations may be restocked at a minimum interval of two days and may be operated continuously 

throughout the year.

BROADCAST APPLICATION – Baits may be hand broadcast or dropped from helicopters and 

fixed-winged aircraft. If the desired bait placement is in the forest canopy, attach a small piece of 
9 

netting or other material to the bait. During one baiting period, apply no more than 90 baits per 

acre (36 baits per hectare) per drop, with a minimum interval between drops of 2 days. No more 

that 15 drops may occur in a single baiting period. No more than two baiting periods per acre or 

hectare may occur in a single year.

Several use restrictions are also included on the pesticide label (EPA Reg. No. 56228-34): 

DO NOT apply this product in a manner that will contaminate food, feed, and water. 

DO NOT apply this product in a manner where it is accessible to children or domestic animals.

DO NOT apply treated baits in areas where there is a danger that threatened or endangered 

species will consume baits unless special precautions are taken to limit such exposures. Such 

precautions shall include applying treated baits inside bait stations that will exclude threatened or 

endangered species that otherwise might feed upon baits. 

2.4 Ecosystems Potentially at Risk

The distribution of mouse pup bait containing acetaminophen tablets for control of the brown 

tree snake is expected to release acetaminophen into the terrestrial and freshwater environments. 

Terrestrial ecosystems would potentially be at risk because non-target terrestrial animals may 

consume the baited mouse pups directly, or by secondary exposure by consuming living or dead 

snakes or other animals that have ingested the bait. Some acetaminophen may leach from the bait 

into the soil, where soil-dwelling animals and plants could be exposed. Small quantities of active 

ingredient may reach freshwater ecosystems, being transported from contaminated soil via 

surface water runoff. While the label for the registered use of acetaminophen for tree snake

control contains the environmental hazard statement “do not apply directly to water,” aerial 

broadcast applications of bait may result in a small amount of inadvertent placement of bait in 

freshwater bodies such as streams, ponds, and wetlands. Bait may also fall in gullies where 

runoff may wash it into streams, ponds, wetlands and lakes. Some exposure to freshwater 

ecosystems on treated islands therefore may occur.

Residues of acetaminophen that enter flowing freshwater would drain into estuaries and marine 

habitats. However, as acetaminophen contamination of freshwater habitats is expected to be 

minor, and contaminated freshwater would be diluted with large amounts of uncontaminated 

seawater in estuary and marine habitats, the concentrations of acetaminophen in estuarine and 

marine habitats is expected to be very small.

2.5 Receptors

In ecological risk assessment, the receptor is the biological entity that is exposed to the stressor 

(USEPA, 1998). For the use of acetaminophen (the stressor) in bait targeting the brown tree 

snake, the highest potential for nontarget exposure is expected to be to terrestrial wildlife, 

including birds, mammals, reptiles, terrestrial-phase amphibians, and terrestrial invertebrates

(e.g., land crabs). In particular, predator and scavenger species are the most significant potential 

nontarget receptors because they may be exposed to a large dose of acetaminophen if they 

consume the dead mouse pups with the acetaminophen tablets intact, or if they consume live or 

dead snakes that have ingested the bait. Soil and aquatic habitat may be exposed as residues that 
10 

can leach out of the remaining baits and carcasses of dead snakes, or are excreted by animals that 

consume the bait. Therefore, soil invertebrates and aquatic organisms are also potential 

receptors.

Assessment endpoints represent the actual environmental value that is to be protected, defined by 

an ecological entity (species, community, or other ecological entity) and its attributes or 

characteristics (USEPA, 1998). For the acetaminophen risk assessment, the ecological entities 

include all species of animals and plants inhabiting Guam and the CNMI. The attributes for each 

of the ecological entities may include growth, reproduction, and survival. 

3.0 Exposure Assessment

The registered use of acetaminophen in bait for the brown tree snake is not predicted to result in 

significant exposure to aquatic organisms from any mechanism of transport. Because the 

acetaminophen is formulated into tablets that are inserted in the bait (mouse pups), transport 

through spray drift is not relevant. Also, because acetaminophen has a very low vapor pressure 

and is formulated as a tablet, any volatilization and inhalation exposure are expected to be 

negligible. Any remaining baits not consumed by the target pest or other opportunistic feeders 

may result in some amounts, albeit small, of acetaminophen to potentially leach out of the baited 

mouse pups into the soil, and then may be transported to water via surface runoff; however, this 

route is expected to result in trivial contributions to aquatic exposure. A few of the placed baited 

mouse pups may inadvertently be dropped in or transported to water bodies, in which case the 

active ingredient may dissolve and disperse into the ambient water. But again, this route is 

expected to result in little contamination of surface water.

3.1Environmental Fate and Transport Characterization

Physical and Chemical Properties

Acetaminophen is a white, highly water soluble (12,780 μg/L), odorless crystalline powder. A 

saturated aqueous solution has a pH of about 6 and is stable but stability decreases in acid or 

alkaline conditions, slowly degrading to acetic acid and p-aminophenol (Paracetamol 

Information Centre, 2011). Acetaminophen (paracetamol) is rapidly transformed in water into 

volatile transformation products (TP) [undefined TP except for CO2], resulting in a DT50 of 3.1 

days. Acetaminophen is highly persistent and mobile in soil. Upon contact with the sediment, 

the majority of acetaminophen was bound to the sediment as non-extractable residues. 

Acetaminophen did not remain in the water phase, but was rapidly bound to the sediment. The 

bound TP residues were more persistent, potentially accumulating in the sediment. In the 30 day 

study, hydrolytic and microbial degradation were not significant degradation pathways, as 

acetaminophen did not degrade in sterile or non-sterile pond water samples stored in the dark. 

Degradation was observed to occur when exposed to direct or indirect light, suggesting that 

exposure to sunlight is an important process in the degradation of acetaminophen. 

Physical/chemical properties and environmental fate data available for acetaminophen are 

summarized in Table 2 and briefly discussed in the following section. The USDA Animal and 

Plant Health Inspection Services (APHIS) requested data waivers for the following guideline 

environmental fate studies (hydrolysis, photodegradation in water, aerobic soil metabolism, 
11 

leaching/adsorption/desorption, and field dissipation-soil studies (2002). They submitted (MRID 

45716708) data from other sources to address the environmental fate and transport of data 

requirements for acetaminophen. The Agency also considered data available in literature (Table 

2). The structure of acetaminophen is shown in Figure 3. 

Table 2. Physical and Chemical Properties of Acetaminophen

Property Value Source of Information

Common name Acetaminophen 

(Paracetamol)

Chemical name N-(4-hydroxyphenyl)-acetamide

N-(4-hydroxyphenyl)acetamide 

N-acetyl-p-aminophenol

MRID 45716708

Pesticide type Acetaminophen is part of the class of 

pharmaceutical drugs known as "aniline

analgesics". Acetaminophen is not 

currently registered as a pesticide for 

uses in food commodities, but is 

registered as a pesticide used to control 

brown tree snake in Guam or the 

Commonwealth of Northern Mariana 

Islands.

Chemical class p-aminophenol, acetic anhydride MRID 45716708

CAS number 103-90-2 MRID 45716708

Empirical formula C8H9NO2 MRID 45716708

Molecular mass (g/mol) 151.16 MRID 45716708; 

El-Obeid and Al-Badr, 1985.

Vapor pressure 1.65 x 10-7 mmHg to 0.0005 mmHg for 

the temperature range 40°C to 74°C

Williams et al. 2011.

Solubility in water 12,780 mg/L @ 20 °C

14,285 mg/L

Granberg and Rasmuson, 

1999

MRID 457167-08

Dissociation Constant (pKa) 9.5 @ 25oC

9.38

MRID 457167-08

Löffler et al., 2005; Hari et 

al., 2005; Lorphensri et al., 

2006

Log Kow 0.27

0.46

Yamamato et al., 2005

Hari et al., 2005

Koc (organic carbon normalized 

partition coefficient)

2.7(±0.0) x102 L/kgoc (soil; %OC=2.7)

3.3 (±0.9) x103 L/kgoc (river sediment; 

%OC=0.79%)

Yamamato et al., 2005

Kd 7.7 (±0.9) L/kg (soil)

2.6 (±0.7) L/kg (river sediment)

0 (Not significant in soils with variable 

charge (i.e., soils with a pH dependent 

charge)

Yamamato et al., 2005

Lorphensri et al., 2006 
12 

Figure 1. Chemical structure of acetaminophen. 

The length of time, or persistence, that a pesticide or metabolism/transformation products will 

remain in the environment is influenced by both biotic and abiotic processes, sorption to soil 

material, and the environmental factors (temperature, precipitation, length of day, water 

turbidity, etc.) which act upon the chemical. Acetaminophen is formulated as a tablet and then 

placed in a “bait” (dead mouse), and, thus, is not directly applied to water, soil, or vegetation. 

Therefore, while in the bait, photolysis is not anticipated to be significant. Based upon the 

limited data noted above, acetaminophen is expected to be fairly stable in the bait (in the dead 

mouse), due to limited photolysis and direct exposure to environmental conditions. Storage and 

field stability data shown in Johnston et al. (2002; Table 2), suggest that the acetaminophen will 

have a half-life of about 90-days in the mouse bait. When exposed to the environment, 

acetaminophen appears to be rapidly transformed in the water phase when exposed to light. It is 

anticipated that based upon the pesticidal use of acetaminophen, the potential to contaminate 

surface water via runoff would be minimal. 

Persistence

Löffler et al. (2011) reported that radiolabeled acetaminophen (paracetamol) transformed in 

water (water (pH 8.5)/sediment (pH 7.7) study) into volatile transformation products (TP) 

[undefined TP except for CO2], resulting in a DT50 of 3.1 days. Upon contact with the sediment, 

the majority of acetaminophen was bound to the sediment as non-extractable residues. The study 

suggested that acetaminophen did not remain in the water phase, but was rapidly bound to the 

sediment. The bound TP residues were more persistent, potentially accumulating in the sediment 

(Löffler et al., 2011). Lam et al. (2009) evaluated the aquatic persistence of eight 

pharmaceuticals, including acetaminophen, in a microcosm study. In the 30 day study, hydrolytic 

and microbial degradation were not significant degradation pathways, as acetaminophen did not 

degrade in sterile or non-sterile pond water samples stored in the dark. Degradation was 

observed to occur when exposed to direct or indirect light, suggesting that exposure to sunlight is 

an important process in the degradation of acetaminophen. The average half-lives [t1⁄2] of

acetaminophen at four treatment levels (n=4) were 1.1, 0.7, 0.9 and 1.0 days for ultra-high to low 

treatment concentrations1. The overall average half-life was 0.9 ± 0.2 days. 

 

1 Treatment concentrations were chosen based upon distributional analysis of the upper exposure centiles (e.g., 95th, 
13 

In addition to the pesticidal use of acetaminophen to control brown tree snakes in Guam and the 

CNMI, acetaminophen is also a widely used pharmaceutical. The pharmaceutical use can also 

introduce acetaminophen into the environment in several ways. One important pathway is 

through discharge of acetaminophen residues remaining in treated effluent from sewage 

treatment plants, most of which predate the identification of pharmaceuticals as potential water 

contaminants and, as a result, are not designed for complete removal of these microcontaminants (Kinney et al., 2006). Septic system effluent discharge and disposal of sewage 

sludge are other potential mechanisms that can introduce pharmaceuticals to surface water, 

groundwater, and soil. Kinney et al. (2006) investigated the presence and distribution of a 

number of pharmaceuticals, including acetaminophen, in soil irrigated with water containing 

acetaminophen (mean acetaminophen concentrations ranged 8.1 to 65.3 ng/L) derived from 

urban waste water treatment sources. Although this study does not allow for determination of a 

half-life (degradation or dissipation), acetaminophen (integrated mass with depth and time) 

tended to “increase in mass during the irrigation period” during the course of the study, 

suggesting that acetaminophen was somewhat persistent.

Mobility

The ionization of acetaminophen (a non-hydrophobic chemical) is strongly dependent upon the 

system pH, pKa (~9.4), hydrophobicity, and the nature of the surface charge of the sorptive media 

(Lorphensri et al., 2006). Lorphensri et al. (2006) found that acetaminophen showed no sorption 

(@ pH 7) to two pure minerals (silica and alumina) and a hydrophobic medium (Porapak P2). 

Yamamato et al. (2005) reported mean Kd of 7.7 and 2.6 L/kg and Koc of 270 and 330 for Elliot 

Silt Loam soil and Suwannee River sediment, respectively. In order to standardize the qualitative 

descriptors used to report the chemical mobility in soil of pesticides in exposure assessments, the 

Agency has adopted the standardized classification scheme published by the Food and 

Agricultural Organization (FAO) of the United Nations (Bradbury, 2006). Based upon this 

scheme, acetaminophen would be classified with a medium mobility class (Koc range of 150 to 

500). 

Monitoring Data

Acetaminophen has been included in a number of surface and ground water monitoring programs 

designed to evaluate the occurrence of pharmaceuticals in water (Table 3). The maximum 

surface water concentration was 10 μg/L and the maximum ground water concentration was 8.1 

μg/L. The source of acetaminophen in these studies was from the pharmaceutical use (no 

monitoring data from Guam). The highest concentration detection was 78.2 μg/L from a sewage 

treatment plant (STP) discharge. 

 

99th, 99.5th, 99.9th) estimated from actual surface water concentrations (0.83, 33.98, 132.0 and 2190 μg/L for low, 

medium, high, and ultrahigh treatment levels, respectively (Lam et al., 2004). 2 Porapak P is a cross-linked polymer of divinyl-benzene/styrene purchased from Supelco.
14 

Table 3. Summary of acetaminophen in detected in drinking, ground and surface water, and waste water. 

Study Water Type Number of 

Samples

Percent 

Detections

Maximum 

Detection 

(μg/L)

Reporting 

Limit

(μg/L)

Barnes et al., 2004 Ground Water 5 0 0 0.009

Barnes et al., 2008 Ground Water 47 6.4 0.38 0.009

Kolpin et al., 2002 Surface Water 84 23.8 10 0.009

Focazio et al. 2008 Ground/Surface 74 23 8.1 0.009

Fram and Belitz, 2011 Ground Water 1231 0.32 1.89 0.06

Lee et al., 2004 All water1 131 15.3 16.0 0.009

Lucia et al., 2010 Surface (STP)2 -

3 - 78.2 - 1 Waste water, surface, ground and drinking water.

2

 STP- Sewage Treatment Plant discharge 3

 – not specified

4.0 Effects Characterization

Table 4 provides a summary of acute toxicity findings for taxonomic groups, along with their 

corresponding surrogate species, that the Agency used to assess acute risk of acetaminophen to 

non-target animals. In addition, the table provides an overview of acute toxicity classifications 

based on these findings. Results for acute oral and sub-acute dietary toxicity to birds are based 

on acceptable guideline studies submitted by the registrant. An acute oral toxicity study 

(guideline number 850.2100) was conducted with the northern bobwhite, and a sub-acute dietary 

study (guideline number 850.2200) was conducted with both the northern bobwhite and the 

mallard. In all three of these studies, no mortality was observed at the highest test concentration, 

yielding non-definitive endpoint estimates, and establishing the acute toxicity category as 

“practically nontoxic.” Results for acute toxicity to mammals were obtained from data published 

in the open literature. Among the species tested, the most sensitive mammal was the house 

mouse (Mus musculus), which was considerably more sensitive than the Norway rat (LD50 range 

1944 to 4450 mg/L, MRID 45716715). Acute toxicity to the house mouse categorizes 

acetaminophen as “moderately toxic” to mammals.

Toxicity findings for aquatic species are limited to acute toxicity tests with aquatic invertebrates 

published in the open literature. These studies are summarized in MRID 45716711. The aquatic 

risk assessment will be based on the most sensitive aquatic invertebrate species, the Sudanese 

fairy shrimp (Streptocephoreum proboscideus). A test with the waterflea (Daphnia magna), 

which is considered a preferred test species, has an only slightly higher EC50 value of 40.7 mg/L 

(Calleja et al., 1993). Results from both of these tests categorize acetaminophen as slightly toxic 

to aquatic invertebrates. No LC50 estimate is available for acute toxicity to fish or marine/ 

estuarine invertebrates.

The toxicity values that are used in the risk assessment are the most sensitive toxicity values 

obtained from scientifically valid studies with mammalian and invertebrate test species. The 

acute toxicity values for mammals and invertebrates are presented in Table 4. 
15 

Table 4. Test Species Evaluated for Assessing Potential Ecological Effects of Acetaminophen and the 

Associated Acute Toxicity Findings

Taxonomic 

Group Surrogate Species Acute Toxicity2 CitationMRID

Acute 

Toxicity 

Classification

Birds1

Northern Bobwhite

(Colinus 

virginianus)

and

Mallard (Anas 

platyrhynchos) 

LD50 > 2250 mg a.i./kg

NOAEL = 2250 mg a.i./kg

MRID 45716712

(bobwhite)

Practically LC50 nontoxic > 5000 mg a.i./kg feed3

NOAEC = 5000 mg a.i./kg 

feed3 

MRID 45717601 

(bobwhite)

MRID 45716713 

(mallard)

Mammals Laboratory mouse

(Mus musculus)

LD50 = 338 mg a.i./kg

95% CI : 306 - 373 mg 

a.i./kg

MRID 45716715

(Starmer et al., 

1971)

Moderately 

toxic

Freshwater 

invertebrates

Sudanese fairy 

shrimp 

(Streptocephalus 

proboscideus)

EC50 = 29.5 mg a.i./L

95% CI : Not reported

MRID 45716711

(Calleja et al., 1993) Slightly toxic

1 Birds represent surrogates for terrestrial-phase amphibians and reptiles; fish serve as surrogate species for the 

aquatic phase.

2 These values may be revised if future studies or literature become available.

3 Results for the northern bobwhite and mallard were identical.

Pen studies with wild caught fish crows (Avery et al., 2004; Johnston et al., 2002) provide 

further information on the toxicity of acetaminophen to birds. One study examined the acute 

toxicity of acetaminophen to the fish crow (Corvus ossifragus), which served as a surrogate for 

the endangered Mariana crow (Corvus kubaryi) of Guam. Five crows were each force-fed two 

40-mg tablets of acetaminophen, for a dose of 80 mg/bird. This is the same amount of active 

ingredient that is placed within a single mouse pup as bait to control brown tree snakes. The 

study was then repeated with a dose of 4 tablets (160 mg/bird). At the 80-mg dose, 4 out of 5 

birds appeared lethargic after dosing and 3 of the birds vomited, but all birds survived. At the 

160 mg dose, all five of the crows regurgitated the acetaminophen tablets. One of the five crows 

died three hours after repeated regurgitation. Based on the weight of recovered vomited tablets, 

compared to weights of intact tablets, 12% to 75% of the administered dose remained ingested 

after vomiting, with the greatest percentage (75%) being in the bird that died. 

While the number of birds tested is too small to draw definitive conclusions, this study indicates 

that that ingestion of two 40-mg tablets of acetaminophen (a.i.) causes lethargy but is not lethal 

to crow-size birds, whereas the ingestion of four 40-mg tablets can be lethal. As the fish crow 

has a body weight of approximately 300 mg (NatureServe, 2011), administration of 160 mg to a 

fish crow is equivalent to an acute oral dose of 533 mg a.i./kg BW. Thus, this study suggests that 

crows may be more sensitive to acetaminophen than the northern bobwhite, for which no 

mortality was observed with acute oral doses up to 2250 mg/kg BW (MRID 45716712). 

In a second study, crows were offered dead mouse pups treated with two 40-mg acetaminophen 

tablets and observed. Ten crows were first observed feeding on an untreated mouse pup in 5 

daily trials, and then were observed feeding on a treated mouse pup on one final trial, for a total 

of 60 trials altogether. The crows were observed to mainly feed on the pups by tearing them 
16 

apart bit by bit. They generally picked the acetaminophen tablets from the carcass and either set 

them aside or drop them from the perch. Only one out of ten birds was observed to swallow the 

whole pup intact, with the tablets inside, and this bird did so only in one out of six instances. 

This avoidance of the acetaminophen tablets by birds feeding on carcasses may limit the 

exposure and risk of the baited mouse pups to scavenging birds in the wild (Avery et al., 2004; 

Johnston et al., 2002). 

Information on the chronic toxicity of acetaminophen is limited to mammals. The registrant 

submitted a report that summarizes data on chronic toxicity to mammals (MRID 45716715). 

Submitted chronic studies that showed frank effects relatable to ecological risk, which included 

effects on growth, survival and/or reproduction, were used to select a chronic toxicity estimate 

that will be used to assess ecological risk to mammals. Studies that provided frank effects from 

chronic exposure include a mouse prenatal development toxicity study (Hess, 1984) and a twogeneration mouse reproduction toxicity study (National Toxicology Program, 1984). The 

prenatal developmental study found that exposure at a daily dose of 250 mg/kg (approximately 

5000 mg/kg-diet) caused a reduction in bodyweight and developmental abnormalities. Exposure 

at 140 mg/kg (approximately 2800 mg/kg-diet) did not affect bodyweight but did result in a low 

frequency of abnormalities. In the two-generation reproduction study, the most sensitive 

endpoints were body weights of F1 pups. Exposure at 2500 mg/kg diet resulted in significant 

decreases in body weights, relative to the control, of male and female pups at 28 and 74 days. 

Body weight gain of pups also was reduced at this exposure level. Relative to the control, pup 

body weights decreased by 6-10% at 2500 mg/kg-diet, 8-16% at 5000 mg/kg-diet, and 16-34% at 

10,000 mg/kg-diet. Reproductive and fertility endpoints were not affected at the expose levels of 

2500 and 5000 mg/kg-diet level. Exposure to 10,000 ppm (1%) resulted in reduced number of 

litters per pair and reduced number of live male pups. While these studies do not provide a 

NOAEL for frank effects on mammals, they do indicate that significant frank effects begin to 

occur at dietary exposures of approximately 2500 mg/kg-diet. Thus, 2500 mg/kg-diet will be 

used as an approximate threshold of chronic effects for assessing chronic risk to mammals.

One published paper provided information on the toxicity of acetaminophen to terrestrial plants. 

Bartha et al. (2009) studied the uptake, translocation, toxicity, and detoxification of 

acetaminophen in the Indian mustard (Brassica juncea). In this study, 8-week-old test plants 

were treated by administration of a 1-mM (151-mg/L) solution of acetaminophen to the growth 

medium for one week. Exposure to this concentration was found to cause oxidative stress that 

resulted in irreversible damage to the plants. Leaves of treated plants showed bleaching and 

lesions after 72 h, and necrosis after 168 h. Between 72 and 168 h, roots of treated plants turned 

dark brown and ceased to show lateral development. Plant pigments (chlorophyll a, b, and 

caratenoids) showed a 50% decrease after 72 h and 70% decrease after 1 week. While this study 

shows that soil exposure to a 1 mM solution of acetaminophen may cause phytotoxicity to plants, 

the relationship of this exposure to environmentally relevant exposure levels resulting from the 

pesticidal use of acetaminophen in mice baits is unknown. 
17 

5.0 Risk Characterization 

Risk Estimation and Risk Description

Methods traditionally used to estimate aquatic and terrestrial exposure from typical applications 

of agricultural pesticides are not appropriate for estimating exposure from the use of 

acetaminophen in bait for tree snake control. The bait are distributed in tablets that are placed 

inside mouse pups, and therefore are not subjected to the same environmental degradation and 

transport mechanisms that would be in play for a pesticide that is broadcasted onto soil or foliage 

as a spray or granule. Therefore, this use is expected to result in limited direct exposure to 

terrestrial plants; likewise for terrestrial invertebrates, with the exception of larger scavenging 

terrestrial invertebrates (e.g., land crabs), very minimal exposure is expected. Much of the 

distributed bait is expected to be consumed by brown tree snakes. When the bait is consumed by 

an animal, only a fraction of acetaminophen that is excreted without transformation, or that is 

released when the animal dies and decomposes, would be introduced into the soil. Only this 

small fraction that is released onto the soil would be subjected to transport by runoff into aquatic 

habitats. Models developed for agricultural uses of pesticides likely would greatly overestimate 

the amount of acetaminophen bait that would reach water bodies, and are therefore not used in 

this assessment. Specific details for terrestrial and aquatic environments are given in the 

following sections. 

5.1 Risk to Terrestrial Animals

Mammals

When using a dose-based approach, one can estimate the number of baited mouse pups that a 

mammal would need to consume to reach the LD50. The LD50 for the laboratory mouse is 338 mg 

a.i./kg-BW (MRID 45716715); therefore, a 1,000-g sized mammal would need to ingest 4.2 

tablets (or at least four baited mouse pups) to reach the LD50. However, when scaling the 

exposure to a smaller mammal (e.g., 50 grams), the LD50 is 16.9 mg a.i./50-g mammal, which is 

less than one acetaminophen tablet. Therefore, since the required ingestion rates are plausible for 

both the small and larger mammals, there is potential concern for exposure to mammals if they 

were to consume acetaminophen-baited mouse pups. 

Birds, Reptiles and Terrestrial-Phase Amphibians

As a first-tier risk assessment to estimate potential acute risk, exposure to birds is modeled based 

on the assumption that the baited mouse pups are consumed in entirety and that all of the active 

ingredient within the contained tablets is ingested. Avian reproduction studies with 

acetaminophen are not available, and therefore, is an uncertainty. Based on information provided 

on the label, 80 mg of acetaminophen active ingredient are inserted into each mouse pup in one 

or two tablets. For calculating the dietary concentration of acetaminophen, the weight of a 

mouse pup is assumed to be 5 g (i.e., 0.005 kg), as was assumed in the previous published risk 

assessment of Johnston et al. (2002). Thus, the dietary concentration of acetaminophen in the 

baited mouse pup is 80 mg a.i./0.00508 kg or 15.7 mg a.i./kg-diet. The dietary LC50 for both the 

northern bobwhite and the mallard is >5000 mg a.i./kg-diet. Thus, the dietary RQ for a bird 

feeding directly on the baited mouse pup is 15.7/>5000, or <0.003. Thus, based on the dietary 
18 

risk quotient assessment, acute risk to birds that may feed directly on the baited mouse pups is 

below the EPA’s level of concern.

A single dose acute oral study with the northern bobwhite found that a dose of 2250 mg a.i./kg 

caused no mortality and no observable adverse effects. As was calculated for estimating 

potential exposure to mammals, when using a dose-based approach, one can estimate the 

number of baited mouse pups that a bird would need to consume to reach the NOAEL dose. The 

Mariana crow will be used as a model species. The average body weight of an adult crow is 242 

g for females and 256 g for males (USFWS, 2005) therefore the average weight for females is

used in this risk assessment. The oral dose of a bird the size of a Mariana crow that swallows 

one baited mouse pup (containing 80 mg of acetaminophen) is 80 mg a.i./0.242 kg-BW or 331 

mg a.i./kg-BW. Dividing the NOAEL dose, 2250 mg a.i./kg, by this dose for one mouse pup 

yields 6.81. This dose-based assessment indicates that a crow would need to consume seven or 

more whole baited mouse pups before there could be a level of concern exceedance. 

Pen studies with the fish crow (Avery et al., 2004), which are discussed in Section 4.0 of this 

document, showed somewhat greater acute toxicity in this species than predicted by the 

quantitative risk assessment. In a trial in which five fish crows were force-fed 80 mg of 

acetaminophen, four of the birds appeared lethargic after dosing and three of the birds vomited, 

but all of them survived. In a second trial in which five fish crows were given a 160-mg dose, 

all of the birds regurgitated and one of the birds died. These results indicate that a crow-sized 

bird may suffer sublethal effects from consuming one whole baited mouse pup, and could die 

from consuming two whole mouse pups within a short period. 

Another study looked at the behavioral response of fish crow to mouse pups baited with 

acetaminophen (Avery et al., 2004; Johnston et al., 2002). In this study, the majority of the birds 

tested (9 out of 10) did not swallow the whole mouse pup with the acetaminophen tablets intact, 

but rather tore the bait apart bit by bit and discarded the tablets with their bill. 

A second factor that is expected to minimize exposure risk to birds is the use of bait traps in 

some areas of baiting programs. The label recommends use of bait stations made of PVC pipe 2-

4 inches in diameter and 12-18 inches in length. Avery et al. (2004) observed the response of 

fish crows to bait stations with a mouse pup inserted half-way in the pipe. They tested PVC pipes 

that were at the extremes of the recommended dimensions (2 or 4 inches in diameter, and 12 or 

18 inches long). They found that the longer pipes significantly reduced the total number of pups 

that were accessed. They found that some crows succeeded in accessing the mouse pups from all 

except the long and narrow bait stations. When short and wide stations were used, 5 out of 10 

crows were able to access at least one pup. Based on these results, use of bait stations likely 

would reduce the ingestion of the baited mouse pups by avian predators , but likely would not 

completely eliminate it unless they are constructed with the longest length and narrowest width 

recommended on the label. 

Birds represent surrogates for terrestrial-phase amphibians and reptiles; therefore, risk 

conclusions for birds also apply to reptiles and terrestrial-phased amphibians on Guam. 
19 

Invertebrates 

The Agency does not have standardized risk assessment method for quantitatively assessing risk 

to terrestrial invertebrates for this type of use pattern; therefore, any potential exposure and risk 

is discussed qualitatively. Exposure to terrestrial invertebrates is expected to be minimal given 

the majority of the baited mouse pups are consumed by the brown tree snake and that the 

acetaminophen tablets are inserted into a dead mouse pup with very limited dispersion 

throughout the mouse pup tissue. Exposure could occur if an invertebrate were to feed on the 

carcass as it decomposes (e.g., land crabs), but would likely be minimal as they would likely 

consume tissue around the acetaminophen tablets without consuming the tablets themselves. 

Exposure through contact with contaminated soil is possible but is also expected to be minimal. 

Therefore, while there is a potential for exposure to terrestrial invertebrates, EFED believes that 

exposure is very limited and accompanying risk is very unlikely. 

5.2 Risk to Aquatic Organisms

Because use of the traditional aquatic exposure models is not appropriate, as described above in 

the exposure assessment, an alternative approach is used to estimate aquatic environmental 

exposure concentrations (EECs) from this use. As a worst-case scenario, baited mouse baits are 

assumed to be placed directly into a body of water, and all of the active ingredient contained 

within that pup (80 mg per pup) is assumed to dissolve into the ambient water. While the 

Environmental Hazards section of the label contains a statement that prohibits direct application 

to water, aerial broadcast application could result in a small number of baited pups inadvertently 

entering bodies of freshwater. Also, a small number of baited pups may be transported into water 

by rainwater runoff or movement by animals.

A first-tier screen for aquatic risk assessment can be conducted by determining the number of 

baited pups that must enter a pond for the water concentration in that pond to reach a level that 

would make the aquatic risk quotient (RQ) equal to the aquatic risk LOC. In this assessment, all 

of the active ingredient in the bait is assumed to dissolve and be distributed evenly within the 

pond with no degradation. As acetaminophen is highly soluble in water (12,780 mg/L), solubility 

is not expected to be a limiting factor. The standard pond the Agency uses to assess risk to 

aquatic organisms is a one hectare by two meter deep pond that contains 20,000 liters of water. 

Based on the acute LC50 of 29.5 mg a.i./L for the Sudanese fairy shrimp, the acetaminophen 

concentration in the pond would need to reach 14.8 mg a.i./L for the acute aquatic RQ to equal 

the LOC of 0.5. 

Based on label information, 80 mg of acetaminophen is placed in each mouse pup, and thus 80 

mg will added to the water per each mouse pup that enters the pond. Therefore the total number 

of mouse pups that must enter the 1-ha pond for the acute aquatic RQ to equal the LOC is 

calculated as follows:

14.8 mg a.i./L x 1 pup/80 mg a.i. x 20,000 L = 3700 pups 

This calculation shows that 3700 mouse pups would need to enter the 1-ha pond for the 

acetaminophen concentration in the pond to reach levels that would be of concern for aquatic 

organisms. Considering that the application mechanism is intended to maximize the number of 
20 

baited pups to remain hanging in tree limbs (i.e., dropping baited-mouse pups that are attached to 

cardboard on paper streamers that become entangled in tree limbs), the label prohibits 

application of this product directly to water, and doing so would clearly make the bait ineffective 

for controlling tree snakes, and the maximum number of baited mice allowed per acre is 90, it is 

unlikely that any inadvertent exposure to acetaminophen in the waterbody would reach or exceed 

the level of concern (i.e., equivalent to 3,700 baited pups relative to the most sensitive aquatic 

animal). Thus, the Agency believes that the concentration of acetaminophen in aquatic habitats is

well below the level of concern for aquatic species. 

5.3 Risk to Terrestrial Plants

The registered use of acetaminophen to control the brown tree snake is expected to result in very 

minimal exposure to terrestrial plants. The bait is distributed as tablets placed inside of mouse 

pups. Most of these pups are expected to be consumed by snakes, and thus, most of the active 

ingredient is not expected to be directly released into the soil. Residues entering the soil would 

be limited to the amount that leach from the baited mouse pups that are not consumed, the 

amount that is excreted from animals that consume the bait, and the amount that is released when 

poisoned brown tree snakes die and decompose. None of these exposure routes are expected to 

result in significant contamination of soil or exposure to terrestrial plants. Therefore, the Agency 

believes that risk to terrestrial plants is negligible. 

6.0 Endangered Species Assessment

Consistent with the Agency’s responsibility under the Endangered Species Act (ESA), EPA has 

evaluated risks to Federally-listed threatened and/or endangered (listed) species from uses of 

acetaminophen for brown tree snake control in Guam. This assessment was conducted in 

accordance with the Overview Document (USEPA, 2004), provisions of the ESA, and the 

Services’ Endangered Species Consultation Handbook (USFWS/NMFS, 1998). 

The assessment of effects associated with registration of acetaminophen is based on an action 

area. The action area is considered the area directly or indirectly affected by the federal action, as 

indicated by the exceedance of LOCs used to evaluate direct or indirect effects. The EPA’s 

approach to defining the action area under the provisions of the Overview Document (USEPA, 

2004) considers the results of the risk assessment process to establish boundaries for that action 

area with the understanding that exposures below the Agency’s defined LOCs constitute a noeffect threshold. For the purposes of this assessment, attention is focused on the footprint of the 

action (i.e., the area where acetaminophen application occurs), plus all areas where offsite 

transport (e.g., runoff, volatilization and long-range atmospheric transport, etc.) may result in 

potential exposure that exceeds the LOCs. As acetaminophen is registered only for the control of 

the brown tree snake, which is currently established only on Guam, both the footprint of action 

and the action area of this endangered species assessment for acetaminophen is the island of 

Guam. Since the brown tree snake persists throughout the island, the entire island of Guam is 

included in the action area. 
21 

While there are various federally listed endangered and threatened mammals, birds and reptiles, 

and plants listed for Guam and the CMNI, specific to the action area (i.e., Guam), listed species 

that occur (or have previously been found) on Guam consist of 6 birds, 2 fruit bats, 2 marine 

turtles, and 1 terrestrial plant (Table 5). The food habits of these species are provided in Table 6. 

Table 5. Federally Listed Threatened and Endangered Species Occurring in Guam and the Commonwealth 

of the Northern Mariana Islands (CNMI) (based on USFWS, 2011)

Common Name Scientific Name Federal Listing

Distribution Critical 

Guam CNMI Habitat

Mammals

Bat, Little Mariana 

fruit

Pteropus tokudae Endangered X ND2

Bat, Mariana fruit Pteropus mariannus Threatened X X Guam3

Birds

Crow, Mariana Corvus kubaryl Endangered X1 X Guam3, 

Rota4 

Kingfisher, Guam 

Micronesian

Todiramphus 

cinnamominus 

Endangered X1 Guam3

Moorhen, Mariana 

Common

Gallinula chloropus 

guami

Endangered X X ND2

Rail, Guam Gallirallus owstoni Endangered X1 ND2

Reed-Warbler,

Nightingale

Acrocephalus luscinia Endangered X1 ND2

Swiftlet, Mariana Aerodramus bartschi Endangered X X ND2

White-eye, Guam 

Bridled

Zosterops conspicillatus Endangered X ND2

White-eye, Rota Zosterops rotensis Endangered X ND2

Reptiles

Turtle, Green Sea Chelonia mydas Threatened X X ND2

Turtle, Hawkbill Eretmochelys imbricate Endangered X ND2

Plants

Hayun Lagu (Guam)

Tronkon Guafi (Rota)

Serianthes nelsonii Endangered X X ND2

(No common name) Nesofenes rotensis Endangered X ND2

(No common name) Osmoxylon mariannense Endangered X ND2

1

Extirpated from the wild in Guam. 2 ND = not defined

3 Critical habitat is designated on the northern tip of Guam, within the Guam National Wildlife Refuge.

4 Rota is an island that is part of the CNMI.
22 

Table 6. Food Habits of Federally Listed Threatened and Endangered Species Occurring in Guam and the 

Commonwealth of the Northern Mariana Islands

Common Name Scientific Name Diet Reference

Bat, Little Mariana 

fruit

Pteropus tokudae A wide variety of plant matter, primarily fruit 1 

Bat, Mariana fruit Pteropus mariannus A wide variety of plant matter, primarily fruit 1

Crow, Mariana Corvus kubaryl Fruit, seeds, foliage, and buds of trees; 

invertebrates (lepidopteran larvae,

grasshoppers, mole crickets, praying mantis, 

earwigs, hermit crabs), reptiles (skinks, 

geckos), immature rats, bird eggs

2 

Kingfisher, Guam 

Micronesian

Halcyon cinnamominus Skinks, geckos, insects, segmented worms 

(Annelida), hermit crabs, chicken chicks, rats 3 

Moorhen, Mariana 

Common

Gallinula chloropus 

guami

Plant matter, insects, and mollusks 4 

Rail, Guam Rallus owstoni Seeds, palm leaves, carrion, gastropods, skinks, 

geckos 5 

Reed-Warbler, 

Nightingale

Acrocephalus luscinia Terrestrial arthropods (insects, spiders), snails, 

lizards 6 

Swiftlet, Mariana Aerodramus bartschi Insects 7

White-eye, Guam 

Bridled

Zosterops conspicillatus Primarily insects, small amount of fruit and 

nectar 8 

White-eye, Rota Zosterops rotensis Insects, fruit, seeds, nectar 9

1. USFWS. 1990. Guam Mariana Fruit Bat and Little Mariana Fruit Bat Recovery Plan. Portland, Oregon, 63 

pp. Available at: http://ecos.fws.gov/docs/recovery_plan/901102.pdf

2. USFWS. 2005. Draft Revised Recovery Plan for the Aga or Mariana Crow, Corvus kubaryi. Portland 

Oregon. 147 pp. Available at: 

http://ecos.fws.gov/speciesProfile/profile/speciesProfile.action?spcode=B05X 

3. USFWS. 1990. Revised Recovery Plan for the Sihek or Guam Micronesia Kingfisher (Haycyon 

cinnamomina cinnamomina). Portland, Oregon. 117 pp. Available at: 

http://ecos.fws.gov/docs/recovery_plan/081114.pdf

4. USFWS. 1991. Recovery plan for the mariana common moorhen (= Gallinule) Gallinula chloropus guami. 

U.S. Fish and Wildlife Service, Portland, Oregon. 55 pp. Available at: 

http://ecos.fws.gov/docs/recovery_plan/910928.pdf 

5. USFWS. 1990. Native Forest Birds of Guam and Rota of the Commonwealth of the Northern Mariana 

Islands Recovery Plan. U.S. Fish and Wildlife Service., Portland, Oregon. 86 pp. 

http://ecos.fws.gov/docs/recovery_plan/900928b.pdf

6. USFWS. 1998. Recovery Plan for the Nightingale Reed warbler (Acrocephalus luscinia). Portland, Oregon. 

62 pp. Available at: http://ecos.fws.gov/docs/recovery_plan/980410a.pdf

7. USFWS. 2009. Mariana Swiftlet or Chachaguak (Aerodramus bartschi): 5-Year Review Summary and 

Evaluation. U.S. Fish and Wildlife Service, Honolulu, Hawaii. 10 pp. Available at: 

http://ecos.fws.gov/docs/five_year_review/doc3352.pdf

8. Species specific recovery plan available on FWS website, 1990. U.S. Fish and Wildlife Service, Portland, 

Oregon. 55 pp. Available at: 

http://ecos.fws.gov/docs/recovery_plan/900928b.pdf

9. USFWS. 2007. Recovery Plan for the Nosa Luta or Rota Bridled White-Eye (Zosterops rotensis). Portland, 

Oregon. 139 pp. Available at: http://ecos.fws.gov/docs/recovery_plan/071019.pdf 
23 

Based on the conceptual model and problem formulation for aquatic wildlife, the registered use 

of acetaminophen is not expected to result in significant exposure to organisms in the aquatic 

environment, and thus, the listed marine turtles are not expected to be affected. 

With the exception of certain mammals and birds (i.e., those that feed on carrion), the registered 

use of acetaminophen also is expected to result in minimal direct exposure to terrestrial 

organisms. The two listed species of fruit bats that occur on Guam feed strictly on plant matter 

and thus would not consume the baited mouse pups or poisoned tree snakes. 

As previously discussed, minimal exposure to terrestrial plants is expected resulting from 

acetaminophen bait applications. The only listed plant identified in Guam is the Serianthes 

nelsonii tree; however, as determined in the 2011 USFWS BiOp, because of the proximity of the 

acetaminophen applications relative to the location of the tree, no adverse effects are expected. 

The natural history and feeding habits of the 8 endangered birds of these islands were 

investigated to determine if they would likely be exposed by the registered use of acetaminophen 

for brown tree snake control. Of these eight species, only the Mariana crow (Corvus kubaryi), 

the Guam Micronesian kingfisher (Halcyon cinnamominus), and the Guam rail (Gallirallus 

owstoni) are known to eat small mammals or carrion (Table 6). All three of these species are now 

extirpated from the wild in Guam. The Guam Micronesian kingfisher and the Guam rail now 

exist only in captivity, and thus would not be exposed by baiting with acetaminophen. Until 

recently, small numbers of Mariana crow existed in extreme northern Guam, but the species now 

have since been extirpated from the wild or solely are in captivity in Guam and currently occurs 

in the wild only on the island of Rota (Earl Campbell, USFWS, personal communication). 

While the Guam Micronesian kingfisher and the Mariana crow no longer exist in the wild in 

Guam, designated critical habitat still exist on that island. The critical habitat that is designated 

on Guam is the same for both species, consisting of 376 acres of protected forest on the extreme 

northern fringe of the island, near Ritidian Point (USDI, 2004). All of the critical habitat lies on 

federal land within the Guam National Wildlife Refuge. Since baiting in this area with 

acetaminophen could cause direct effects to these two species, if they were present, registered 

use of acetaminophen for brown tree snake control could cause an adverse modification of the 

critical habitat. However, this adverse modification in the habitat is expected to be very transient, 

lasting only as long as the dead mouse pups remained intact in the environment. The dead mouse 

pups are expected to soon disappear primarily by consumption by tree snakes and 

decomposition. The acetaminophen tablets located within the dead mouse pups, once exposed to 

the environment, are expected to dissipate rapidly into the soil due to its high water solubility. 

Therefore, any potential adverse modification to the habitat from the use of acetaminophen is 

expected to be transient. 

Therefore, we expect that the Guam Micronesian kingfisher and Mariana crow could be 

reintroduced into this critical habitat without suffering any direct or indirect adverse effect from 

the acetaminophen baits, as long as the reintroduction effort does not overlap with baiting events. 

Because direct adverse effects are not expected for any of the reptiles, amphibians, mammals, 

fish, and aquatic organisms found on Guam, and there is no reasonable expectation of effects to 

plants or terrestrial invertebrates, the EPA makes a No Effects (NE) determination for all 
24 

Federally-listed species. While there is a potential for acetaminophen exposure to occur to birds 

that may feed on carrion (and swallow an entire acetaminophen bait), all known listed birds have 

since either been extirpated from Guam, or are in captivity, and are not expected to feed on the 

acetaminophen baits (i.e., dead mouse pups) that are used in the baiting programs to control the 

brown tree snake. Given that any current exposure is expected to be transient in nature (i.e., does 

not persist for extensive periods in the environment), and consistent with the determination in the 

2011 USFWS BiOp for the Mariana crow, a No Habitat Modification determination is made for 

all designated critical habitats for the currently registered uses of acetaminophen.

6.1 Formal Consultation on the Aerial Application of Acetaminophen for Brown 

Tree snake Control on Guam

At the time of the issuance of the November 2011 Biological Opinion, there was one male 

Mariana crow remaining in Guam, the USDA-APHIS requested formal consultation on the 

proposed aerial application of acetaminophen (in baited mouse pups) for the control of brown 

tree snake on Guam, and the potential exposure and effects to the endangered Mariana crow

(Corvus kubaryi), the Mariana fruit bat (Pteropus mariannus), and the Serianthes nelsonii, an 

endangered tree. On November 2, 2011 USFWS issued a Biological Opinion, which completed 

the formal consultation request from USDA APHIS WS, initiated June 5, 2011. This Biological 

Opinion included information that outlined the avoidance, minimization, and conservation 

measures necessary to minimize potential impacts to these endangered species. The conclusions 

of EPA’s preliminary risk assessment are consistent with the findings in the Biological Opinion 

that described the potential exposure and impact to each listed organism, and the corresponding 

effects. The determinations made by the USFWS in 2011 are summarized below. Since then, 

this last Mariana crow has been removed from the wild, and thus the species currently occurs in 

the wild only on the island of Rota (Earl Campbell, USFWS, personal communication). 

Mariana crow (Corvus kubaryi) 

The November 2011 Biological Opinion provided a summary of the proposed acetaminophen 

application project (i.e., baited mouse pups aerially dropped into tree canopies into targeted areas 

surrounding the Munitions Storage Area of Andersen Air Force Base), along with detailed 

information that outlined conservation measures to minimize potential impacts to the Mariana 

crow. The USFWS analysis considered the potential impacts from any direct exposure to the 

Mariana crow from the acetaminophen-baited mouse pups, as well as the indirect effect 

attributed to any noise produced from the flying of a helicopter during an application, or in 

proximity adjacent to the application (drop) site. 

This Biological Opinion was completed for a specific project involving the aerial application of 

acetaminophen in dead neonatal mice (DNM) in the Andersen Air Force Base on for control of 

the brown tree snake. The conclusions do not directly address use of acetaminophen in bait 

stations; however, that use would be expected to incur less exposure to Mariana crow than the 

proposed aerial application.

 Amount of Extent of Take Anticipated and Effect of the Take: Based on the proposed 

aerial application of acetaminophen-baited mice, incidental take of one Mariana crow was

expected to occur during the capture and translocation effort. Incidental take was expected in the 
25 

form of harassment, injury and mortality may occur during the capture and translocation effort. 

If the capture was unsuccessful, incidental take was expected to take the form of harassment for 

the 16-month implementation of the acetaminophen action due to noise and human disturbance 

caused from the helicopter, supplemental feeding, and monitoring. More relevant to the current 

assessment, the Biological Opinion also stated that there was the chance of incidental take due to 

the ingestion of DNM (dead neonatal mice, i.e., the acetaminophen-baited mouse pups). USFWS 

concluded that after reviewing the current status of the Mariana crow, the environmental baseline 

of this species in the acetaminophen application area, and the effects of the proposed action, 

including cumulative effects, it is of [their] Biological Opinion that the proposed aerial toxicant 

research project on AAFB, Guam may adversely affect the Mariana crow, but will not jeopardize 

their survival and recovery in the wild, nor will the critical habitat for this species be affected.

Reasonable and Prudent Measure: Minimize impacts of the proposed aerial toxicant 

drops [acetaminophen-containing baits] on the Mariana crow.

Mariana fruit bat (Pteropus mariannus) 

At the time of the consultation, there was a small population of the Mariana fruit bat (Pteropus 

mariannus) that existed in northern Guam. In the May 13, 2011 informal consultation request 

(Ref. 2011-I-0270), which included proposed aerial applications of acetaminophen, there was the 

potential that low-flying aircraft could adversely affect fruit bats flying in the area due to 

collisions or noise disturbance. However, since flights for the acetaminophen-baited mouse pup 

applications were scheduled to occur at times when the fruit bats are less active, the very low 

population densities of fruit bats on Guam, and few reported incidents of fruit bat strikes by an 

aircraft [(one recorded incident in 1982 (Morton 1996)], the USFWS concurred that the effect of 

the acetaminophen action to this fruit bat would be discountable. 

Serianthes nelsonii Tree

In the May 13, 2011 informal consultation request (Ref. 2011-I-0270), USFWS concluded that 

considering the distance of the tree from the action area, the acetaminophen application is not 

likely to adversely affect the one endangered tree remaining on Guam, because of the distance of 

the tree from the action area of the proposed project. 

 
26 

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