From: TSS (216-119-144-14.ipset24.wt.net)
Subject: Infectivity Studies of Both Ash and Air Emissions from Simulated Incineration of Scrapie-Contaminated Tissues [FULL TEXT]
Date: December 29, 2004 at 10:50 am PST
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Subject: Infectivity Studies of Both Ash and Air Emissions from Simulated Incineration of Scrapie-Contaminated Tissues [FULL TEXT]
Date: Wed, 29 Dec 2004 12:47:54 -0600
From: "Terry S. Singeltary Sr."
Reply-To: Bovine Spongiform Encephalopathy
##################### Bovine Spongiform Encephalopathy #####################
Infectivity Studies of Both Ash
and Air Emissions from Simulated
Incineration of Scrapie-
P A U L B R O W N , * , E D W A R D H . R A U , !
P A U L L E M I E U X , § B R U C E K . J O H N S O N ,
A L F R E D E . B A C O T E , A N D
D . C A R L E T O N G A J D U S E K |
Laboratory of Central Nervous System Studies,
National Institute of Neurological Disorders and Stroke, and
Division of Environmental Protection, Office of Research
Facilities Development and Operations, National Institutes of
Health, United States Department of Health and Human
Services, Bethesda, Maryland 20892, National Homeland
Security Research Center, Office of Research and Development,
United States Environmental Protection Agency, Research
Triangle Park, North Carolina 27711, and Institut Alfred
Fessard, Centre National de la Recherche Scientifique,
91198 Gif sur Yvette, France
We investigated the effectiveness of 15 min exposures to
600 and 1000 °C in continuous flow normal and starvedair
incineration-like conditions to inactivate samples of pooled
brain macerates from hamsters infected with the 263K
strain of hamster-adapted scrapie with an infectivity titer
in excess of 109 mean lethal doses (LD50) per g. Bioassays
of the ash, outflow tubing residues, and vented emissions
from heating 1 g of tissue samples yielded a total of
two transmissions among 21 inoculated animals from the
ash of a single specimen burned in normal air at 600 °C. No
other ash, residue, or emission from samples heated at
either 600 or 1000 °C, under either normal or starved-air
conditions, transmitted disease. We conclude that at
temperatures approaching 1000 °C under the air conditions
and combustion times used in these experiments,
contaminated tissues can be completely inactivated, with
no release of infectivity into the environment from
emissions. The extent to which this result can be realized
in actual incinerators and other combustion devices will
depend on equipment design and operating conditions during
the heating process.
Safe disposal of medical wastes, carcasses of cattle with bovine
spongiform encephalopathy (BSE), cervids with chronic
wasting disease (CWD), sheep with scrapie, and more
generally, anyhumanor animal tissue infected or potentially
infected with one of the agents that cause transmissible
spongiform encephalopathy (TSE) continues to be an issue
of concern. High temperature incineration has been the
method of choice for treatment of medical and veterinary
wastes by virtue of its proven ability to inactivate all types
of conventional pathogens, high throughput capacity, and
significant volume reduction. However, TSE agents are
uniquely resistant to most physical and chemical methods
of disinfection, including dry heat (1-3).
In a previous series of experiments (4), we showed that
transmission could occur even after ashing infected tissue
in a covered crucible at 600 °C: the ash from one sample of
fresh brain tissue heated for 15 min transmitted to five of 18
animals (another sample heated for 5 min did not transmit
to any of the 15 animals), and one formalin-fixed sample
heated for 5 min transmitted to one of 24 animals. As no
transmissions occurred from any sample heated to 1000 °C,
the infectivity extinction point was somewhere between 600
and 1000 °C, most probably very close to 600 °C, approaching
the operating temperature of some incineration units.
Because of concerns about the reproducibility of these
unprecedented results, and about the possibility that some
infectivity might be entrained in stack gases vented during
incineration, we designed an experimental apparatus to
produce conditions that reflect more closely actual incineration
conditions, in which gases flowed across a heated open
crucible containing contaminated tissue, oxidizing or pyrolyzing
the tissue, and partially entraining some of the ash;
wealso performed infectivity bioassay measurements of both
ash and emissions. The 263K strain of hamster-adapted
scrapie was chosen because the concentration of infectivity
in brain tissue of terminally ill animals is as high or higher
than in any other TSE, natural or experimental, and thus
allows the maximum measure of reduction, and because
this strain shows resistance to heat that is comparable to
that of BSE and superior to other tested TSE strains (refs 5-8
and personal communication from Dr. David Taylor,
We here report that once again, despite the nearly total
destruction of over 109 LD50, and individual bioassay animal
caging to avoid any possibility of cross-contamination, an
ashed sample of scrapie-infected tissue transmitted disease
after having been exposed to 600 °C for 15 min, and once
again, we found no survival after exposure to 1000 °C. We
also show that no infectivity escaped into air emissions from
15 min test burns at either 600 or 1000 °C.
Whatever the mechanism of this minimal level of survival
in extreme heatswhether a result of incomplete combustion,
the existence of a mineralized template for replication, or
some other unimagined phenomenonsit may be concluded
that the exposure under carefully controlled laboratory
conditions of a small sample of contaminated tissue to 1000
°C, under either an oxidizing or reducing atmosphere, will
ensure complete sterilization of the ash and emissions.
Exposure at 600 °C allows a minimal level of infectivity to
persist in the ash but generates air emission products that
Tissue Samples. Brains from 20 terminally ill hamsters
infected with the 263K strain of hamster-adapted scrapie
were pooled, homogenized, and distributed into 1 g aliquots.
The same procedure was used for a small pool of uninfected
control brains. Samples were frozen until the test burns were
Simulated Incineration. The incineration simulation
apparatus was constructed, and test burns were performed
at the U.S. Environmental Protection Agencys National Risk
* Corresponding author phone: (301)652.5940; fax: (301)652-4312;
Laboratory of Central Nervous System Studies, National Institute
of Neurological Disorders and Stroke, National Institutes of Health.
! Division of Environmental Protection, National Institutes of
§ National Homeland Security Research Center.
| Centre National de la Recherche Scientifique.
Environ. Sci. Technol.2004, 38,6155-6160
10.1021/es040301z CCC: $27.50 ã 2004 American Chemical Society VOL. 38,
NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 6155
Published on Web 10/19/2004
Management Research Laboratory located at Research
Triangle Park, NC. Tissue samples were heated in a 2.54 cm
(1 in.) diameter quartz reactor (Prism Research Glass,
Research Triangle Park, NC) placed inside a Lindberg furnace
(Blue M Model 542 32-V). Three separate, nearly identical
quartz reactors were used. One reactor was only used for the
1000 °C tests, and the other two were alternated for the 600
°C tests. Gas flow through the experimental system was
controlled with a rotameter (Gilmont Instruments, a Barnant
Company, Barrington, IL).
A two-stage impinger was used to collect emissions from
the gas stream exiting the reactor: the first stage discharged
the gas through deionized water in a tube held in an ice bath;
gas exhausted from the first impinger trap flowed into a
second trap suspended in dry ice within a polystyrene foam
container. The apparatus is shown schematically in Figure
1 and is photographed in Figure 2. All components of the
impinger system were made of quartz glass and connected
with Teflon couplings. Metal components were avoided
because of their tendency to bind amyloid protein (9). The
entire apparatus was located in a fume hood.
The experimental matrix included testing of normal and
infected tissues at both 600 and 1000 °C. Experiments were
performed under oxidative (combustion) and reducing
FIGURE 1. Schematic view of incineration simulation apparatus showing,
from left to right, the gas inlet, Lindberg furnace surrounding
a removable combustion chamber (quartz reactor tube), quartz exhaust
tube, emission impingers (ice water bath followed by dry ice bath),
and exhaust through filter into fume hood.
FIGURE 2. Photograph of the incineration simulation apparatus.
6156 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 22, 2004
(pyrolytic) conditions: oxidative conditions utilized humidified
air, and reducing conditions utilized humidified nitrogen
(N2) as the reactor inlet gas. These parameters were selected
to simulate conditions within incinerators commonly used
for destruction of medical waste.
Before each test, the reactor was immersed for 30 min in
a 1:1 aqueous solution of freshly prepared sodium hypochlorite
(Clorox bleach) and then extensively rinsed with
deionized water and allowed to dry. The clean, dry reactor
was then placed into the Lindberg furnace, and the furnace
temperature controls were adjusted to the desired setting
based on calibrations that were performed prior to the
experiments. Each 1 g tissue sample was thawed and placed
into anewquartz crucible. Before beginning the experiments,
a type-K thermocouple (Omega, Model No. KQXL-18G-12)
was used to measure the gas temperature at the axial location
of the reactor where the crucible would be inserted. The
temperature was allowed to equilibrate until no significant
temperature change occurred. The furnace temperature was
then logged, and the thermocouple was removed.
After removal of the thermocouple, the impinger train
(Prism Research Glass, Research Triangle Park, NC) was
installed onto the outlet of the reactor, the inlet gas system
was connected to the reactor, and the temperature of the
impinger train was logged. A bubble meter was used to
perform a leak check by setting the gas rotameter to the
desired flow rate and checking the impinger train outlet flow
rate. After the leakage rate was determined for each test and
airflow was found to be within the acceptable range (50-70
mL/min), the tissue sample contained in the quartz crucible
was placed into the crucible holder and inserted into the
reactor, and a clip was placed around the joint between the
reactor and the crucible holder. All samples were heated for
Collection and Processing of Ash and Emission Samples.
At the end of each test burn, the clip holding the crucible
holder to the reactor was removed, and the crucible holder
with crucible inside was removed and immediately cooled.
The crucible with its contained ash was then placed in a
labeled sample vial. It was noted that the reactor walls and
crucible were coated with opaque, glasslike surface deposits
after the 1000 °C tests. When found in the reactor, these
deposits were dislodged using a stainless steel spatula,
collected from the reactor as thoroughly as possible, and
placed in the labeled sample vial along with the ash from
each crucible. The collected ash and deposits were then
transferred to a Tenbroeck tissue grinder and homogenized
in 1 mL of distilled water.
Following each test burn, the impinger train was removed,
labeled, and sealed. The entire impinger train was placed at
4 °Cfor storage and later transported to the National Institutes
of Health in Bethesda, Maryland for bioassay of the impinged
emission materials that were recovered separately from the
glass tubing leading from the burner to the first impinger
and from the traps. Visible deposits from the tubing were
assiduously scraped, rinsed into a tissue grinder, and
homogenized in 1mL of distilled water. Water from the first
trap was allowed to evaporate inside a laminar flow hood to
a volume of approximately 1 mL, which was transferred
together with all associated tube residues from both traps to
a tissue grinder and homogenized. Each test burn yielded
three samples: (1) residue collected from the crucible and
deposits from the inside of the heated zone of the reactor
(ash); (2) residue from the exhaust zone of the reactor tube
to the first impinger trap (exit tube residue); and (3)
commingled water and residues from the two impinger traps
(air emission samples).
Bioassays. The total volume of each sample was inoculated
undiluted into groups of healthy female weanling
hamsters (0.05 mL per animal by the intracerebral route;
approximately 20 animals per sample). Twenty uninoculated
sentinel animals were randomly positioned among the
inoculated bioassay animals, all of which were individually
caged, to avoid fighting and any possibility of crosscontamination.
Animals were observed for a period of 12
months for clinical signs of scrapie, at which point the
survivors were euthanized. The brains of all animals, whether
dying during the observation period, or surviving to its
conclusion, were examined for the presence of proteinaseresistant
protein (PrPres) by Western blot immunoassays.
The 12 month observation period was mandated by
considerations of cost and space associated with prolonged
care of the largenumberof animals (450) needed to conduct
this study. The occurrence of rare transmissions after longer
incubation periods in rodents inoculated intracerebrally with
low dose infectious material has been documented (10, 11),
but this possibility was mitigated in our experiment by the
examination of all brains for the presence of PrPres, which
is visible well before the onset of symptomatic disease (12,
Western BlotImmunoassays. Approximately 0.1 gof brain
tissue was extracted per sample by the phosphotungstic acid
method described by Safar et al. (14) and blotted using the
monoclonal anti-hamster PrP antibody 3F4 at a dilution of
1:2000. Samples giving a questionable positive result were
reextracted using the purification/concentration method of
Xi et al. (15): all six such samples were found to be clearly
negative on retesting.
Results and Discussion
Bioassay results for each tested sample are summarized in
Table 1. It is important to note that the all material recovered
from each test burnsapproximately 1 mL volumes of
resuspended ash, residues, or emissionsswas inoculated to
avoid any sampling error that can be significantwhendealing
with very low levels of infectivity.
Two unheated 263K brain tissue samples were assayed,
yielding levels of infectivity of 109.2 and 109.7 LD50/g of tissue
macerate. Incubation periods in the lowest dilution (10-1)
groups were between 50 and 60 days; incubation periods in
the highest positive dilution groups (titration end point)
ranged from 120 to 180 days.
The residual ash from the 1 g sample of 263K brain
macerate heated at 600 °C in normal air transmitted disease
to two of 21 inoculated animals after incubation periods of
261 and 303 days, and their brains were positive for PrPres.
The clinical signs and PrPres patterns in both hamsters were
TABLE 1. Bioassay Results for Combustion Products from
Heated Infected Hamster Brain Tissue Macerates and
test conditions bioassay specimen
tissue gas °C crucible exit tube traps
normal air ambient NA NA NA
normal air 600 0/20 NT NT
normal N2 603 0/21 0/18
normal air 1015 0/23 NT NT
normal N2 1000 0/20 0/18
infected air ambient NA NA NA
infected air 612 2/21 0/22 0/24
infected N2 598 0/20 0/19 0/26
infected air 996 0/15 0/26 0/23
infected N2 997 0/23 0/18 0/23
a For each test group, fractions represent number of PrPres-positive
animals over total number of inoculated animals. Residues from the
exit tubes and emissions from the impinger traps were combined for
bioassays of the uninfected control samples subjected to 600 and 1000
°C under N2. NA ) not applicable; NT ) not tested.
VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 6157
indistinguishable from those of the positive control animals
that received unheated inocula.
No other heated samples were infectious, based on the
absence of symptomatic disease and brain PrPres, including
reactor exit tube residues and emission samples from tissues
heated to 600 °C; ash, exit tube residues, and emissions from
tissues heated to 1000 °C; and normal brain tissue heated to
600 °C (bioassays were not done on normal tissue heated to
1000 °C). In particular, no clinically healthy animal surviving
to the observation end point was found to have PrPres in the
brain (i.e., no preclinical or subclinical infections were
detected 12 months after inoculation). All uninoculated
sentinel animals also remained asymptomatic and PrPresnegative.
Comparison of Experimental and Actual Incineration
Conditions. The question as to whether medical waste
incinerators and other types of combustion units used for
disposal of contaminated materials provide the conditions
necessary for inactivation of TSE cannot be completely
answered by laboratory experimentation. It is acknowledged
that experiments such as these cannot duplicate the dynamic
operating conditions and complex rheology of incinerators
and the myriad of interactions with other waste constituents
that occur in a combustion environment. However, smallscale
simulations can provide valuable qualitative information
regarding the behavior of materials in a high temperature
combustion environment under tightly controlled conditions.
With this limitation in mind, we offer the following comparisons
of our experimental conditions with those expected
in actual incinerators and comment on the implications of
our data for potential environmental releases of infectivity
from combustion processes.
Types of Incinerators and Operating Temperatures. In
the U.S., three types of incinerators are typically used for
disposal of medical wastes: controlled-air two-stage modular
systems, excess air batch systems, and rotary kilns (16, 17).
Of these, the controlled-air (also referred to as starved-air)
systems are the most widely used today (18). In these units,
combustion of wastes occurs in two stages. In the first stage,
waste is fed into the primary chamber, which is operated
with less than the stoichiometric amount of air required for
combustion. Air enters from both above and below the
burning bed of waste, which is dried, volatilized, pyrolized,
and partially combusted. In this chamber, the air temperature
above the waste is typically 760-980 °C. In the second stage,
air is added to the gases produced in the first chamber to
complete combustion, and the gas temperature is higher,
typically 980-1095 °C. The partial combustion of the waste
in the first stage yields a gas with sufficient heating value to
operate the combustion process in the second stage without
the need for additional fuel. Gas temperatures in each
chamber of controlled-air incinerators are thus higher than
the temperatures observed in our experiments to achieve,
respectively, near-total and total inactivation of the agent.
Excess air medical waste incinerators are typically small
modular units, usually designed with two chambers and
provisions for manual loading of waste into the primary
chamber and removal of ash. Burners are ignited to bring
the secondary chamber to an operating temperature of 870-
980 °C. When this temperature is reached, the burner in the
primary chamber is ignited. The unit is operated with levels
of air that are approximately 100% higher than the stoichiometric
amount of air required for combustion.
The operating temperatures of modern medical waste
incinerators, which typically operate well above 600 °C,
should reduce TSE infectivity concentrations to levels at or
very close to extinction. However, it should be noted that
these temperatures are usually measured in the gases above
the bed of burning waste, not in the bed itself. Themaximum
temperatures achieved in the bed may be as much as 100 °C
lower than the gases, depending on the bed depth, composition
of the input material, and other factors. Accordingly,
incinerators used to dispose of TSEs should not be operated
at lower temperatures. Indeed, studies by theEPAhaveshown
that incinerators operated at air temperatures of about 600
°C may not even inactivate conventional pathogens that are
much less resistant to thermal inactivation than TSEcontaminated
Large-volume, nonmedical waste streams that may contain
TSE-contaminated livestock and wildlife carcasses or
meat and bone meal(MBM)have been disposed of by various
methods. In the U.K., all carcasses are incinerated in dual
chamber facilities with primary and secondary chamber
temperatures of approximately 850 and 1000 °C, respectively.
These incinerators typically operate at a carcass input batch
rate ranging between 100 and 1000 kg/h (average 450 kg/h),
with a solid-phase residence time of 1 h (personal communication,
Dr. Stephen Wyllie, Department for Environment,
Food and Rural Affairs, U.K.). MBM may also be
incinerated, may be subjected to other thermal technologies
including rotary kilns, fluidized beds, and cement plants, or
co-fired in power plants with fuels such as coal, lignite, and
other wastes (19). If these nonincineration systems operate
under conditions similar to incinerators, they may be
expected to provide a similar level of inactivation. Evaluations
by the German Federal Institute for Viral Illnesses in Animals
and the Institute for Biological Safety indicate that the
incinerators used inGermanyfor disposal ofMBNcan achieve
a temperature of 600 °C for 15 min in the waste if specific
operating conditions are met (20, 21). Incinerators used in
the U.S. for disposal of municipal waste operate at temperatures
above 1000 °C (22).
Concentration of the Infectious Agent. In these experiments,
the waste load consisted of pure brain tissue with an
extremely high concentration of infectivity (>109LD50/g). In
actual incinerators, the concentration of infectivity in the
waste load will be much lower than that in our experiments
because the brain infectivity concentration in hamsters
infected with the 263K strain of scrapie is at least 2-3 logs
higher than in livestock infected with either BSE or scrapie
(CWD brain has not been titered) and also because high
infectivity central nervous system tissues are diluted in the
mix of peripheral carcass (orMBM)tissues that contain little
or no infectivity.
In medical waste incinerators, mixing of TSE tissues and
contaminated items with other materials also dilutes the
concentration of the agent in the waste load. Tissues usually
comprise only a small percentage of the total volume of most
hospital waste streams; almost all of the mass of material
that is classified as medical waste is comprised of noninfectious
materials such as paper and plastic (23); and medical
wastes are often burned together with noninfectious, nonmedical
Dilution of the infectious agent in a much larger volume
of noninfectious material is theoretically advantageous
because it reduces the probability of the agent being in
localized areas of the incinerator, which may have less than
optimal conditions for inactivation. An example of such an
area is the zone near the incinerator walls, which may be
cooler than the rest of the chamber. Conversely, mixing TSEcontaminated
materials with other wastes could adversely
impact inactivation by insulating the agent and decreasing
its total time of exposure to inactivation temperatures.
Combustion Gases. In some of these experiments, pure
nitrogen gas was used to simulate the combustion gas in the
primary chamber of a controlled-air incinerator. Oxygen in
the small volume of air that entered the reactor during the
few seconds when the crucible holding the tissue sample
was inserted would have been rapidly purged from the
system, probably before the sample was dried out and heated
6158 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 22, 2004
to the target temperature. Thus, virtually all of the test burns
using nitrogen were carried out under anoxic conditions.
This differs somewhat from actual starved-air incineration
conditions where limited amounts of air enter the chamber
throughout the combustion cycle, and partial oxidation of
waste constituents occurs. Volatilized organic and particulate
materials from the primary chamber enter the secondary
chamber where excess air is added to complete oxidization
of these materials.
The ash from controlled-air incinerators has a relatively
high carbon content, typically from 3 to 6% and values as
high as 30% are common (17). The high carbon content is
of concern because there is some evidence (24) that the
presence of carbon may protect TSE infectivity, and some of
the residues observed in the reactor exit tube and impinger
traps in these experiments were similar in color and form to
Although the nitrogen used as a reactor carrier gas did
not contain any oxygen, as would be present in actual
controlled-air incinerators, the results yielded information
relevant to inactivation mechanisms at higher temperatures.
No transmissions were detected in ash or emissions from
infected issues in the test burns performed in nitrogen,
confirming that the presence of oxygen is not required to
inactivate the agent and that any carbon formed was not
sufficiently protective to prevent its inactivation. The lack of
transmission from test burns in anoxic conditions also
suggests that denaturation or some inactivation mechanism
other than chemical oxidation may be operative at incineration
temperatures. These results may have potential application
in selection of waste processing technologies,
particularly for high-volume waste streams such as MBM
and animal carcasses. High-temperature, anoxic waste
pyrolysis systems that can yield biofuels and other useful
byproducts could be considered as alternatives for incineration,
which is usually a strictly destructive process.
SecondaryChamber.Another aspect of these experiments
that differed from actual incineration conditions was that
the vented gases from the reactor tube, which is functionally
similar to the primary chamber of an incinerator, were
exhausted directly into a cold impinger train. In actual
incinerators, the vented emissions from the primary chamber
typically enter a secondary chamber, which is usually
operated at a temperature higher than the primary chamber,
providing additional opportunity for the inactivation of any
pathogens carried in the gas phase. Depending on the system
design, gases exiting the secondary chamber may then be
cooled and passed through scrubbers or other types of air
pollution control equipment before they are released to the
environment. In our experiments, no infectivity was detected
in emission deposits collected directly from the exhaust end
of the reactor tube. This suggests that the agent would be
inactivated or retained in the ash in the first chamber of an
incinerator and that the potential for contamination of
residues, wastewater, and other effluents generated by gas
cooling and air pollution control systems is minimal.
Any thermal treatment processs whether incineration or
alternative technologysthat ensures exposure of TSE wastes
to temperatures of 1000 °C for at least 15 min should result
in sterile output products, as the minimum temperature
required to achieve sterility is probably only marginally above
600 °C. Treatment at 600 °C may thus produce an ash that
is either sterile or contains a level of residual infectivity well
within regulatory requirements for reductions of conventional
pathogens in sterile products. In our experiments, over one
billion LD50 of scrapie infectivity were reduced to less than
a single LD50 (two transmissions among 21 inoculated
animals) by a 15 min exposure to dry heat at 600 °C. Although
it may be objected that even this degree of reduction does
not achieve zero risk, it is approximately 10-fold greater than
the most stringent process validation guidelines issued by
the FDA to ensure the safety of biological products (up to 8
log virus removal) (25) or than the standard used by the EPA
for registration of sterilants (no growth of Bacillis subtilis in
720 carriers each having at least 2 105 spore counts) (26).
For TSE inactivation conditions to be met, incinerators
and other thermal treatment systems must be properly
selected and operated. In actual incinerators, inactivation
conditions can be adversely affected byanarray of operational
factors, such as overloading, cold start-ups and shut-downs,
inadequate control of air flow, insufficient or excessive
turbulence, and loss of partially burned material through
grates. Under these failure mode conditions, inactivation
may be incomplete.
Given a hypothetical potential for survival of trace
amounts of TSE infectivity in the combustion products of
incinerators operated under suboptimal conditions, the
likelihood of disease transmission via environmental media
is minimized by several factors including the following:
dilution; hydrophobic properties of agents that would be
expected to reduce their mobility in water and soils;
containment provided by ash landfill design and operations;
biological degradation; species barriers; and the inefficiency
of likely routes of exposure (27-32). It should also be noted
that neither humans nor animals appear to be susceptible
to air-borne TSE infections, further diminishing any potential
risk from incinerator emissions.
Prior to this study, data on inactivation of TSE-infected
tissues in incinerator emissions were not available for risk
assessment, and probabilistic approaches were used to assess
the risks of combustion processes, including the burning of
carcasses in open pyres. These approaches led to conclusions
that the risk of transmission to humans was extremely low
(33, 34). Our study provides actual data on inactivation under
incineration conditions and offers further reassurance that
TSE materials can be safely disposed of via incineration.
The authors thank Mr. David Liles of ARCADIS G&M, who
set up and operated the reactor and impinger train, and Mr.
George Nelson, who conscientiously scraped all tubing and
traps for emission residues. The combustion test portion of
this work was performed under Interagency Agreement
RW75938614 between the National Institutes of Health and
the U.S. Environmental Protection Agency.
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Received for review January 2, 2004. Accepted June 14, 2004.
6160 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 22, 2004TSS
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