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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)
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)
Image: A flushing toilet launches contaminated water droplets into the air
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 collected by a vacuum cleaner
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.
The larger airborne particles that are inhaled (greater than about 30 micrometres) are mainly caught in the upper airways, especially when breathing through the nose with low breathing rates. They are caught in the mucus lining the nasal passages and can be expelled. The particles of medium size are caught in the airways between the head area and the upper lungs. These can also be easily ejected by the action of the cilia (hair-like cells) that line the air passages and mucus. This action is impaired by smoking, however.
The particles that reach the deepest part of the lungs, the alveolar region, are mainly less than 10 micrometres in size. The particles deposited there peak at 2 micrometres while smaller particles tend to be exhaled again. Breathing through the mouth greatly increases the amount of dust and larger particles deposited in the lower airways. (11)
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)
Image: Patients in operating theatres are vulnerable to infection from airborne microorganisms originating from various sources including medical staff, clothing and bed linen
People being cared for in hospitals and especially those in operating rooms and intensive care units are vulnerable to infections. Patients who have infectious diseases and severe injuries have a lowered immune system that makes them susceptible to microorganisms that are generally no threat to healthy people. In addition, the hospital environment may contain pathogenic microorganisms derived from ill patients that are not common in homes and general public areas.
Surgical personnel and patients release aerosols from their respiratory systems, skin particles and textile fibres from clothing. Clothing, bed linen, pajamas and privacy curtains can become contaminated from being close to or touching an infectious patient and release infected fibres into the air. Gut, hair and skin associated bacteria are more common in items that have been close to patients.
Antibiotic resistant Staphylococcus strains on bed linen can be made airborne by normal handling of bedding and be transmitted through the air. Dust on mobile surgery lamps used in operating rooms has been found to contain Staphylococcus bacteria.
In operating rooms, studies have shown that the number of airborne particles is related to the number of people present, their movement and opening and closing of doors. Some medical instruments produce smoke from human tissue, such as ultrasonic scalpels and lasers used for tissue coagulation. Cleaning regimes, including air filtration are the most important means of preventing airborne infection. (17)
Image: NASA simulation showing: dust blown from the Sahara westwards across the Atlantic; smoke from the Pacific northwest blown east to Europe, together with salt spray from the north Atlantic Ocean. Source: NASA Goddard Space Flight Center (https://svs.gsfc.nasa.gov/12772)
Wind can carry microorganisms from soil into the local environment and over long distances in dust. A genetic analysis of dust particles up to 10 micrometres extracted from a Beijing smog found a common soil bacterium was the most abundant microorganism. Pneumonia-causing Streptococcus pneumonia and the allergenic fungus Aspergillus fumigatus, along with faecal bacteria, were also present. (13)
Dust storms in Africa and Asia have significantly increased in recent years, with storms in the Sahara affecting Southern and Central Europe, the Caribbean and Florida — where half of airborne particles in summer come from North Africa. Saharan storms have been shown to cause an increase in asthma in Greece, Italy and Trinidad. Other studies have shown Asian dust storms have increased respiratory diseases, including asthma in East China, the Korean peninsula, Japan, Kuwait, Iraq. (12)
The composition of the particles varies with geology of the source location, but their complex structure can also contain pollutants, organic matter and microorganisms. Asthma-causing allergens found in desert dusts include fungal spores, dust mites, pollen, pollutants and organic detritus, with house dust mites the major cause.
Microorganisms in dusts have been shown to withstand harsh environmental conditions of transport through the atmosphere of high and very low temperatures, UV radiation and desiccation. (12)
Avian flu has also been linked to dust storms from central Asia. Kawasaki disease is a sometimes fatal condition that causes inflammation of the blood vessels in young children. So far has not been linked to any microorganism but outbreaks have been linked to winds blowing from central Asia across Japan, and also reaching Hawaii and California. (14)
All these cases show that wind-borne dust could play a major role in the spread of diseases and potentially contamination in the food supply chain.
Fungus growing on a damp wall
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:
Wind-borne fungal spores lifted from soil can also be a health threat over large geographic areas, but has been little studied. In some arid areas of California, Utah, Nevada, Arizona, New Mexico and Texas an infection called Valley Fever or desert rheumatism caused by airborne spores of a soil fungus, Coccidioides immitis, causes around 2000 – 20,000 reported infections a year, but is thought to be widely under-reported. The fungus is also known to be present in Washington, Mexico and parts of South America.
The spores are disturbed by wind and activities such as farming, which has implications for the food supply chain, and construction. In the southwestern US, high concentrations of the fungus have been found in certain soil types and around rodent burrows. Domestic and wild animals are also susceptible to the disease. (13) This kind of infection could be greatly under-reported worldwide as even in the US health authorities are not very aware of the risk.
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