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:
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:
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)
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:
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.
Dust is considered to consist of solid particles with dimensions ranging from below 1 micrometer to at least 100 micrometres in diameter. Particles greater than 50 micrometres diameter do not remain airborne for long in still air, dropping about 7 cm per second. Larger airborne particles can stay in the air longer, however, depending on origin, physical characteristics and ambient conditions. The settling rate for airborne dust particles less than 1 micrometre in size is considered to be negligible, so effectively float in the air. (11)
Dust is made airborne by physical processing of materials, such as sawing, sanding, grinding, cutting, drilling, crushing and friction. Handling of particulate material also produces dust, for example bagging, mixing or filling containers from a hopper. Movement of the particles also generates dust as particles break up into smaller pieces from friction and impact.
There are many types of dust in the natural and human environment and a wide range of health problems caused by them. Examples of airborne dusts (also termed suspended particulate matter) that can cause health problems include:
Farming activities can generate large amounts of inorganic and organic dust from ploughing, combine harvesting, grass cutting, grain moving. Cleaning and maintenance activities in building can generate dust in sweeping, cleaning. Infestations of birds and rodents can result in build-up of material with hazardous microorganisms such as Salmonella and Leptospira, which can become airborne when disturbed.
People in the developed world spend most of their lives inside buildings yet relatively little is known about the microbes commonly present in homes and offices. A study of 1200 homes across the US collected settled dust inside and outside homes. The researchers found that there were distinct bacterial communities inside and out, but that fungal communities found inside were more related to those found in the environment outside the home. (10)
The fungal communities varied with climatic and geographical region, but the bacterial communities in dust found indoors were dependent on the number of people, the female–male ratio and the presence of pets. Other studies have found a relationship to household insects, differences in ventilation, building design, the environmental characteristics found within buildings and prior water damage from flooding.
Fungi that were more abundant in the home than outdoors included common household moulds such as Aspergillus and Penicillium. Bacteria found indoors were mainly associated with human skin (eg Staphylococcus, Streptococcus) and faeces. There were also different bacteria if women were present (eg Lactobacillus, Bifidobacterium) and in male dominated households where there where more Corynebacterium, Dermabacter (skin-associated) and Roseburia (faecal-associated). When pets were present, bacteria associated with mouths and faeces of dogs and cats were more abundant. (10)
Fungal spores are common in the outdoor and indoor environment. Many species of fungus have tiny spores that disperse through air currents, which also means the spores are of suitable size (a few micrometres) to be inhaled into the lungs. Fungi feed on organic matter, so any organic material in the right conditions can provide a growing medium for fungi and be a source of fungal spores — usually moist and warm conditions are conducive to growth.
Healthy people are unlikely to be affected in normal conditions where the concentration of spores in the air is low and the species present are not regarded as pathogenic. There are situations, however, where the concentration of spores and the species of fungus greatly increase the risk of infection or an allergic reaction causing asthma.
In the outdoor environment fungal spores are produced on decaying organic matter such as:
Any disturbance of these can create a high concentration of spores in the air and pose a health risk to people nearby and become sources of contamination of food prepared and stored in the vicinity.
In the indoor environment the fungal spores will be similar to those outdoors if there is airflow through windows, doors and ventilation systems. There are also many products used in buildings on which fungi will grow if they are wet or damp, including:
Inside buildings conditions that produce damp or wet conditions and encourage growth of fungi are:
The most common fungi that cause disease are:
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