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Fumigation of a Y-shaped tunnel using a portable

CO generator

A report to Wildlife Species and Conservation Division,

Defra

20 December 2006


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

Department for Environment, Food and Rural Affairs

Website: www.defra.gov.uk

© Crown copyright

Copyright in the typographical arrangement and design rests with the

Crown.

This publication (excluding the royal arms and departmental logos) may

be re-used free of charge in any format or medium provided that it is

re-used accurately and not used in a misleading context. The material

must be acknowledged as crown copyright and the title of the

publication specified.

Published by the Department for Environment, Food and Rural Affairs


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

1 Executive summary

1. Based on available scientific literature on the euthanasia of rabbits and other

mammals, carbon monoxide (CO) has been identified as a potential fumigant for

humane euthanasia of badgers in their sett.

2. Defra (2005c) initially identified an idling, de-tuned petrol vehicle engine without

catalytic converter as a feasible source of CO. Further investigations identified a

prototype portable CO generator (PCOG), based on a small, four-stroke engine

run on liquid petroleum gas (LPG). This prototype is currently being developed in

Australia for the fumigation of rabbit warrens and provides a more practical

method of CO production.

3. The aim of this research was to determine if a target CO concentration and

duration (1% for 1 hour) can be achieved in an artificial tunnel structure using the

PCOG.

4. The artificial tunnel structure had two entrances, tunnels of 30 cm diameter, and a

large chamber situated 13 m from the furthest entrance. The total volume of the

structure was 1.2 m 3 . Gas sampling lines and temperature thermocouples were

placed at various points through the structure. This was the same Y-shaped tunnel

and gas measuring set up used to assess the de-tuned petrol engine as reported in

Defra (2006).

5. The levels of CO and other by-products were measured in the exhaust from the

PCOG, this confirmed CO output similar to, if not greater than, the previously

measured detuned petrol engine.

6. A series of four trials were carried out whereby the Y-shaped tunnel was

fumigated for a period of 60 minutes using the PCOG. Target concentrations of

1% CO for one hour were exceeded during all trials and levels of O2 and CO2

were significantly decreased and increased respectively. NO2 was not detected

while levels of NO were thought to be acceptable. Measured levels of

hydrocarbons were slightly lower than those found with the petrol engine.

7. Temperatures measured in the Y-shaped tunnel were not significantly above

ambient.

8. The PCOG is relatively small, light and portable and therefore could be used in


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

situations where vehicular access is restricted.

9. The production of smoke by the PCOG is an important feature for enhancing

efficacy of operations and in ensuring operator safety.

10. The feasibility of achieving target CO concentration and duration within different

size, complexity, and soil types of setts needs to be determined, although

concentrations of CO achieved in these trials are thought to be sufficient to kill

badgers. A computational fluid dynamics (CFD) model is being developed to

address this.

11. To determine that CO is a humane method of killing badgers further studies

would need to be completed to assess: a) the minimum lethal concentration of CO

required for badgers, b) the impact of the measured oxygen and carbon dioxide

concentrations on toxicity and humaneness, c). the humaneness of the flow rate of

CO and d) the aversiveness to the elevated levels of hydrocarbons.

12. In conclusion, fumigation of a Y-shaped tunnel, which mimicked a small, simple

badger sett, using the exhaust from the PCOG reached and sustained the target

level of CO. This is a promising start but further work needs to be carried out

before this method can be considered either feasible or humane in the field.


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

2 Introduction

Defra has closed the consultation on “Controlling the spread of bovine tuberculosis in

cattle in high incidence areas in England: badger culling” (March 2006). This

consultation sought views on the principle of whether to introduce badger culling to help

control bovine TB (bTB), and on options for implementing a cull. Responses are

currently being considered. If Ministers were to decide that the culling of badgers was

appropriate to prevent the spread of bTB, then it is important to have appropriate

evidence on the effectiveness and humaneness of possible culling methods. Gassing

badgers in their setts is one such potential method.

Carbon monoxide (CO) has been identified as the best potential fumigant for use within

badgers setts, in terms of feasibility and humaneness (Defra, 2005c). When other

mammals are exposed to concentrations of over 1% CO for at least 60 minutes, they

gradually loose consciousness, followed by death. Various papers have reported

successful CO fumigation of a range of burrow dwelling species (Oliver & Blackshaw,

1979; Deng & Chang, 1986; Pelz & Gemmeke, 1988; Ross et al., 1998). However,

computational fluid dynamics (CFD) modelling (Defra, 2005a; Defra, 2005b) has raised

concern over the penetration of significant levels of CO further into the tunnels and in

particular into blind tunnels. Previous field trials showed that it was possible to sustain a

target concentration of 1% CO over 1 hour in an artificial fox earth den in calm

conditions, using CO generating cartridges as the delivery mechanism (CSL, 2001). Ross

et al (1998), working on rabbit burrow systems and using a similar delivery system,

found that although target, lethal concentration could generally be sustained for the

required duration at entrances and in tunnels, this was not always possible at blind ends,

particularly under windy conditions.

Defra (Defra, 2005c) reviewed the possible delivery methods of CO into a sett and

suggested a detuned, idling vehicle petrol engine without catalytic converter as a feasible

delivery method. Subsequent discussions with the inventors of a portable CO generator

(PCOG) in Australia (Gigliotti et al., 2005), suggest that this device may be easier to use


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

and to approve the gas produced than a vehicle petrol engine.

However, questions still remain regarding the use of gas produced from this generator as

a fumigant for badgers. Specifically, further work needs to be carried out to investigate 1)

whether it is possible to obtain potentially lethal concentrations of CO throughout a

complex sett and in different soil conditions, 2) the welfare implications of gas flow rate,

3) the effects of by-products in the fumes, and 4) the lethal concentration of CO for

badgers.

The aim of this project was to determine if a target CO concentration and duration could

be achieved in a simple, Y-shaped tunnel without animals after fumigation with the

PCOG and what, other by-products of combustion were detectable within the tunnel

structure.

3 Material and methods

3.1 Design

The Y-shaped tunnel constructed for the previous trials with a vehicle petrol engine

(Defra 2006) was used to allow a direct comparison of results. Blind tunnel lengths of

various excavated setts had been calculated (Defra, 2005c), and it was decided to base the

constructed tunnel system on maximum tunnel and chamber dimensions measured, to

reflect a worst-case scenario. The design of the Y-shaped tunnel is represented from

different angles in Figure 1. The distance between the end of the chamber (at a blind end)

and the fumigated entrance (entrance I) was 13m, incorporating a right angle bend. An

extra branch with a second entrance was added 4 metres into the tunnel, so the gas would

have an option of least resistance. Tunnels at the entrances were sloped steeply to

resemble natural setts (Roper et al., 1991), and entrance I faced south-south-west.


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

Figure 1. Design of the Y-shaped tunnel.


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

For the construction methods used see Defra (2006). Before the present trials commenced

an underground camera was manoeuvred through the structure to guarantee all tunnels

had remained unblocked.

3.2 Fumigation

3.2.1 Generator

Figure 2. The PCOG pumping gas into the tunnel structure.

The PCOG (Figure 2) is comprised of a 49cc, four-stroke engine (Honda® WX15)

coupled to the fan system of a commercial two-stroke air blower (Talon® AB3201). The

engine is modified to run on propane gas (LPG) as this fuel is regarded to be clean

burning, readily available and relatively cheap. A new combustion chamber was


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

designed and manufactured for the engine to allow for the gas conversion and enable the

engine to run reliably using a rich air/fuel mixture. This produces a gas rich in CO,

typically over 4%.

The original engine fuel tank is utilised for the smoke trace liquid that is regulated into

the combustion chamber where it is vaporised to produce a clearly visible white smoke.

The smoke trace is important for safety and efficacy in that it allows operators to locate

all the exit points and provides a visual indication of where the gas has reached.

3.2.2 Gas temperature and airflow measurement

The same apparatus and procedures as described in Defra (2006), were used.

3.2.3 Fumigation protocol

The engine was run for 10 minutes to enable it to warm up before fumigation. This was to

ensure stable concentrations of CO were being produced. The exhaust pipe flexible

extension was then placed into the tunnel entrance and earth used to seal the entrance

around the pipe. The second entrance was sealed with earth prior to any trials.

Due to time constraints a maximum of six trials could be carried out. The first four trials

aimed to provide a direct comparison between fumigation using the PCOG and

fumigation using the petrol engine (Defra 2006). The final two trials aimed to begin to

investigate an appropriate practical fumigation protocol.

For the initial four trials the engine pumped gas into the Y-shaped tunnel for 60 minutes

under low wind conditions. During the fifth trial gas was pumped into the Y-shaped

tunnel for 15 minutes. In the final trial, the usefulness of the smoke trace component of

the PCOG was investigated. Unfortunately, the engine could only be run for a very short

time under this final scenario due to a need for further refinement of this feature and no

gas measurements were taken.


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

3.2.4 Gas sampling protocol

The sampling procedure was similar to that used in the previous vehicle petrol engine

study (Defra, 2006). Initial measurements of CO were taken from the 6m sampling point

until the concentration exceeded 1%; sampling then commenced from the 10 m point

until 1% CO was exceeded; finally, the sampling was moved to the 13 m sampling point.

Monitoring at the 13 m point then continued for 2-3 hours after the engine had been

switched off, during which time the entrances were left closed. The entrances were then

opened at the end of each trial to let the gases escape, so that another trial could be

carried out the following day CO concentrations in the tunnel system were always found

to be zero when checked after the entrances had been left open overnight.

As only one gas could be measured at any one time, the other gases (carbon dioxide

(CO2), oxygen (O2), lower- and higher-class hydrocarbons (LCH and HCH respectively),

and nitrous oxides (NOX)) were measured intermittently throughout the tunnel structure.

4 Results

4.1 Exhaust emissions

The results from analysis of the gases generated by the PCOG clearly show that sufficient

CO was produced and at a comparable flow rate to the vehicle engine (Table 1 and Defra,

2006).

Table 1. Gases produced from the PCOG.

CO (%) O 2 (%) CO 2 (%) NO 2

(ppm)

NO

(ppm)

5.0 11.1 2.8 0 0.7 4.0

The daily outside ambient temperatures throughout the trials were warm, on average

20°C. The highest temperature recorded in the tunnel structure, close to the inlet point,


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

during fumigation was 38.6°C. The temperature in the chamber never increased by more

than 0.8°C above the ambient temperature (16°C).

4.3 Gas pumped into the Y-shaped tunnel for 60 minutes

In all four trials the target concentration of 1% CO for 1 hour was achieved.

In the first trial the highest CO concentration recorded in the chamber (13m) was 4.1%,

25 minutes after the PCOG had been switched off (Figure 3). At the end of the trial, 197

minutes after the engine had been switched off the concentration of CO in the chamber

was still considerably higher (2.9%) than the target concentration. Oxygen concentrations

in the chamber were measured as low as 12.5%, 61 minutes after the start of the trial.

Whereas the highest carbon dioxide concentrations in the chamber were recorded much

later, 2.4% at 197 min after the start of the trial. LCH and HCH were measured at 0.1%

and >0.7% (calibration of gas sampling tubes only allowed measurement of HCH

concentrations up to 0.7%, Defra (2006)) respectively within the chamber.

CO concentration (%)

6

5

4

3

2

1

0

Wind speed 2.2 m/s

0 30 60 90 120 150 180 210 240 270

6 metres 13 metres 12.5 metres

Time (min)

Figure 3. Gas pumped into the Y-shaped tunnel for 60 minutes (red bar), trial one.


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

For the second trial the highest CO concentration measured in the chamber (13m) was

considerably higher (5.4%) than that in the first trial, although the timing of this peak was

very similar to that in the first trial, 85 minutes from the start of the trial (Figure 4). The

rate of the rise in CO concentration in the chamber could also be measured, with a

concentration of 1% being reached within 1.75 minutes. The lowest oxygen

concentrations measured in the chamber were 10.8 % at 101 minutes after the start of

fumigation, whereas the highest carbon dioxide levels reached 5.2 % at 22 minutes. The

concentration of LCH was greater in this trial, reaching 0.6 % at 6 m and 0.45% in the

chamber.

CO concentration (%)

6

5

4

3

2

1

0

Wind speed 1.1 m/s

0 20 40 60 80 100 120 140

6 metres 13 metres

Time (min)

Figure 4. Gas pumped into the Y-shaped tunnel for 60 minutes (red bar), trial two.

In the third trial CO concentrations at 13m peaked at about the same time as in the two

previous trials (81 minutes) (Figure 5). The highest CO concentration recorded at the

13m point was 4.3%, and even after 300 minutes was still above 3.5%. The highest

carbon dioxide concentrations measured in the chamber were 3.1%, 289 minutes after the

start of fumigation. The lowest oxygen level recorded in the chamber (13m) was 12.3 %,


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

after 41 minutes of the trial. LCH were found to reach 0.05% throughout all of the tunnel

structure, whereas HCH again were found to be over 0.7%.

CO concentration (%)

6

5

4

3

2

1

wind speed 0.7 m/s

0

0 30 60 90 120 150 180 210 240 270 300 330

6 metres 13 metres Time (min)

Figure 5. Gas pumped into the Y-shaped tunnel for 60 minutes (red bar), trial three.

In the fourth trial the highest CO concentration was measured approximately 20 minutes

later than in the previous three trials (Figure 6). The recording of 5.1% in the chamber,

was reached after 108 minutes of fumigation. Oxygen levels were recorded as low as

11.7% in the chamber and at a similar time as in the previous trial, 48 minutes. Unlike the

previous trial the highest carbon dioxide concentrations in the chamber were recorded at a

similar time as the highest oxygen concentrations. Carbon dioxide was recorded at 2.7%,

41 minutes after the start of the trial. LCH were measured at 0.1% throughout the tunnel

structure and HCH were at levels higher than 0.7%.


CO concentration (%)

6

5

4

3

2

1

0

Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

wind speed 2.9 m/s

0 20 40 60 80 100 120

6 metres 13 metres

Time (min)

Figure 6. Gas pumped into the Y-shaped tunnel for 60 minutes (red bar), trial four.

4.4 Gas pumped into the Y-shaped tunnel for 15 minutes

All measurements were taken from the 13 m sample point for this trial, and it took

approximately 12 minutes for CO to be detected. A concentration of 1% was achieved

within a further 6.5 minutes after CO was first detected (Figure 7). The highest recorded

concentration was 2.3% at 67 minutes after the start of the trial, and the concentration

stayed over 1% for longer than 135 minutes. Measured oxygen concentrations did not

decrease as much as in previous trials, 15.2% at 35 minutes. The highest measured carbon

dioxide concentrations were also lower, 1.7% at 79 minutes. Hydrocarbon levels were

similar to previous trials, with LCH levels being measured at 0.1% and HCH levels at

over 0.7%.


CO concentration (%)

6

5

4

3

2

1

0

Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

wind speed 2.7 m/s

0 15 30 45 60 75 90 105 120 135 150 165

Time (min)

13 metres

Figure 7. Gas pumped into the Y-shaped tunnel for 15 minutes (red bar).

Table 2. Summary of concentrations of gases measured within the chamber (13m) in the

different trials with the PCOG.

Trial

1

Trial

2

Trial

3

Trial

4

Trial

5

Duration

gas

pumped

(min)

Peak

CO

(%)

Duration

of CO

>1%

(min)

Highest

measured

NOx

(ppm)

Highest

measured

CO2 (%)

Lowest

measured

O2 (%)

Highest

measured

LCH (%)

60 4.1 >220 2 2.4 12.5 0.1 >0.7

60 5.4 >120 2 5.2 10.8 0.6 >0.7

60 4.3 >260 2 3.1 12.3 0.05 >0.7

60 5.1 >100 2 2.7 11.7 0.1 >0.7

Highest

measured

HCH (%)

15 2.3 >135 2 1.7 15.2 0.1 >0.7


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

5 Discussion

This report describes the distribution of gasses measured throughout a Y-shaped tunnel,

built to mimic a simple badger sett, after fumigation with the PCOG.

Compared with the detuned petrol engine without catalytic converter, used in previous

trials (Defra, 2006), the PCOG generated a higher concentration of CO. Given the similar

flow rate from the two types of generator, higher concentrations would thus have been

expected within the tunnel structure from the PCOG, and this was what was found. The

concentrations in the tunnel structure were consistently higher after fumigation with the

PCOG. As found in the previous trials, presumably leakage of the gas out of the tunnel

system was relatively low, with concentrations being maintained over several hours. In

contrast to the petrol engine, lower concentrations of carbon dioxide and higher

concentrations of oxygen were measured in the gas generated by the PCOG. In the

detuned petrol engine, the changes in concentration of both these gases were thought to

have the potential to enhance the lethal effects of CO. This effect would not be quite so

marked with the gas mixture produced from the PCOG. NO2 levels were similar in the

gases produced by both methods, but NO levels were much lower in the gas from the

PCOG. The components of exhaust gas that were of most concern with the petrol engine,

are present in similar concentrations in the gas generated by the PCOG. These are the

lower and higher class hydrocarbons. Lower concentrations of LCH were measured from

the PCOG but similar levels of HCH were found in the gas generated by the PCOG and

petrol engine. It is known that hydrocarbon emissions can be altered by tuning the engine,

however they would not be reduced to such a level that they would not require further

consideration.

There are three main advantages of the PCOG over a detuned petrol engine without

catalytic converter. These are a) portability, b) smoke trace and c) ease of licensing. The

PCOG weighs 15 kg and therefore can be manually carried over any terrain to all badger

setts. The additional weight of the gas bottle varies depending on size used. In Australia


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

the proposed gas bottle size to be used during rabbit warren fumigation is 4 kg, which can

be easily carried in a back-pack arrangement. Laboratory trials conducted in Australia

have shown LPG consumption to be 0.65 kg of propane in 30 minutes under full

fumigating conditions. In contrast, it would be difficult to get a vehicle close enough to

many setts to enable fumigation to take place. The addition of the smoke trace is an

important efficacy and safety feature. The operator will be able to tell when the smoke

has reached all exits, and therefore when to stop adding further gas to the sett, and

accidental inhalation of CO, a very dangerous gas with strict health and safety

precautions, should be avoided by not standing in areas where the smoke appears. The

third advantage of the PCOG is that it is a standard machine and would be relatively easy

to license. It could be supplied from the manufacturer, with a certificate guaranteeing

output of a specific concentration of CO, that could be checked regularly Although any

petrol engine without catalytic converter could be detuned on farm, the concentration of

CO produced from the engine would be unknown. The engine would then need to be

taken to a garage to determine the CO output from the exhaust, before it could be reliably

and legally used to fumigate a sett.

The target concentration of CO, 1% for one hour, was reached in all trials. After

reviewing the literature CSL (CSL, 1993) concluded that any proposed use of CO for the

fumigation of mammals should seek to ensure exposure to concentrations greater than 1%

for one hour and to have a gradual increase in concentration, to prevent the onset of

convulsions before insensibility. This target concentration and duration was a

conservative estimate based on the fact that rabbits have a greater tolerance to carbon

dioxide as an adaptation to burrow living. The ability to tolerate higher levels of carbon

dioxide may be important in metabolism of COHb, leading to increased resistance to CO

poisoning. For example, rabbits exposed to sub-lethal doses of CO over an extended

period were able to eliminate COHb much more quickly than guinea-pigs or dogs

(Semerak & Bacon, 1930). One rabbit exposed to 0.25% CO for over 3 hours showed no

signs of distress, whereas at the same concentration a dog collapsed in 10-15 minutes

(Burrell et al., 1914). However, at higher concentrations the differences are not so clear-

cut (Oliver & Blackshaw, 1979). The possible ability of badgers to tolerate higher levels


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

of carbon dioxide, and their metabolism of CO has not been investigated. Hence, the

target CO concentration and duration that has been set may or may not be lethal for

badgers and this needs to be evaluated.

The rate of increase in CO concentration is an important factor in the humaneness of CO

euthanasia. The rate of increase in CO concentration measured in the tunnels varied

greatly, with the fastest increase being from 0 to 1% in 1.75 minutes in the chamber. The

rate of increase tended to decrease the further into the sett the gas travelled. When pigs

were exposed to gradual rises of CO, i.e. 0.5% over 30 min, convulsions were not

observed (Lambooy & Spanjaard, 1980). Convulsions had been observed when CO

concentrations reached at least 1% within 1 min. Convulsions are an important

consideration in the assessment of humaneness if they occur while the animal is still

conscious. In other trials with mink where EEG was monitored, convulsions during

euthanasia with CO occurred both prior to and after onset of unconsciousness (Lambooy

et al., 1985). The rate of increase in CO reported in the current trials falls close to that

where convulsions were observed in pigs. Further investigations on the exact responses of

badgers to the rates of CO concentration increases produced by the PCOG are needed to

determine the humaneness of the technique.

Carbon monoxide causes death through depriving the brain of oxygen. This can also be

achieved by decreasing oxygen levels and/or increasing carbon dioxide levels in the air.

Lowered oxygen levels were found in all trials, although more so in some than others. In

humans, atmospheric levels higher than 14%, but below 20%, affect physical and

intellectual performance, 10-14% oxygen results in faulty judgement, possible diminished

sense of pain, easily aroused ill-temped and rapid fatigue on exertion. At 11% oxygen

there is risk of death. (Compressed Gas Association, 2001). Although badgers are thought

to be more tolerant to lower levels of oxygen than humans (Roper & Kemenes, 1997), the

lowered levels measured in these trials would still be expected to have some impact on

them.

Carbon dioxide concentrations were also increased in the Y-shaped tunnel, varying


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

between 1.7 and 5.2%. In normal air the concentration of this gas is 0.03%, and in

blocked badger setts containing live badgers has been found as high as 0.58% (Roper &

Kemenes, 1997). The levels of carbon dioxide found in these trials could increase

susceptibility to CO poisoning. In rats exposed to 0.25% CO in the presence of 5.25%

CO2, more animals died during and post exposure and the rate of formation of COHb

concentrations was 1.5 times faster than with CO alone (Levin et al., 1987). Acidosis was

more pronounced and prolonged, and recovery periods were considerably longer (60

minutes as opposed to 5 minutes). This effect was most pronounced at 0.25% CO and 5%

CO2. Exposure to increased concentrations of CO2 causes an increase in respiratory rate,

and the synergy effect of these two gases was attributed to a combination of respiratory

and metabolic acidosis. Oliver and Blackshaw (1979) also mention the putative increased

toxicity of exhaust fumes, but in a direct comparison Moreland (1974) did not observe a

shorter time to death when comparing time to death from two sources of CO. The

magnitude of the effect of high levels of CO2 in badgers, that are probably adapted to

living with slightly elevated levels of carbon dioxide, is currently unknown and needs to

be quantified.

One of the concerns regarding fumigation with CO is the risk of sub-lethal effects. The

long-term effects of sub-lethal concentrations of CO are dependent on the degree of

anoxia. At low to moderate levels full recovery is normal, however if severe anoxia is

experienced permanent brain damage can occur. Humans that have been found suffering

from CO poisoning are always treated by moving them into an oxygen rich environment

(Kao & Nanagas, 2005; Hardy & Thom, 1994). However, even in an oxygen rich

environment (100%), it can take 80 minutes to remove the CO from haemoglobin. In

normal air this increases to 5.5 hours (Stone, 1999). We therefore do not know the likely

outcome if a badger were to receive a sub-lethal dose of CO.

Several reports state that the high temperature and other pollutants in exhaust fumes have

a detrimental effect on animals (Scientific Committee on Animal Health and Welfare,

2001; Scientific Panel on Animal Health and Welfare, 2004; Close et al., 1996) and CO

for euthanasia should therefore not be produced from a petrol engine. However, an


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

extensive literature search found no published scientific studies carried out regarding this

(Defra, 2005c). In one paper that describes the responses of rabbits to fumigation with

petrol engine exhaust fumes, no signs of aversion or distress were observed before

collapse (Oliver & Blackshaw, 1979). In a different study that directly compared CO

produced from petrol engine exhaust and pure CO, Lambooy et al. (1985) found slight

differences in behaviour between the two sources of CO. However, these differences

could have been due solely to the significantly different concentrations of CO in the

chamber, in combination with the animals being placed directly into these relatively high

concentrations. The only published study that compared petrol exhaust gases filtered

through water with a similar final concentration and flow rate of pure CO, found no

differences in behaviour or signs of distress (Moreland, 1974). This was perhaps

unsurprising as filtering the gases did not change their composition. In addition, they

speculated that the NO (up to 80 ppm) and unburnt hydrocarbons (up to 0.025%) in the

exhaust gas were unlikely to cause any irritation in the short time (3 minutes) before loss

of consciousness occurred. Temperature increases in the Y-shaped tunnel were shown to

be minor, and would not be expected to have any detrimental effects on badgers in a sett.

The components of exhaust fumes that are believed to be irritant and have detrimental

effects are NO, NO2 and hydrocarbons. NO2 was not found in the gases produced by the

PCOG. Levels of NO were well below the short-term exposure limit (STEL) for humans

(25 ppm) throughout the tunnel.

The concentrations of hydrocarbons measured in the tunnel were much higher than were

found when Moreland (1974) euthanised dogs using petrol engine fumes (0.025%). The

majority of toxicity data relating to hydrocarbons is specific to individual compounds

rather than to combinations of several as measured here. Hydrocarbons are a group of

compounds that can be released during incomplete combustion of organic matter. The

following information is taken from the WHO task group on environmental health criteria

for selected non-heterocyclic polycyclic aromatic hydrocarbons (1998): Levels of

individual compounds in exhaust fumes may vary over time, though removing a catalytic

converter increases the levels. Acute toxicity is thought to be moderate to low, although


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

many hydrocarbons are carcinogenic, genotoxic, and immunosuppressive- however this is

of little consequence if lethal concentrations of CO are reached, as the badgers will die

before these chronic symptoms have time to take effect. Fluoranthene and pyrene

constitute 70-80% of total hydrocarbon emissions from petrol engines, and both have

relatively low subacute and subchronic toxicity. Anthracene, another compound found in

petrol exhaust fumes from vehicles without a catalytic converter, is a primary irritant

however, and can cause mild irritation of the skin, eyes, mucous membranes and

respiratory tract. Levels of hydrocarbons are related to both the starting and ambient

temperature, being 3-10 times higher when it is cold. They are also thought to be higher

when the air:fuel ratio is lower. In both the PCOG and vehicle petrol engines, to produce

maximal concentrations of CO the engines need to be run fuel rich, therefore leading to

higher concentrations of hydrocarbons. The degree of pain and aversiveness of the high

levels of hydrocarbons to badgers is unknown, however, there is some indication that

exhaust fumes are at least irritating to the eye and airways. With respect to humans a

suicide note, which was written as the car filled with carbon monoxide, mentioned

irritation of the eyes and throat after seven minutes (published in Stone, 1999).

There still remain questions as to how aversive animals find inhalation of exhaust fumes.

To be certain that CO from the PCOG provides a humane method of euthanasia aversion

trials similar to those recently conducted investigating humaneness of carbon dioxide

euthanasia need to be undertaken (Leach et al., 2002).

Although the levels of CO achieved in the Y-shaped tunnel were above the target, we

cannot be sure that these levels can be achieved consistently in other setts. There are

several factors that may influence the dispersion of the gas within a sett and the

maximum concentration achievable, such as complexity, soil type and size of the sett.

The total volume of the Y-shaped tunnel used for this study was 1.21 m 3 . Previous

excavations of five badger setts has provided data on a range of characteristics, one being

volume of the sett. The volumes of these setts ranged from 0.7 to 25.2 m 3 with 4 out of

the five being greater than 5 m 3 . One of these setts was only partially excavated and the

estimated total volume of this sett was 38.7m 3 (Roper et al., 1991). There is a requirement


Fumigation of a Y-shaped tunnel using a portable CO generator

A report to Wildlife Species and Conservation Division, Defra, 20 December 2006

to determine to what extent lethal concentrations can be achieved in these sizes of setts,

and to determine the impact of different soil types. This could be done by carrying out

experimental trials, or by using a validated CFD model. A validated CFD model would

be the most efficient and robust method of assessing what concentrations of CO can be

achieved in all the different scenarios.

6 Conclusion

In conclusion, the concentration of CO throughout the Y-shaped tunnel designed to

mimic a simple badger sett was sustained at or above 1% for at least 1 hour using the

PCOG. The PCOG provides a more practical and safer method of CO production than a

de-tuned petrol engine and would be relatively straightforward to control using a

licensing system. However, presuming a valid CFD model can be developed, which

predicts with confidence that the target CO concentration and duration can be achieved in

a wide variety of complex setts in different soil types, further assessment of CO

euthanasia will need to be carried out before this method can be advocated as humane and

effective. Specifically this will require validating the target CO concentration and

duration for badgers; determining how reduced oxygen and elevated carbon dioxide

influence the toxicity and humaneness of carbon monoxide; assessing the effect of rate of

increase of CO concentration on humaneness; and assessing the aversiveness of the gases

(particularly the hydrocarbons). Finally appropriate operating procedures and guidance

for use under field conditions would need to be devised.

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