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How can you catch diseases through the air?
The air around us may appear to be clean, but carries many types of microscopic particles, such as dust, water droplets and pollen, that cannot be seen by the human eye. These particles can transport microorganisms that cause a wide range of diseases in both humans and animals.
Bacteria, viruses and fungal spores in infected solids and liquids and on surfaces can become airborne by fast moving water and wind or other physical disturbance, then remain suspended for long periods. As water droplets move through the air they evaporate and shrink, which can leave behind particles small enough to float through the air and be carried on air currents inside or outside buildings. Inside buildings they can be drawn into ventilation systems and around the sewerage and drainage system.
The smallest particles can be breathed in and are then trapped on mucus membranes in the mouth, nose, throat and lungs where they can cause an infection. Until relatively recently it was unclear which were the most important means of airborne infection and how far bacteria and viruses could travel through air in the human environment. But with recent outbreaks of diseases such as influenza, SARS, Norovirus, Ebola virus and greater awareness of their impact, there has been more research on this topic. There are several ways in which infectious particles can become airborne:
breathing
coughing and sneezing
vomiting
flushing toilets
physical disturbance such as grinding, pouring, ploughing, sweeping
wind
Breathing
Studies on patients with influenza show that low-speed air flow produced by inhaling and exhaling can create aerosol particles in the lungs and emit them in exhaled breath. Breathing is also thought to be a greater source of infectious particles than coughing.
Coughing produces a large number of virus particles in each cough. Breathing expels fewer virus particles per breath, but as it is done more frequently than coughing, it produces a greater quantity of infectious material overall. (7)
The lower speed of air from breathing may mean that the infectious particles are not carried as far, but the risk of infection from breathing depends on:
distance from the infected person
length of time spent in the vicinity
infectious dose produced by the person the
airflow in the room
type of bacteria or virus — the viability and quantity of organisms needed to cause an infection vary greatly
Coughing and sneezing
Coughing and sneezing produce high-speed air flow through the lungs, throat, nose and mouth. This can dislodge infected particles of mucus and saliva and project them at high speed into the surrounding air. These range in size from large visible blobs that quickly land nearby, to microscopic particles at micrometer scale that behave like clouds and swirl through the air for several metres.
Not all people infected with viruses such as flu are infectious, however, as there are critical periods during illness when virus production peaks. Less than half of flu patients released flu viruses into the air in a study by Wake Forest Baptist Medical Center. Around a fifth of patients studied were classified as “super spreaders” or “super emitters” because they produced up to 32 times more viruses than other emitters. They also had more severe illness, producing greater viral load in their bodies. (4)
High speed videos taken by researchers at MIT show that coughs and sneezes produce gas clouds in addition to flying droplets and sheets of mucus and saliva. They found that tiny droplets in the clouds travelled 5-200 times further than previously thought. The turbulence of the cloud keeps the smaller droplets suspended, while the larger ones fall out.
The MIT research showed that droplets 100 micrometres (about the width of human hair) in diameter travelled five times farther, while droplets 10 micrometres in diameter (size of a typical cloud droplet) travelled 200 times further. Droplets less than 50 micrometers in size can remain airborne long enough in buildings to reach ventilation systems. (16)
Vomiting
Mild vomiting (non-projectile) can infect people nearby by producing infectious airborne particles. This was shown by a study of a Norovirus outbreak that found that some of the diners in a restaurant developed acute gastroenteritis following vomiting by another diner. There was a decreasing rate of infection with increasing distance from the source: 91% at the same table, 71% and 56% at the two adjacent tables and lower rates farther away. As none of these diners came into contact with the vomit it was most likely caused by airborne transmission. (5)
Toilet flush
The risk of airborne disease transmission from toilets, even from one building to another through the sewerage system, was first demonstrated in 1907. In an experiment in the 1950s a toilet was seeded with bacteria and agar plates used to collect aerosols settling out of the air. This found that the amount of aerosols increased with increasing flush energy and that the bacteria were still in the air eight minutes after the flush. (5)
Research has shown, directly or indirectly, that several types of bacteria and viruses can contaminate the air from a flushing toilet:
E. coli: In the 1970s, it was discovered that aerosols containing E. coli bacteria remained airborne and viable for at least 4-6 hours after flushing. In another experiment, toilets seeded with a bacterium and a virus (MS2 bacteriophage and poliovirus) were not completely free from the microorganisms after seven flushes and attempts to clean the bowl were only minimally effective in eliminating them. (9)
Salmonella: More recently, in 2000, it was found that Salmonella could be cultured from air samples near a toilet bowl after flushing. Salmonella also remained in the bowl water for more than 12 days and in a biofilm below the water line for 50 days after seeding the toilet with the bacteria. This shows that a biofilm may be able to maintain a supply of bacteria that infects the toilet bowl water and the aerosols created on flushing for much longer periods. (5)
Norovirus: The spread of Norovirus on ships, even after attempts to sanitise them after an outbreak, is thought to be a result of the ability of toilets to continue generating contaminated aerosols after multiple flushes and the resistance of Norovirus to cleaning and disinfection. (5)
Influenza: The flu virus may also be spread through the air by toilet flushing. A recent study conducted in healthcare facilities, day care centres, and aircraft found influenza A virus in aerosols of the size that can enter lungs. In the early stages of infection with the H1N1 strain of flu virus, symptoms include diarrhoea and vomiting, which means the virus may be spread by toilet flushing. (3)
Poorly designed building drainage systems
Leaking pipes
In apartment blocks the shared sewerage system can lead to the spread of infections between apartments and also by aerial dispersal to other buildings. Following the 2003 SARS outbreak in Hong Kong’s Amoy Gardens apartment complex it was found that the spread of the virus was likely caused by virus-laden aerosols originating in the sanitary system.
The sewerage system was contaminated with SARS coronavirus (SARS-CoV) when an infected person who was suffering from diarrhoea visited one of the apartments and used the toilet. It was concluded that contaminated aerosol was drawn through dry U-tube traps in the bathroom floor drains of other apartments by bathroom exhaust fans. Some aerosol particles may have then have been expelled to the outside of the multistory building and carried upward to other apartments. People in nearby buildings were also infected, thought to be from particles carried by the wind.(8) This could also mean that toilet flushing can generate aerosol particles contaminated with SARS, but it has not been determined experimentally yet.
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