Environmental Engineering Reference
In-Depth Information
air recirculation units, which can capture aerosol and particulate material as small as
0.3 microns.
For most biological agents, there is no data related to dose and response except
for the case of tuberculosis (TB). TB is endemic in some countries (during 1996,
3.8 million new cases of TB were reported to the WHO), while it is relatively rare in
others, and the potential for its transmission and introduction amongst disease-free
areas is high. TB is the leading cause of death worldwide from infectious disease,
and owing to the mode of transmission experts have raised particular concerns about
exposure amongst passengers and aircrew. TB infection is acquired through inhala-
tion of
Mycobacterium tuberculosis
in aerosolized respiratory secretions from an infec-
tious person coughing, talking or sneezing. The risk of infection is related to the
proximity and duration of exposure to the source patient. Decreased ventilation in
crowded and confined environments is often a contributing risk factor. Although a
single flight carries the risk of a relatively limited exposure, prolonged sojourn in a
confined aircraft cabin may increase the risk of transmission of
Mycobacterium tuber-
culosis
(WHO, 1998).
During the past few years, several episodes of potential transmission of TB infec-
tion during air travel have been reported, some of which have raised great anxiety
amongst the general population, health authorities, mass media and airline compa-
nies. No case of active TB has been identified as a result of exposure while on a com-
mercial air flight; however, there is evidence that transmission of
Mycobacterium
tuberculosis
may occur during long-haul flights (ie more than eight hours) from an
infectious source to other passengers or crew members. Decreased ventilation and con-
fined environment are risk factors. However, a single flight contains minimal risk of
exposure, while prolonged travel may increase the risk of transmission (Driver et al,
1994; Anon, 1995; Kenyon et al, 1996).
The reason that transmission of TB appears to be rare is that most modern air-
craft recirculate up to 50 per cent of the cabin air, with the remainder made up from
fresh bleed air.
Mycobacterium tuberculosis
is between 0.5 and 1.0 microns; therefore,
the HEPA filters used in recirculation (0.3 microns) should remove the tubercle bacilli
from the cabin air during the recirculation process, providing the HEPA filters are
well maintained. As a result, the potential risk from a person with infectious TB in
an aircraft with recirculation should therefore be limited to droplet spread to a small
number of seats in the immediate vicinity of the infected passenger before recircula-
tion takes place (Ormerod, 2000).
Control and prevention of disease transmission
During 1998 the WHO produced guidelines for the control and prevention of TB
transmission via air travel that would apply to all domestic and international airlines.
The main guidelines were:
•
Once informed by health authorities, airline companies should inform passen-
gers and crew of possible infection and encourage the passengers to establish
whether or not infection has occurred, and whether or not preventative treatment
is required. To facilitate this, airlines should maintain complete passenger con-
tact records.
