Transmission of infectious diseases during commercial air travel
Transmission of infectious diseases during commercial air travel Alexandra Mangili, Mark A GendreauLancet 2005; 365: 989–96
See Comment page 917 Because of the increasing ease and affordability of air travel and mobility of people, airborne, food-borne, vector- Division of Geographic borne, and zoonotic infectious diseases transmitted during commercial air travel are an important public health Medicine and Infectious Diseases, Tufts-New England issue. Heightened fear of bioterrorism agents has caused health ofﬁcials to re-examine the potential of these agents Medical Center, Boston, MA, to be spread by air travel. The severe acute respiratory syndrome outbreak of 2002 showed how air travel can have an USA (A Mangili MD); and important role in the rapid spread of newly emerging infections and could potentially even start pandemics. In Department of Emergency addition to the ﬂight crew, public health ofﬁcials and health care professionals have an important role in the Medicine, Lahey Clinic Medical Center, Burlington, MA 01805, management of infectious diseases transmitted on airlines and should be familiar with guidelines provided by local USA(MAGendreau MD) and international authorities. Introduction
cabin can be manipulated by the ﬂight deck.1 When firstname.lastname@example.org
Over 1 billion passengers travel by air annually;
parked at the terminal, fresh air is supplied to the
50 million of these travel to the developing world.1,2
aircraft by auxiliary power units. During ﬂight, fresh air
Although infrequently reported and very difﬁcult to
is supplied into the cabin from the engines where the air
assess accurately, there is a risk of disease transmission
is heated, compressed, cooled, and passed into the cabin
during commercial air travel and this risk has become
to be circulated by the ventilation system.3 The outside
the focus of heightened attention. The growing mobility
air is assumed to be sterile at typical cruising altitudes.
of people and popularity of airline transportation has
Air circulation patterns aboard standard commercial
ampliﬁed the potential for disease to be transmitted to
aircraft are side-to-side (laminar) with air entering the
passengers not only during but also before and after
cabin from overhead, circulating across the aircraft, and
ﬂights. Here, we review knowledge about transmission
exiting the cabin near the ﬂoor (ﬁgure 1). Little front-to-
of infectious diseases associated with commercial air
back (longitudinal) airﬂow takes place.3–9 This air
travel, with particular emphasis on transmission within
circulation pattern divides the air ﬂow into sections
within the cabin, thereby limiting the spread of airborneparticles throughout the passenger cabin. The aircraft cabin environment
Most commercial aircraft in service recirculate 50% of
During ﬂight, the aircraft cabin is a ventilated, enclosed
the air delivered to the passenger cabin for improved
environment that exposes passengers to hypobaric
control of cabin circulation, humidity, and fuel
hypoxia, dry humidity, and close proximity to fellow
efﬁciency.5–9 This recirculated air usually passes through
passengers. This space is regulated by an environmental
high efﬁciency particulate air ﬁlters (HEPA) before
system that controls pressurisation, temperature,ventilation, and air ﬁltration on the aircraft. Althoughthis system is wholly automated, the number of air-conditioning packs in operation, zone temperatures, andthe mixture of fresh and re-circulated air delivered to the
Search strategy and selection criteria
We searched MEDLINE database for articles using: “aircraft”,“cabin air quality“, “infectious diseases“, “tuberculosis“,“severe acute respiratory syndrome“, “measles“, “inﬂuenza“,“common cold“, “food-borne“, “water-borne“, and “vector-borne“. We reviewed technical and governmental reportsfrom the UK House of Lords, UK Department of Transport, UKCivil Air Authority, US National Research Council, US Federal Aviation Administration, WHO, US Department of
Transportation, and US House of Representatives. Wesearched the internet using the Google search engine withthe same terms. Finally, we contacted medicalrepresentatives from the Civil Air Authority, the FederalAviation Administration, and several international air carriers. Figure 1: Air circulation pattern in typical airline passenger cabin From WHO4 with permission of the publisher. Arrows show air currents.
www.thelancet.com Vol 365 March 12, 2005
delivery into the cabin. Normal airline cabin air exchange
rates range from 15 to 20 air changes per hour compared
with 12 air changes per hour for a typical ofﬁce
building.3–9 Ventilation capacity varies substantially,
Positive TB skin test only. No active TB.
dependent on the aircraft type but typically averages
10 (4·7 L/s) cubic feet per minute.3,6–10 Ventilation rates
can also vary within the different cabin sections, such as
ﬁrst and economy class.7,9 In general, HEPA ﬁlters used
on commercial airlines have a particle-removing
efﬁciency of 99·97% at 0·3 microns.4,6,7,9,11 These ﬁlters
remove dust, vapours, bacteria, and fungi. HEPA ﬁlters
also effectively capture viral particles because viruses
usually spread by droplet nuclei.7,11 No ventilation
operational standards for commercial aircraft are
available. Although a survey showed that most air
carriers equip their large aircraft with HEPA ﬁlters,
neither the Civil Aviation Authority nor the Federal
Aviation Administration require their use.9,12,13
Likely to be airport, not aircraft, transmission
Cabin air quality has been the focus of many media
investigations and criticism from special interest
groups.14–16 Most of this concern is associated with the
perception that airborne particles are distributed
throughout the entire cabin by the ventilation system. Table: Reported infections transmitted on commercial airlines
However, no peer-reviewed scientiﬁc work links cabinair quality and aircraft ventilation rates to heightenedhealth risks compared with other modes of transport or
happens by aerolisation of an infectious agent through
with ofﬁce buildings,4,7,9,12,13,17–22 so this work is limited.7,9,12
droplet nuclei (residua of large droplets containing
These shortcomings motivated the UK government to
microorganisms that have evaporated to Ͻ5 microns).
form the Aviation Health Working Group in 2000 to
These residual droplets become aerosolised and disperse
further investigate health aspects of air travel, including
widely, dependent on environmental conditions, and
cabin air quality and safety. In a detailed study into cabin
remain suspended in air for indeﬁnite periods.5,27,28
air quality released in 2004, the group concluded that
Common vehicle transmission involves one inanimate
temperature, humidity, air speed, and concentrations of
vehicle, which transmits infection to many hosts, and
carbon monoxide, carbon dioxide, and microbiological
typically applies to microorganisms spread by food and
ﬂora aboard 14 commercial ﬂights using British
water. Vector-borne transmission results from the
Aerospace 146 and Boeing 300 aircraft were similar to
spread of disease by insects and vermin. The table shows
other reported studies.23 The European Cabin Air study
the infectious diseases that have been transmitted on
coordinated by the Building Research and Consultancy
commercial airlines. All types of disease transmission
continues to investigate environmental aspects within
the passenger cabin. These efforts will probably lead to
Large droplet and airborne mechanisms probably
improved international regulations for the certiﬁcation,
represent the greatest risk for passengers within the
inspection, and maintenance of aircraft environmental
aircraft because of the high density and close proximity
of passengers. In addition to proximity, successfulspread of contagion to other hosts is dependent on many
Modes of disease transmission
factors, including infectiousness of the source;
Four routes for the spread of microorganisms exist:
pathogenicity of the microorganism; duration of
contact, airborne, common vehicle, and vector-borne.25,26
Contact transmission involves direct contact in which
humidity, temperature); and host-speciﬁc factors such as
body-to-body contact takes place, or indirect in which the
general health and immune status.4,7,9,22 How these
susceptible person comes into contact with a
factors affect risk of disease transmission within the
contaminated intermediate host (fomite). Large droplet
transmission is judged a form of contact transmission inwhich large droplets (Ͼ5 microns) contaminated with
Risk of transmission
microorganisms are generated when an infected person
The risk of disease transmission within the conﬁned
sneezes, coughs, or talks. These droplets are propelled
space of the aircraft cabin is difﬁcult to determine.
short distances (Ͻ1 m) and deposited on a susceptible
Insufﬁcient data prohibits meta-analysis, which would
host’s conjunctiva or mucosa. Airborne transmission
allow an idea of the probability of disease transmission
www.thelancet.com Vol 365 March 12, 2005 Figure 2: Schematic diagram of SARS outbreak aboard Hong Kong to Beijing ﬂight From reference 31with permission of the publisher.
for each respective contagion. Many of the available
Airborne and large droplet-transmitted diseases
epidemiological studies are compromised by reporting
bias caused by incomplete passenger manifests, thereby
Tuberculosis is a serious global threat, and estimates
complicating risk assessment. Despite these limitations,
suggest that about a third of the world’s population has
data suggest that risk of disease transmission to other
the disease.4,38 The transmission of Mycobacterium
symptom-free passengers within the aircraft cabin is
tuberculosis is the most studied model of the spread of
associated with sitting within two rows of a contagious
airborne pathogens aboard aircraft. Several studies
passenger for a ﬂight time of more than 8 h.3,4,7,9,12,29–31 This
about in-ﬂight transmission of tuberculosis have been
association is mainly derived from investigations of in-
reported, with most being done in the mid-1990s.4,29,30,39–41
ﬂight transmission of tuberculosis, but is believed to be
Two of the seven investigations revealed a probable link
relevant to other airborne infectious diseases.3,4,7,9,32–34 Some
of onboard transmission. In the ﬁrst occurrence, a ﬂight
variation in this association has been reported, with one
attendant was the index case, and two documented
outbreak of severe acute respiratory syndrome (SARS) in
tuberculin skin test conversions occurred during
which passengers seated as far as seven rows from the
5 months in 1992 in 212 fellow crew members and
59 frequent ﬂyer passengers.39 The second, and largest,
Risk of disease transmission within the aircraft cabin
incident was of a passenger with pulmonary
also seems to be affected by cabin ventilation.4,5,7,9,12,29,30,35–37
tuberculosis travelling from Baltimore to Chicago and
In general, proper ventilation within any conﬁned space
then on to Honolulu.29 Four of 15 fellow passengers
reduces the concentration of airborne organisms in a
seated within two rows of the index passenger had
logarithmic fashion, and one air exchange removes 63%
positive tuberculin skin test conversion. Although there
of airborne organisms suspended in that particular
is a risk of tuberculosis within the aircraft cabin, no
space.27,28 The main laminar ﬂow pattern within the
cases of active disease have been reported as a result of
aircraft cabin (ﬁgure 1) with the practice of frequent cabin
air travel. Transmission within the aircraft cabin seems
air exchanges and use of HEPA ﬁltration for recirculated
to be more likely with close proximity to a contagious
air clearly limits transmission of contagion.4,7,9,12,36
passenger (within two rows) over a long time (greater
Transmission becomes widespread within all sections of
than 8 h) and not as a result of the practice of
the passenger cabin when the ventilation system is
recirculating 50% of the cabin air.4,9,12,29 An overall
nonoperational, as shown by an inﬂuenza outbreak when
probability of infection in the order of one in 1000 when
passengers were kept aboard a grounded aircraft with an
a symptomatic source is present has been suggested,35
and this probability of risk is similar to, if not less than,
Risk assessment incorporating epidemiological data
into mathematical models may show how proximity andventilation
commercial airlines. Deterministic modelling with data
SARS is a non-typical pneumonia caused by a
from an in-ﬂight tuberculosis investigation revealed that
coronavirus.32,43–45 The global spread by air travellers and
doubling ventilation rate within the cabin reduced
in-ﬂight spread of SARS has been documented.31,32,46–48
infection risk by half.35 Risk also reduced exponentially to
The disease is believed to usually be spread by large
almost zero in passengers seated 15 seats from the
aerosolised droplets or by direct and indirect contact, but
infectious source.35 Clearly ventilation provides a crucial
airborne or small droplet transmission better explains
determinant of risk, and efforts to increase ventilation will
the distribution of SARS cases that has occurred on
www.thelancet.com Vol 365 March 12, 2005
transmission of SARS during the Amoy Gardens
The fact that inﬂuenza outbreaks worldwide have been
outbreak in Hong Kong was a result of airborne spread via
affected by inﬂuenza strains imported by air travel is well
a viral plume.49 A total of 40 ﬂights have been investigated
established;52–54 however, only three studies of in-ﬂight
for carrying SARS-infected passengers.31,32,46–48 Five of
transmission of inﬂuenza have been reported.33,37,55 The
these ﬂights have been associated with probable on-board
ﬁrst was in an outbreak of inﬂuenza A/Texas strain
transmission of SARS in 37 passengers.31,32,46 Most of those
aboard a commercial carrier in 197937 that resulted in 72%
passengers were seated within ﬁve rows of the index
of all passengers aboard the airline contracting inﬂuenza
case.31,32 One 3-hour ﬂight carrying 120 passengers
within 72 h. The secondary attack rate in their families
travelling from Hong Kong to Beijing on March 15,
was estimated to be 20% within 2 weeks. The high
2003,31 began a superspreading event accounting for 22 of
transmission rate in this particular case was believed
the 37 people who contracted SARS after air travel.31,32,48
attributable to passengers being kept aboard the aircraft
for 3 h with an inoperative ventilation system while repair
occurred in 16 people, two passengers had a diagnosis of
work was being done. The second study described an
probable SARS, and four were reported to have SARS but
outbreak of inﬂuenza A/Taiwan/1/86 at a naval air station
could not be interviewed.31 The number of secondary cases
in 1989 in military personnel who were returning from
from that ﬂight remains under investigation, but more
temporary duty.55 Transmission of inﬂuenza occurred
than 300 people might have been affected.32,50 Figure 2
both on the ground and aboard two DC-9 aircraft that
shows the distribution pattern of the SARS outbreak
transported the squadron from Puerto Rico to a Florida
naval station. The third outbreak happened in 1999 in
This pattern could be important because it did not
mine workers travelling on a 75-seat aircraft.33 15
follow the typical example of in-ﬂight transmission of
passengers travelling with the index case developed
airborne pathogens—ie, risk of disease transmission is
symptoms within 4 days. Nine of the 15 were seated
associated with a ﬂight time of more than 8 h and sitting
within two rows, and all were seated within ﬁve rows, of
within two rows of the index passenger.4,7,9,12,29 The
the index case. No further inﬂuenza outbreaks aboard
duration of the Hong Kong to Beijing ﬂight was 3 h and
commercial aircraft have been reported since 1999.
affected passengers were seated seven rows in front andﬁve rows behind the index passenger.31 Possible
Other airborne diseases
explanations for this outbreak distribution include:
Meningococcal disease occurs after direct contact with
airborne transmission rather than direct contact spread; a
respiratory secretions and is associated with high
malfunctioning cabin ﬁltration system; and passengers
morbidity and mortality rates.42 A case of meningococcal
infected before or after the ﬂight.31 No on-board
disease associated with air travel is deﬁned as the
transmissions have occurred since late March, 2003,
development of the illness within 14 days of travel on a
when the WHO issued speciﬁc guidelines for in-ﬂight
ﬂight lasting at least 8 h, including ground delay, take off,
containment of SARS.47,51 As the ﬁrst severe contagious
and landing.34 The US Centers for Disease Control
disease of the 21st century, SARS exempliﬁes the ever-
received 21 reports of suspected air travel-associated
present threat of new infectious diseases and the real
meningococcal disease from 1999 to 2001.34 In all cases
potential for rapid spread made possible by the volume
the index person was contagious while aboard a
and speed of air travel. Finally, the distribution pattern of
commercial ﬂight, but no secondary cases of the disease
SARS transmission aboard the ﬂight emphasises the need
were reported. People seated next to an ill passenger
to study airborne transmission patterns aboard
should be quickly contacted and given chemoprophylaxis
within 24 h of identifying the index case. Chemoprophylaxis given more than 14 days after onset of
illness in the index case is probably of little or no value.56
Common cold outbreaks as a result of air travel have not
Measles is an airborne and highly contagious viral
been reported, which could be attributable to the
infection with an attack rate of about 80%.57 Transmission
difﬁculties of investigating such outbreaks in view of the
can occur during the prodromal illness and passengers
ubiquitous nature of the common cold. One study
might be unaware of their diagnosis at the time of travel.
compared the risk of developing an upper respiratory tract
Measles is no longer endemic in the USA and
infection during air travel in passengers ﬂying on aircraft
importations from developing countries account for most
that recirculated 50% cabin air versus aircraft using 100%
outbreaks. Imported measles and associated cases
fresh air in the passenger cabin.36 Recirculation of aircraft
accounted for at least 17% of all reported cases within the
cabin air was not a risk factor for contracting upper
USA during 1982.58 From 1996 to 2000 30% of all
imported measles cases were estimated to be in peoplewho ﬂew while symptomatic with the disease.59,60
Three case studies have described measles
The aircraft as a vector for global spread of inﬂuenza
transmission during commercial air travel.58,60–62 A case
strains is a greater concern than is in-ﬂight transmission.
report in 1982 identiﬁed seven secondary cases of
www.thelancet.com Vol 365 March 12, 2005
measles epidemiologically linked to the index case as a
attributable to greater use of prepackaged frozen meals,
result of in-transit exposure; one was a passenger ﬂying
improved food handling, and inspection, but might
on the same aircraft as the index case and ﬁve others
represent under-reporting by passengers or reporting
were people who had visited at least one common
departure gate with the index case.58 In the same year,another study reported an index passenger who infected
Vector-borne and zoonotic diseases
two fellow passengers on a ﬂight from Venezuela to
Vector-borne diseases are very frequent causes of
Miami.61 A third report recorded eight cases of in-ﬂight
morbidity and mortality in many parts of the world, and
transmitted measles during a 10-h ﬂight from New York
the potential of their importation via commercial aircraft
to Tel Aviv in 1994.62 The source case was not identiﬁed
remains a risk. Many cases of malaria occurring in and
but was speculated to be a crew member. In 2004, a
around airports all over the world in people who had not
passenger with measles travelled from Japan to Hawaii,
travelled to endemic areas, known as airport malaria, is
but this did not result in any transmission to other air
evidence that malaria-carrying mosquitoes can be
travellers.59 International adoptions have a signiﬁcant
imported on aircraft.69,70 A total of 87 cases of airport
role in the number of imported measles cases. In April,
malaria have been reported, 75 of which happened in
2004, an outbreak in children adopted from an
Europe.70 Dengue and yellow fever are both transmitted
orphanage in China resulted in temporary suspension of
by mosquitoes of the genus Aedes. The Aedes mosquito
adoptions from that particular orphanage.60 Nine of the
has been introduced into countries where it had not
ten conﬁrmed cases were thought to be infectious while
previously been present and many of these mosquitoes
travelling. One secondary case was a female student in
were likely to have spread by aircraft.69 A case of dengue
close contact with an infected child.
fever was reported in Germany in a couple returningfrom a trip to Hawaii.71 Airport transmission of dengue
Common vehicle diseases
fever was suspected in this particular case. The
The most commonly reported diseases transmitted on
disinsection of aircraft—spraying aircraft before landing
aircraft have been spread by the fecal-oral route via
to kill insects—and vector control around airports, as
contaminated food. A total of 41 in-ﬂight food-borne
well as immunisation requirements, seem to have been
outbreaks resulting in 11 deaths were documented
effective in preventing outbreaks in non-endemic areas.
between 1947 and 1999.63–68 Salmonella is the most
Although International Health Regulations recommend
usually reported food-borne pathogen spread via a
disinsecting aircraft travelling from countries with
commercial airline, with ﬁfteen documented outbreaks
malaria and other vector-borne diseases, only ﬁve
between 1947 and 1999 infecting nearly 4000 passengers
countries do so.69,72,73 Common and exotic animals are
and resulting in seven deaths.63,64 Eight food-borne
regularly transported on aircraft and may carry disease.
outbreaks caused by Staphylococcus and one associated
Many zoonotic pathogens cause emerging and re-
death were reported between 1947 and 1999.63,65 One of
emerging diseases.74,75 Up to now, no zoonotic outbreaks
the largest cases involved 57% of the passengers served a
associated with air travel have been reported; however,
ham omelette on an international ﬂight in 1975.65
continued monitoring of air transport of animals,
Surprisingly, only one viral-induced enteritis outbreak
especially from developing countries, is needed.
has been described.66 In this incident, contaminatedorange juice was the vehicle of transmission and a
Norwalk-like agent was isolated from fecal samples of
The potential for spread of bioterrorism agents via air
travel exists. A well written, comprehensive review of
There have been a few reported cases of ill passengers
bioterrorism agents and the implications of air transport
as a result of food or water contaminated with Vibrio
is available,5 and a new interest in previously eradicated
cholerae consumed during international air travel.63,67,68
smallpox has emerged. In-ﬂight spread of this disease
The ﬁrst documented in-ﬂight outbreak was in 1972 on a
has been documented, and so the potential for in-ﬂight
ﬂight from London to Sydney via Singapore.67 Of the
spread of smallpox is of public health concern.76,77 In
47 people who developed cholera, which was attributed
1963 an epidemic was reported in Sweden, in which the
to a cold appetiser served during the ﬂight, one died. The
index case was attributed to in-transit exposure, either at
largest outbreak of airline-associated cholera occurred in
the air terminal or on the aeroplane, and caused
1992, during a cholera epidemic in Latin America.68
During a ﬂight from Buenos Aires via Lima to Los
Viruses causing viral haemorrhagic fever, such as
Angeles, 75 passengers developed cholera, resulting in
ebola and lassa, have also been the focus of media
ten passengers being admitted and one death. A cold
attention and have been investigated for potential
seafood dish, prepared in one of the cholera-affected
transmission while aboard an aircraft.5 Although aerosol
countries, was implicated as the source of transmission.
spread of ebola has not been documented in man, this
No food-borne or water-borne outbreaks have been
mode of transmission occurs in non-human primates.
reported over the past 5 years, which is probably
Lassa, known to be transmitted via large droplets, is
www.thelancet.com Vol 365 March 12, 2005
thought to have an incubation period of up to 3 weeks,
of passengers.4,29 Although air carriers are under no
making infectious passengers potentially symptom-free
obligation to archive passenger manifests, most have
and unaware of their status at the time of travel.76–81
internal policies to do so for up to 3 months.24
A study of passengers exposed to an index case of lassafever in-ﬂight noted no evidence of transmission, even
in the 19 passengers seated within two rows of the index
Commercial airlines are a suitable environment for the
passenger.81 Because both lassa and ebola viruses have
spread of pathogens carried by passengers or crew. The
frequent fatal outcomes and no vaccine is available,
environmental control system used in commercial
appropriate infection control procedures should be
aircraft seems to restrict the spread of airborne
followed to prevent the transmission of these
pathogens, and the perceived risk is greater than the
diseases.79,80 Lassa fever is treatable with ribavirin if the
actual risk.12 Transmission of infectious diseases probably
drug is given within the ﬁrst 6 days of illness, making
happens more frequently than reported for various
reasons, including reporting bias and the fact that mostdiseases have a longer incubation period than air travel. Management of infectious disease exposures
Important questions include: what factors affect the
aboard commercial aircraft
transmission of infectious diseases within the aircraft
Early recognition and appropriate infection control
cabin? How effective are the ventilation systems used
measures are needed when passengers become exposed
within commercial aircraft with regard to emerging
to an infectious or potentially infectious passenger.
infections? Further assessment of risk through
Government and international laws provide legal
mathematical modelling is needed and will provide
authority to control the movement of passengers with
insight into disease transmission within the aircraft
communicable diseases.82,83 This authority ranges from
cabin as well as control of outbreaks of different diseases.
issuing travel alerts to quarantine of passengers arriving
The International Health Regulations adopted
at airports. Although air carriers have the right to refuse
worldwide in 1969 to limit the international spread of
to take passengers who are ill with a communicable
disease are being revised to provide a means for
disease or medically unﬁt for air travel, systematic
immediate notiﬁcation of all disease outbreaks of
screening of passengers for contagious diseases and
excluding passengers with infectious symptoms is
characterised by clinical syndrome rather than speciﬁc
impractical.84 Health care professionals are expected to
diagnosis to expedite reporting.88 These new regulations
identify individuals who are unﬁt for air travel, or advise
and continued vigilance by countries, health authorities,
the ﬂying public of how to safely travel by air. Prevention
airlines, and passengers will keep to a minimum but not
of a disease outbreak is the most important means of
eliminate the risk of disease spread by aircraft. The
control, and travellers should therefore be advised to
aviation industry and medical community should
postpone any air travel when they are ill.
educate the general public on health issues related to air
Good hand hygiene has been proven to reduce the risk
of disease transmission, and air travellers should make it
Conﬂict of interest statement
part of their normal travel routine.85 Although masks
We declare that we have no conﬂict of interest.
play a crucial part in infection control in health care
settings, their use is unproven in disease control within
We thank our colleagues for their helpful suggestions.
the aircraft cabin. However, a mask (NIOSH N 95)
should be placed on a passenger suspected of having
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Consultant Physician, Dept of GU Medicine• To distinguish normal/variants of normal • Different causes of genital lumps and of oily secretions that keep the skin healthy & mistaken for warts –they do not change in size but warts do(especially in women) & may be mistaken for warts ‘Cord like’ swelling, usually just below the head of the penisUsually ca
American Academy of Periodontology Statement on LocalDelivery of Sustained or Controlled Release Antimicrobialsas Adjunctive Therapy in the Treatment of Periodontitis*Sustained or controlled release local delivery (e.g.,smokers,patientswithaggressiveperiodonti-antimicrobial agents (LDAs) are available fortis, or who are medically compromised). Additionaluse as adjuncts to scaling and root plan