COMMENTARY: Health workers need optimal respiratory protection for Ebola
E
ditor's Note:
Today's commentary was submitted to CIDRAP by the authors, who are
national experts on respiratory protection and infectious disease
transmission. In May they published a similar commentary on MERS-CoV.
Dr Brosseau is a Professor and Dr Jones an Assistant Professor in the
School of Public Health, Division of Environmental and Occupational
Health Sciences, at the University of Illinois at Chicago.
Healthcare
workers play a very important role in the successful containment of
outbreaks of infectious diseases like Ebola. The correct type and level
of personal protective equipment (PPE) ensures that healthcare workers
remain healthy throughout an outbreak—and with the current rapidly
expanding Ebola outbreak in West Africa, it's imperative to favor more
conservative measures.
The precautionary
principle—that any action designed to reduce risk should not await
scientific certainty—compels the use of respiratory protection for a
pathogen like Ebola virus that has:
- No proven pre- or post-exposure treatment modalities
- A high case-fatality rate
- Unclear modes of transmission
We
believe there is scientific and epidemiologic evidence that Ebola virus
has the potential to be transmitted via infectious aerosol particles
both near and at a distance from infected patients, which means that
healthcare workers should be wearing respirators, not facemasks.1
The
minimum level of protection in high-risk settings should be a
respirator with an assigned protection factor greater than 10. A powered
air-purifying respirator (PAPR) with a hood or helmet offers many
advantages over an N95 filtering facepiece or similar respirator, being
more protective, comfortable, and cost-effective in the long run.
We
strongly urge the US Centers for Disease Control and Prevention (CDC)
and the World Health Organization (WHO) to seek funds for the purchase
and transport of PAPRs to all healthcare workers currently fighting the
battle against Ebola throughout Africa—and beyond.
There has been a
lot of on-line and published controversy about whether Ebola virus can
be transmitted via aerosols. Most scientific and medical personnel,
along with public health organizations, have been unequivocal in their
statements that Ebola can be transmitted only by direct contact with
virus-laden fluids
2,3 and that the only modes of transmission we should be concerned with are those termed "droplet" and "contact."
These
statements are based on two lines of reasoning. The first is that no
one located at a distance from an infected individual has contracted the
disease, or the converse, every person infected has had (or must have
had) "direct" contact with the body fluids of an infected person.
This
reflects an incorrect and outmoded understanding of infectious
aerosols, which has been institutionalized in policies, language,
culture, and approaches to infection control. We will address this
below. Briefly, however, the important points are that virus-laden
bodily fluids may be aerosolized and inhaled while a person is in
proximity to an infectious person and that a wide range of particle
sizes can be inhaled and deposited throughout the respiratory tract.
The
second line of reasoning is that respirators or other control measures
for infectious aerosols cannot be recommended in developing countries
because the resources, time, and/or understanding for such measures are
lacking.
4
Although there are some important barriers to
the use of respirators, especially PAPRs, in developing countries,
healthcare workers everywhere deserve and should be afforded the same
best-practice types of protection, regardless of costs and resources.
Every healthcare worker is a precious commodity whose well-being ensures
everyone is protected.
If we are willing to offer infected US
healthcare workers expensive treatments and experimental drugs free of
charge when most of the world has no access to them, we wonder why we
are unwilling to find the resources to provide appropriate levels of
comparatively less expensive respiratory protection to every healthcare
worker around the world.
How are infectious diseases transmitted via aerosols?
Medical
and infection control professionals have relied for years on a paradigm
for aerosol transmission of infectious diseases based on very outmoded
research and an overly simplistic interpretation of the data. In the
1940s and 50s, William F. Wells and other "aerobiologists" employed now
significantly out-of-date sampling methods (eg, settling plates) and
very blunt analytic approaches (eg, cell culturing) to understand the
movement of bacterial aerosols in healthcare and other settings. Their
work, though groundbreaking at the time, provides a very incomplete
picture.
Early aerobiologists were not able to measure small
particles near an infectious person and thus assumed such particles
existed only far from the source. They concluded that organisms capable
of aerosol transmission (termed "airborne") can only do so at around 3
feet or more from the source. Because they thought that only larger
particles would be present near the source, they believed people would
be exposed only via large "droplets" on their face, eyes, or nose.
Modern
research, using more sensitive instruments and analytic methods, has
shown that aerosols emitted from the respiratory tract contain a wide
distribution of particle sizes—including many that are small enough to
be inhaled.5,6 Thus, both small and large particles will be present near an infectious person.
The
chance of large droplets reaching the facial mucous membranes is quite
small, as the nasal openings are small and shielded by their external
and internal structure. Although close contact may permit large-droplet
exposure, it also maximizes the possibility of aerosol inhalation.
As
noted by early aerobiologists, liquid in a spray aerosol, such as that
generated during coughing or sneezing, will quickly evaporate,
7
which increases the concentration of small particles in the aerosol.
Because evaporation occurs in milliseconds, many of these particles are
likely to be found near the infectious person.
The current
paradigm also assumes that only "small" particles (less than 5
micrometers [mcm]) can be inhaled and deposited in the respiratory
tract. This is not true. Particles as large as 100 mcm (and perhaps even
larger) can be inhaled into the mouth and nose. Larger particles are
deposited in the nasal passages, pharynx, and upper regions of the
lungs, while smaller particles are more likely to deposit in the lower,
alveolar regions. And for many pathogens, infection is possible
regardless of the particle size or deposition site.
It's time to
abandon the old paradigm of three mutually exclusive transmission routes
for a new one that considers the full range of particle sizes both near
and far from a source. In addition, we need to factor in other
important features of infectivity, such as the ability of a pathogen to
remain viable in air at room temperature and humidity and the likelihood
that systemic disease can result from deposition of infectious
particles in the respiratory system or their transfer to the
gastrointestinal tract.
We recommend using "aerosol transmissible"
rather than the outmoded terms "droplet" or "airborne" to describe
pathogens that can transmit disease via infectious particles suspended
in air.
Is Ebola an aerosol-transmissible disease?
We recently published a
commentary
on the CIDRAP site discussing whether Middle East respiratory syndrome
(MERS) could be an aerosol-transmissible disease, especially in
healthcare settings. We drew comparisons with a similar and more
well-studied disease, severe acute respiratory syndrome (SARS).
For
Ebola and other filoviruses, however, there is much less information
and research on disease transmission and survival, especially in
healthcare settings.
Being at first skeptical that Ebola virus
could be an aerosol-transmissible disease, we are now persuaded by a
review of experimental and epidemiologic data that this might be an
important feature of disease transmission, particularly in healthcare
settings.
What do we know about Ebola transmission?
No one
knows for certain how Ebola virus is transmitted from one person to the
next. The virus has been found in the saliva, stool, breast milk,
semen, and blood of infected persons.
8,9 Studies of
transmission in Ebola virus outbreaks have identified activities like
caring for an infected person, sharing a bed, funeral activities, and
contact with blood or other body fluids to be key risk factors for
transmission.
10-12
On the basis of epidemiologic
evidence, it has been presumed that Ebola viruses are transmitted by
contaminated hands in contact with the mouth or eyes or broken skin or
by splashes or sprays of body fluids into these areas.
Ebola viruses
appear to be capable of initiating infection in a variety of human cell
types,13,14 but the primary portal or portals of entry into susceptible hosts have not been identified.
Some
pathogens are limited in the cell type and location they infect.
Influenza, for example, is generally restricted to respiratory
epithelial cells, which explains why flu is primarily a respiratory
infection and is most likely aerosol transmissible. HIV infects T-helper
cells in the lymphoid tissues and is primarily a bloodborne pathogen
with low probability for transmission via aerosols.
Ebola virus,
on the other hand, is a broader-acting and more non-specific pathogen
that can impede the proper functioning of macrophages and dendritic
cells—immune response cells located throughout the epithelium.15,16
Epithelial tissues are found throughout the body, including in the
respiratory tract.
Ebola prevents these cells from carrying out their
antiviral functions but does not interfere with the initial inflammatory
response, which attracts additional cells to the infection site. The
latter contribute to further dissemination of the virus and similar
adverse consequences far beyond the initial infection site.
The
potential for transmission via inhalation of aerosols, therefore, cannot
be ruled out by the observed risk factors or our knowledge of the
infection process. Many body fluids, such as vomit, diarrhea, blood, and
saliva, are capable of creating inhalable aerosol particles in the
immediate vicinity of an infected person. Cough was identified among
some cases in a 1995 outbreak in Kikwit, Democratic Republic of the
Congo,11 and coughs are known to emit viruses in respirable particles.17 The act of vomiting produces an aerosol and has been implicated in airborne transmission of gastrointestinal viruses.18,19 Regarding diarrhea, even when contained by toilets, toilet flushing emits a pathogen-laden aerosol that disperses in the air.20-22
Experimental
work has shown that Marburg and Ebola viruses can be isolated from sera
and tissue culture medium at room temperature for up to 46 days, but at
room temperature no virus was recovered from glass, metal, or plastic
surfaces.
23 Aerosolized (1-3 mcm) Marburg, Ebola, and Reston
viruses, at 50% to 55% relative humidity and 72°F, had biological decay
rates of 3.04%, 3.06%. and 1.55% per minute, respectively. These rates
indicate that 99% loss in aerosol infectivity would occur in 93, 104,
and 162 minutes, respectively.
23
In still air, 3-mcm
particles can take up to an hour to settle. With air currents, these and
smaller particles can be transported considerable distances before they
are deposited on a surface.
There is also some experimental evidence that Ebola and other filoviruses can be transmitted by the aerosol route. Jaax et al24
reported the unexpected death of two rhesus monkeys housed
approximately 3 meters from monkeys infected with Ebola virus,
concluding that respiratory or eye exposure to aerosols was the only
possible explanation.
Zaire Ebola viruses have also been transmitted in the absence of direct contact among pigs25 and from pigs to non-human primates,26
which experienced lung involvement in infection. Persons with no known
direct contact with Ebola virus disease patients or their bodily fluids
have become infected.12
Direct injection and exposure
via a skin break or mucous membranes are the most efficient ways for
Ebola to transmit. It may be that inhalation is a less efficient route
of transmission for Ebola and other filoviruses, as lung involvement has
not been reported in all non-human primate studies of Ebola aerosol
infectivity.27 However, the respiratory and gastrointestinal
systems are not complete barriers to Ebola virus. Experimental studies
have demonstrated that it is possible to infect non-human primates and
other mammals with filovirus aerosols.25-27
Altogether,
these epidemiologic and experimental data offer enough evidence to
suggest that Ebola and other filoviruses may be opportunistic with
respect to aerosol transmission.
28 That is, other routes of
entry may be more important and probable, but, given the right
conditions, it is possible that transmission could also occur via
aerosols.
Guidance from the
CDC and
WHO
recommends the use of facemasks for healthcare workers providing
routine care to patients with Ebola virus disease and respirators when
aerosol-generating procedures are performed. (Interestingly, the 1998
WHO and CDC infection-control guidance for viral hemorrhagic fevers in
Africa, still available on the
CDC Web site, recommends the use of respirators.)
Facemasks,
however, do not offer protection against inhalation of small infectious
aerosols, because they lack adequate filters and do not fit tightly
against the face.
1 Therefore, a higher level of protection is necessary.
Which respirator to wear?
As described in our earlier
CIDRAP commentary,
we can use a Canadian control-banding approach to select the most
appropriate respirator for exposures to Ebola in healthcare settings.
29 (See
this document
for a detailed description of the Canadian control banding approach and
the data used to select respirators in our examples below.)
The control banding method involves the following steps:
- Identify the organism's risk group (1 to 4).
Risk group reflects the toxicity of an organism, including the degree
and type of disease and whether treatments are available. Ebola is in
risk group 4, the most toxic organisms, because it can cause serious
human or animal disease, is easily transmitted, directly or indirectly,
and currently has no effective treatments or preventive measures.
- Identify the generation rate.
The rate of aerosol generation reflects the number of particles created
per time (eg, particles per second). Some processes, such as coughing,
create more aerosols than others, like normal breathing. Some processes,
like intubation and toilet flushing, can rapidly generate very large
quantities of aerosols. The control banding approach assigns a
qualitative rank ranging from low (1) to high (4) (eg, normal breathing
without coughing has a rank of 1).
- Identify the level of control.
Removing contaminated air and replacing it with clean air, as
accomplished with a ventilation system, is effective for lowering the
overall concentration of infectious aerosol particles in a space,
although it may not be effective at lowering concentration in the
immediate vicinity of a source. The number of air changes per hour (ACH)
reflects the rate of air removal and replacement. This is a useful
variable, because it is relatively easy to measure and, for hospitals,
reflects building code requirements for different types of rooms. Again,
a qualitative ranking is used to reflect low (1) versus high (4) ACH.
Even if the true ventilation rate is not known, the examples can be used
to select an appropriate air exchange rate.
- Identify the respirator assigned protection factor.
Respirators are designated by their "class," each of which has an
assigned protection factor (APF) that reflects the degree of protection.
The APF represents the outside, environmental concentration divided by
the inside, facepiece concentration. An APF of 10 means that the outside
concentration of a particular contaminant will be 10 times greater than
that inside the respirator. If the concentration outside the respirator
is very high, an assigned protection factor of 10 may not prevent the
wearer from inhaling an infective dose of a highly toxic organism.
Practical examples
Two
examples follow. These assume that infectious aerosols are generated
only during vomiting, diarrhea, coughing, sneezing, or similar
high-energy emissions such as some medical procedures. It is possible
that Ebola virus may be shed as an aerosol in other manners not
considered.
Caring for a patient in the early stages of disease (no bleeding, vomiting, diarrhea, coughing, sneezing, etc).
In this case, the generation rate is 1. For any level of control (less
than 3 to more than 12 ACH), the control banding wheel indicates a
respirator protection level of 1 (APF of 10), which corresponds to an
air purifying (negative pressure) half-facepiece respirator such as an
N95 filtering facepiece respirator. This type of respirator requires fit
testing.
Caring for a patient in the later stages of disease (bleeding, vomiting, diarrhea, etc). If
we assume the highest generation rate (4) and a standard patient room
(control level = 2, 3-6 ACH), a respirator with an APF of at least 50 is
needed. In the United States, this would be equivalent to either a
full-facepiece air-purifying (negative-pressure) respirator or a
half-facepiece PAPR (positive pressure), but standards differ in other
countries. Fit testing is required for these types of respirators.
The
control level (room ventilation) can have a big effect on respirator
selection. For the same patient housed in a negative-pressure airborne
infection isolation room (6-12 ACH), a respirator with an assigned
protection factor of 25 is required. This would correspond in the United
States to a PAPR with a loose-fitting facepiece or with a helmet or
hood. This type of respirator does not need fit testing.
Implications for protecting health workers in Africa
Healthcare
workers have experienced very high rates of morbidity and mortality in
the past and current Ebola virus outbreaks. A facemask, or surgical
mask, offers no or very minimal protection from infectious aerosol
particles. As our examples illustrate, for a risk group 4 organism like
Ebola, the minimum level of protection should be an N95 filtering
facepiece respirator.
This type of respirator, however, would only
be appropriate only when the likelihood of aerosol exposure is very
low.
For healthcare workers caring for many patients in an epidemic
situation, this type of respirator may not provide an adequate level of
protection.
For a risk group 4 organism, any activity that has the
potential for aerosolizing liquid body fluids, such as medical or
disinfection procedures, should be avoided, if possible. Our risk
assessment indicates that a PAPR with a full facepiece (APF = 50) or a
hood or helmet (APF = 25) would be a better choice for patient care
during epidemic conditions.
We recognize that PAPRs present some
logistical and infection-control problems. Batteries require frequent
charging (which requires a reliable source of electricity), and the
entire ensemble requires careful handling and disinfection between uses.
A PAPR is also more expensive to buy and maintain than other types of
respirators.
On the other hand, a PAPR with a loose-fitting
facepiece (hood or helmet) does not require fit testing. Wearing this
type of respirator minimizes the need for other types of PPE, such as
head coverings and goggles. And, most important, it is much more
comfortable to wear than a negative-pressure respirator like an N95,
especially in hot environments.
A recent report from a Medecins Sans Frontieres healthcare worker in Sierra Leone
30
notes that healthcare workers cannot tolerate the required PPE for more
than 40 minutes. Exiting the workplace every 40 minutes requires
removal and disinfection or disposal (burning) of all PPE. A PAPR would
allow much longer work periods, use less PPE, require fewer doffing
episodes, generate less infectious waste, and be more protective. In the
long run, we suspect this type of protection could also be less
expensive.
Adequate protection is essential
To summarize,
for the following reasons we believe that Ebola could be an
opportunistic aerosol-transmissible disease requiring adequate
respiratory protection:
- Patients and procedures generate aerosols, and Ebola virus remains viable in aerosols for up to 90 minutes.
- All sizes of aerosol particles are easily inhaled both near to and far from the patient.
- Crowding,
limited air exchange, and close interactions with patients all
contribute to the probability that healthcare workers will be exposed to
high concentrations of very toxic infectious aerosols.
- Ebola targets immune response cells found in all epithelial tissues, including in the respiratory and gastrointestinal system.
- Experimental data support aerosols as a mode of disease transmission in non-human primates.
Risk
level and working conditions suggest that a PAPR will be more
protective, cost-effective, and comfortable than an N95 filtering
facepiece respirator.
Acknowledgements
We
thank Kathleen Harriman, PhD, MPH, RN, Chief, Vaccine Preventable
Diseases Epidemiology Section, Immunization Branch, California
Department of Public Health, and Nicole Vars McCullough, PhD, CIH,
Manager, Global Technical Services, Personal Safety Division, 3M
Company, for their input and review.
References http://www.cidrap.umn.edu/news-perspective/2014/09/commentary-health-workers-need-optimal-respiratory-protection-ebola
As
the Ebola virus continues to spread throughout the country like
wildfire, the elite presidential guard- the Executive Protection
Service, formerly the Special Security Service is the latest hit by the
deadly disease.
