Reading 2
How can you catch diseases through the air?
Dust particles
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:
minerals: crystalline silica, coal, cement
toxic metals: lead, cadmium, nickel, beryllium
other chemicals: many bulk chemicals, pesticides
organic dusts: flour, wood, cotton, tea, pollen
biohazards: bacteria and viruses, moulds and spores (infection and allergies)
sand and soil
volcanic ash
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.
Inhaling airborne particles
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)
Microbes in dust
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)
Hospitals
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)
Wind-borne dust
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.
Fungal spores
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:
compost and manure
hay and grass
soil
dead wood
fallen leaves
old fruit and vegetables
faeces and dead animals
living plants infected with parasitic fungi
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:
soil in potted plants
bathroom fixtures such as shower heads and curtains
carpets and furniture
food and food waste
paper and cardboard products
textiles and leather
moist surfaces such as walls
Inside buildings conditions that produce damp or wet conditions and encourage growth of fungi are:
poor air circulation
direct spray around showers, on curtains and walls
condensation on windows, walls and ceilings
leaking plumbing and drainage systems
air conditioning and heating systems
roof leaks
ground seepage through walls and floors
flooding
The most common fungi that cause disease are:
Aspergillus fumigatus: very widespread in soil and most decaying organic matter and can cause a group of conditions called aspergillosis, the most common cause of air-borne fungal disease; it can affect lungs, eyes, skin, sinuses and other organs
Histoplasma capsulatum: thrives on bird and bat droppings and can survive for years in dry conditions
Cryptococcus neoformans: present in pigeon and bat dropping. Can cause a serious illness called cryptococcal meningitis in some people
Wind-borne fungal spores
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.
Bibliography
Deacon J. Airborne microorganisms. Institute of Cell and Molecular Biology, The University of Edinburgh. (link accessed 19-02-2018)
Airborne and Direct Contact Diseases. (link)
Wan Yang et al. 2011. Concentrations and size distributions of airborne influenza A viruses measured indoors at a health centre, a day-care centre and on aeroplanes. J R Soc Interface. 2011; 8:1176–84. (link)
Bischoff WE, Swett K, Leng I et al. Exposure to influenza virus aerosols during routine patient care. J Infect Dis 2013 Jan 30. (link)
Johnson DL et al. Lifting the lid on toilet plume aerosol: A literature review with suggestions for future research. Am J Infect Control. 2013 Mar; 41(3): 254–258. doi: 10.1016/j.ajic.2012.04.330. (link)
Prussin AJ et al. Seasonal Dynamics of the Airborne Bacterial Community and Selected Viruses in a Children’s Daycare Center. PLosOne 4 March, 2016. (link)
Lindsley, et al. 2016. Viable influenza A virus in airborne particles expelled during coughs versus exhalations. Influenza and Other Respiratory Viruses 10(5), 404–413. DOI: 10.1111/irv.12390. (link)
Hong Kong Special Administrative Unit Department of Health. 2003. Outbreak of severe acute respiratory syndrome (SARS) at Amoy Gardens, Kowloon Bay, Hong Kong: main findings of the investigation. Hong Kong Special Administrative Region Department of Health. (link accessed 19-02-2018)
Gerba CP, Wallis C, Melnick JL. 19975. Microbiological hazards of household toilets: droplet production and the fate of residual organisms. Appl Microbiol. 1975;30:229–37. (link)
Barberán A et al. The ecology of microscopic life in household dust. Proc R Soc B Biol Sci 282:20151139. doi:10.1098/rspb.2015.1139. (link)
WHO. Hazard Prevention and Control in the Work Environment: Airborne Dust. WHO/SDE/OEH/99.14. (link)
Esmaeil N et al. Dust events, pulmonary diseases and immune system. Am J Clin Exp Immunol. 2014; 3(1): 20–29. (link)
Peplow, M. 2014. Beijing smog contains witches' brew of microbes. Nature. doi:10.1038/nature.2014.14640. (link)
Frazer J. Infectious disease: Blowing in the wind. Nature, 484, 21–23, 05 April 2012. doi:10.1038/484021a. (link)
CDC. Valley Fever (coccidioidomycosis). (link accessed 19-02-2018)
Bourouiba L et al. Violent expiratory events: on coughing and sneezing. J. Fluid Mech. (2014), vol. 745, pp. 537-563. 2014 doi:10.1017/jfm.2014.88. (link)
Mora M, et al. Microorganisms in Confined Habitats: Microbial Monitoring and Control of Intensive Care Units, Operating Rooms, Cleanrooms and the International Space Station. Front Microbiol. 2016 Oct 13;7:1573. eCollection 2016. DOI: 10.3389/fmicb.2016.01573. (link)