Hygiene

Washing one's hands, a form of hygiene, is the most effective overall way to prevent the spread of infectious disease.

Astronaut taking a hot bath in the crew quarters of the Orbital Workshop (OWS) of the Skylab space station cluster in Earth orbit. In deploying the shower facility the shower curtain is pulled up from the floor and attached to the ceiling. The water comes through a push-button shower head attached to a flexible hose. Water is drawn off by a vacuum system.

Hygiene refers to the set of practices associated with the preservation of health and healthy living.

Concept of hygiene

Hygiene is an old concept related to medicine, as well as to personal and professional care practices related to most aspects of living. In medicine and in home (domestic) and everyday life settings, hygiene practices are employed as preventative measures to reduce the incidence and spreading of disease. In the manufacture of food, pharmaceutical, cosmetic and other products, good hygiene is a key part of quality assurance i.e ensuring that the product complies with microbial specifications appropriate to its use. The terms cleanliness (or cleaning) and hygiene are often used interchangeably, which can cause confusion. In general, hygiene mostly means practices that prevent spread of disease-causing organisms. Since cleaning processes (e.g., hand washing) remove infectious microbes as well as dirt and soil, they are often the means to achieve hygiene. Other uses of the term appear in phrases including: body hygiene, personal hygiene, mental hygiene, dental hygiene, and occupational hygiene, used in connection with public health. The term "hygiene" is derived from Hygeia, the Greek goddess of health, cleanliness and sanitation. Hygiene is also the name of a branch of science that deals with the promotion and preservation of health, also called hygienics. Hygiene practices vary widely, and what is considered acceptable in one culture might not be acceptable in another.

Medical hygiene

Medical hygiene pertains to the hygiene practices related to the administration of medicine, and medical care, that prevents or minimizes disease and the spreading of disease.

Medical hygiene practices include:

• Isolation or quarantine of infectious persons or materials to prevent spread of infection.
• Sterilization of instruments used in surgical procedures.
• Use of protective clothing and barriers, such as masks, gowns, caps, eyewear and gloves.
• Proper bandaging and dressing of injuries.
• Safe disposal of medical waste.
• Disinfection of reusables (i.e. linen, pads, uniforms)
• Scrubbing up, hand-washing, especially in an operating room, but in more general health-care settings as well, where diseases can be transmitted^[1]

Most of these practices were developed in the 19th century and were well established by the mid-20th century. Some procedures (such as disposal of medical waste) were tightened up as a result of late-20th century disease outbreaks, notably AIDS and Ebola.

Home and everyday life hygiene

Home hygiene pertains to the hygiene practices that prevent or minimize disease and the spreading of disease in home (domestic) and in everyday life settings such as social settings, public transport, the work place, public places etc.

Hygiene in home and everyday life settings plays an important part in preventing spread of infectious diseases.^[2] It includes procedures used in a variety of domestic situations such as hand hygiene, respiratory hygiene, food and water hygiene, general home hygiene (hygiene of environmental sites and surfaces), care of domestic animals, and home healthcare (the care of those who are at greater risk of infection).

At present, these components of hygiene tend to be regarded as separate issues, although all are based on the same underlying microbiological principles. Preventing the spread of infectious diseases means breaking the chain of infection transmission. The simple principle is that, if the chain of infection is broken, infection cannot spread. In response to the need for effective codes of hygiene in home and everyday life settings the International Scientific Forum on Home Hygiene has developed a risk-based approach (based on Hazard Analysis Critical Control Point (HACCP), which has come to be known as “targeted hygiene”. Targeted hygiene is based on identifying the routes of spread of pathogens in the home, and applying hygiene procedures at critical points at appropriate times to break the chain of infection.

The main sources of infection in the home^[3] are people (who are carriers or are infected), foods (particularly raw foods) and water, and domestic animals (in western countries more than 50% of homes have one or more pets). Additionally, sites that accumulate stagnant water—such as sinks, toilets, waste pipes, cleaning tools, face cloths—readily support microbial growth, and can become secondary reservoirs of infection, though species are mostly those that threaten “at risk” groups. Germs (potentially infectious bacteria, viruses etc.) are constantly shed from these sources via mucous, faeces, vomit, skin scales, etc. Thus, when circumstances combine, people become exposed, either directly or via food or water, and can develop an infection. The main “highways” for spread of germs^[3] in the home are the hands, hand and food contact surfaces, and cleaning cloths and utensils. Germs can also spread via clothing and household linens such as towels. Utilities such as toilets and wash basins, for example, were invented for dealing safely with human waste, but still have risks associated with them, which may become critical at certain times, e.g., when someone has sickness or diarrhea. Safe disposal of human waste is a fundamental need; poor sanitation is a primary cause of diarrhoeal disease in low income communities. Respiratory viruses and fungal spores are also spread via the air.

Good home hygiene means targeting hygiene procedures at critical points, at appropriate times, to break the chain of infection i.e. to eliminate germs before they can spread further.^[3] Because the “infectious dose” for some pathogens can be very small (10-100 viable units, or even less for some viruses), and infection can result from direct transfer from surfaces via hands or food to the mouth, nasal mucosa or the eye, 'hygienic cleaning' procedures should be sufficient to eliminate pathogens from critical surfaces. Hygienic cleaning can be done by:

• Mechanical removal (i.e. cleaning) using a soap or detergent. To be effective as a hygiene measure, this process must be followed by thorough rinsing under running water to remove germs from the surface.
• Using a process or product that inactivates the pathogens in situ. Germ kill is achieved using a “micro-biocidal” product i.e. a disinfectant or antibacterial product or waterless hand sanitizer, or by application of heat.
• In some cases combined germ removal with kill is used, e.g. laundering of clothing and household linens such as towels and bedlinen.

Hand hygiene

Defined as hand washing or washing hands with soap and water or using a waterless hand sanitizer.

Hand hygiene is central to preventing spread of infectious diseases in home and everyday life settings.^[4] The most critical situations where hand hygiene is needed are:

• After using the toilet (or disposing of human or animal faeces)
• After changing a baby’s diaper (nappy) and disposing of the faeces
• Immediately after handling raw food (e.g. chicken, meat)
• Before preparing and handling cooked/ready-to-eat food
• Before eating food or feeding children.

Hand hygiene is also important:

• After contact with contaminated surfaces (e.g. rubbish bins, cleaning cloths, food-contaminated surfaces)
• After handling pets and domestic animals
• After wiping or blowing the nose or sneezing into the hands
• After handling soiled tissues (self or others, e.g. children)
• After contact with blood or body fluids (e.g. vomit)
• Before and after dressing wounds
• Before giving care to an “at risk” person
• After giving care to an infected person.

The accepted procedure for hand washing with soap is as follows:

1. Always wash hands under running water
2. Apply soap
3. Rub hands together for 15–30 seconds, paying particular attention to fingertips, thumbs and between the fingers
4. Rinse well and dry thoroughly.

In situations where hand washing with soap is not an option (e.g. when in a public place with no access to wash facilities), a waterless hand sanitizer such as an alcohol hand gel can be used. They can also be used in addition to hand washing, to minimise risks when caring for “at risk” groups. To be effective, alcohol hand gels should contain not less than 60%v/v alcohol. Hand sanitizers are non-options in most developing country settings; in situations where availability of water is a problem, there are appropriate solutions such as tippy-taps, which use much less water and are very low-cost to make, with local materials. In low income communities mud or ash is sometimes used as an alternative to soap.

Respiratory hygiene

Correct respiratory and hand hygiene when coughing and sneezing reduces the spread of germs particularly during the cold and flu season.^[2]

• Carry tissues and use them to catch colds and sneezes
• Dispose of tissues as soon as possible
• Clean your hands by hand washing or using an alcohol hand sanitizer.

Food hygiene at home

Food hygiene pertains to the hygiene practices that prevent food poisoning . The five key principles of food hygiene, according to WHO, are^[5]:

1. Prevent contaminating food with pathogens spreading from people, pets, and pests.
2. Separate raw and cooked foods to prevent contaminating the cooked foods.
3. Cook foods for the appropriate length of time and at the appropriate temperature to kill pathogens.
4. Store food at the proper temperature.
5. Use safe water and raw materials

Household water treatment and safe storage

Household water treatment and safe storage are practices which can be used by the family at home and in the community to ensure that drinking water is safe for consumption.

Drinking water quality remains a significant problem, not only in developing countries^[6] but also in developed countries^[7]; even in the European region it is estimated that 120 million people do not have access to safe drinking water. Point-of-use water quality interventions can reduce diarrhoeal disease in communities where water quality is poor, or in emergency situations where there is a breakdown in water supply.^{[6] [7] [8] [9]} Since water can become contaminated during storage at home (e.g. by contact with contaminated hands or using dirty storage vessels), safe storage of water in the home is also important.

Methods for treatment of drinking water ^[10], ^[11] include:

1. Chemical disinfection using chlorine or iodine
2. Boiling
3. Filtration using ceramic filters^{[12] [13]}
4. Solar disinfection - Solar disinfection is an effective method, especially when no chemical disinfectants are available.^{[14] [15]}
5. UV irradiation - community or household UV systems may be batch or flow-though. The lamps can be suspended above the water channel or submerged in the water flow.
6. Combined flocculation/disinfection systems – available as sachets of powder which act by coagulating and flocculating sediments in water followed by release of chlorine.
7. Multibarrier methods – Some systems use two or more of the above treatments in combination or in succession to optimize efficacy.^[16]

Hygiene in the kitchen, bathroom and toilet

Routine cleaning of “contact” (hand, food and drinking water) sites and surfaces (such as toilet seats and flush handles, door and tap handles, work surfaces, bath and basin surfaces) in the kitchen, bathroom and toilet reduces the risk of spread of germs.^[10] The infection risk from the toilet itself is not high, provided it is properly maintained, although some splashing and aerosol formation can occur during flushing, particularly where someone in the family has diarrhoea. Germs can survive in the scum or scale left behind on baths and wash basins after washing and bathing.

Water left stagnant in the pipes of showers can be contaminated with germs that become airborne when the shower is turned on. If a shower has not been used for some time, it should be left to run at a hot temperature for a few minutes before use.

Thorough cleaning is important in preventing the spread of fungal infections.^[17] Moulds can live on wall and floor tiles and on shower curtains. Mould can be responsible for infections, cause allergic responses, deteriorate/damage surfaces and cause unpleasant odours. Primary sites of fungal growth are inanimate surfaces, including carpets and soft furnishings.^[18] Air-borne fungi are usually associated with damp conditions, poor ventilation or closed air systems.

Cleaning of toilets and hand wash facilities is important to prevent odours and make them socially acceptable. Social acceptability is an important part of encouraging people to use toilets and wash their hands, particularly in low income communities.

Laundry hygiene

Laundry hygiene pertains to the practices that prevent or minimize disease and the spreading of disease via soiled clothing and household linens such as towels.^[10] Items that are most likely to be contaminated with pathogens are those which come into direct contact with the body e.g. underwear, personal towels, facecloths, nappies. Micro-organisms can also be transferred between contaminated and uncontaminated items of clothing and linen during laundering. Of concern are the new “community” strains of MRSA.^[19] Experience in the USA suggests that these strains are transmissible within families, but also in community settings such as prisons, schools and sport teams. Skin-to-skin contact (including unabraided skin) and indirect contact with contaminated objects such as towels, sheets and sports equipment seem to represent the mode of transmission.^[19]

Two processes are considered suitable for hygienic cleaning of clothing and linen:^[10]

• Washing or laundering at 60°C or above
• Washing or laundering at 30-40°C using a bleach-based product: This produces decontamination of fabrics by a combination of physical removal and chemical inactivation. However, some types of fungi and viruses that are harder to inactivate, may not be removed.

Washing at temperatures of 40ºC or below with a non-bleach product is considered to carry a risk of inadequate decontamination.

Medical hygiene at home

Medical hygiene pertains to the hygiene practices that prevents or minimizes disease and the spreading of disease in relation to administering medical care to those who are infected or who are more “at risk” of infection in the home. Across the world, governments are increasingly under pressure to fund the level of healthcare that people expect. Care of increasing numbers of patients in the community, including at home is one answer, but can be fatally undermined by inadequate infection control in the home. Increasingly, all of these “at-risk” groups are cared for at home by a carer who may be a household member who thus requires a good knowledge of hygiene. People with reduced immunity to infection, who are looked after at home, make up an increasing proportion of the population (currently up to 20%).^[2] The largest proportion are the elderly who have co-morbidities, which reduce their immunity to infection. It also includes the very young, patients discharged from hospital, taking immuno-suppressive drugs or using invasive systems, etc. For patients discharged from hospital, or being treated at home special “medical hygiene” (see above) procedures may need to be performed for them e.g. catheter or dressing replacement, which puts them at higher risk of infection.

Antiseptics may be applied to cuts, wounds abrasions of the skin to prevent the entry of harmful bacteria which can cause sepsis.

Day-to-day hygiene practices, other than special medical hygiene procedures^[20] are no different for those at increased risk of infection than for other family members. The difference is that, if hygiene practices are not correctly carried out, the risk of infection is much greater.

Home Hygiene in low income communities

In the developing world, for decades, universal access to water and sanitation has been seen as the essential step in reducing the preventable ID burden, but it is now clear that this is best achieved by programmes that integrate hygiene promotion with improvements in water quality and availability, and sanitation. About 2 million people die every year due to diarrhoeal diseases, most of them are children less than 5 years of age.^[21] The most affected are the populations in developing countries, living in extreme conditions of poverty, normally peri-urban dwellers or rural inhabitants. Providing access to sufficient quantities of safe water, the provision of facilities for a sanitary disposal of excreta, and introducing sound hygiene behaviours are of capital importance to reduce the burden of disease caused by these risk factors.

Research shows that, if widely practiced, hand washing with soap could reduce diarrhoea by almost fifty percent^{[22] [23] [24]} and respiratory infections by nearly twenty-five percent^{[25] [26]} Hand washing with soap also reduces the incidence of skin diseases^{[27] [28]}, eye infections like trachoma and intestinal worms, especially ascariasis and trichuriasis. ^[29]

Other hygiene practices such as safe disposal of waste, surface hygiene, care of domestic animals are also important in low income communities in order to break the chain of infection transmission.^[30]

Disinfectants and antibacterials in home hygiene

Chemical disinfectants are products that kill germs (harmful bacteria, viruses and fungi). If the product is a disinfectant, the label on the product should say “disinfectant” and/or “kills” germs or bacteria etc. Some commercial products, e.g. bleaches, even though they are technically disinfectants, say that they “kill germs”, but are not actually labelled as “disinfectants”. Not all disinfectants kill all types of germs. All disinfectants kill bacteria (called bactericidal). Some will also kill fungi (fungicidal), bacterial spores (sporicidal) and/or viruses (virucidal).

An antibacterial product is a product that acts against bacteria in some unspecified way. Some products labelled “antibacterial” will kill bacteria whilst others may contain a concentration of active ingredient which will only prevent them multiplying. It is therefore important to check whether the product label states that it “kills” bacteria” An antibacterial is not necessarily anti-fungal or anti-viral unless this is stated on the label.

The term sanitizer has been used to define substances which both clean and disinfect. More recently this term has been applied to alcohol-based products which are used to disinfect the hands (alcohol hand sanitizers). Alcohol hand sanitizers however are not considered to be effective on soiled hands.

The term biocide is a broad term for a substance that kills, inactivates or otherwise controls living organisms. It includes antiseptics and disinfectants, which combat micro-organisms, and also includes pesticides.

Body hygiene

Body hygiene pertains to hygiene practices performed by an individual to care for one's bodily health and well being, through cleanliness. Motivations for personal hygiene practice include reduction of personal illness, healing from personal illness, optimal health and sense of well being, social acceptance and prevention of spread of illness to others.

Personal hygiene practices include: seeing a doctor, seeing a dentist, regular washing/bathing, and healthy eating. Personal grooming extends personal hygiene as it pertains to the maintenance of a good personal and public appearance, which need not necessarily be hygienic.

Body hygiene is achieved by using personal body hygiene products including: soap, hair shampoo, toothbrushes, tooth paste, cotton swabs, antiperspirant, facial tissue, mouthwash, nail files, skin cleansers, toilet paper, and other such products.

Excessive body hygiene

The benefits of body hygiene can be diminished by the risks of excessive body hygiene, which is hypothesized to cause allergic disease and bodily irritation.

Excessive body hygiene and allergies

The hygiene hypothesis was first formulated in 1989 by Strachan who observed that there was an inverse relationship between family size and development of atopic allergic disorders – the more children in a family, the less likely they were to develop these allergies.^[31] From this, he hypothesised that lack of exposure to “infections” in early childhood transmitted by contact with older siblings could be a cause of the rapid rise in atopic disorders over the last thirty to forty years. Strachan further proposed that the reason why this exposure no longer occurs is, not only because of the trend towards smaller families, but also “improved household amenities and higher standards of personal cleanliness”.

Although there is substantial evidence that some microbial exposures in early childhood can in some way protect against allergies, there is no evidence that we need exposure to harmful microbes (infection) or that we need to suffer a clinical infection.^{[32] [33] [34]} Nor is there evidence that hygiene measures such as hand washing, food hygiene etc. are linked to increased susceptibility to atopic disease ^{[29] [30]} If this is the case, there is no conflict between the goals of preventing infection and minimising allergies. A consensus is now developing among experts that the answer lies in more fundamental changes in lifestyle etc. that have led to decreased exposure to certain microbial or other species, such as helminths, that are important for development of immuno-regulatory mechanisms. ^[35] There is still much uncertainty as to which lifestyle factors are involved.

Although media coverage of the hygiene hypothesis has declined, a strong ‘collective mindset’ has become established that dirt is ‘healthy’ and hygiene somehow ‘unnatural’. This has caused concern among health professionals that everyday life hygiene behaviours, which are the foundation of public health, are being undermined. In response to the need for effective hygiene in home and everyday life settings, the International Scientific Forum on Home Hygiene has developed a “risk-based” or targeted approach to home hygiene which seeks to ensure that hygiene measures are focussed on the places, and at the times which are most critical for infection transmission.^[3] Whilst targeted hygiene was originally developed as an effective approach to hygiene practice, it also seeks, as far as possible, to sustain “normal” levels of exposure to the microbial flora of our environment to the extent that is important to build a balanced immune system.

Excessive body hygiene of external ear canals

Excessive body hygiene of the ear canals can result in infection or irritation. The ear canals require less body hygiene care than other parts of the body, because they are sensitive, and the body system adequately cares for these parts. Attempts to clean the ear canals through the removal of earwax can actually reduce ear canal cleanliness by pushing debris and foreign material into the ear that the natural movement of ear wax out of the ear would have removed.

Excessive body hygiene of skin

Excessive body hygiene of the skin can result in skin irritation. The skin has a natural layer of oil, which promotes elasticity, and protects the skin from drying. When washing, unless using aqueous creams with compensatory mechanisms, this layer is removed leaving the skin unprotected.

Excessive application of soaps, creams, and ointments can also adversely affect certain of the natural processes of the skin. For examples, soaps and ointments can deplete the skin of natural protective oils and fat-soluble content such as cholecalciferol (vitamin D3), and external substances can be absorbed, to disturb natural hormonal balances.

Culinary (food) hygiene

Culinary hygiene pertains to the practices related to food management and cooking to prevent food contamination, prevent food poisoning and minimize the transmission of disease to other foods, humans or animals. Culinary hygiene practices specify safe ways to handle, store, prepare, serve and eat food.

Culinary practices include:

• Cleaning and disinfection of food-preparation areas and equipment (for example using designated cutting boards for preparing raw meats and vegetables). Cleaning may involve use of chlorine bleach, ethanol, ultraviolet light, etc. for disinfection.
• Careful avoidance of meats contaminated by trichina worms, salmonella, and other pathogens; or thorough cooking of questionable meats.
• Extreme care in preparing raw foods, such as sushi and sashimi.
• Institutional dish sanitizing by washing with soap and clean water.
• Washing of hands thoroughly before touching any food.
• Washing of hands after touching uncooked food when preparing meals.
• Not using the same utensils to prepare different foods.
• Not sharing cutlery when eating.
• Not licking fingers or hands while or after eating.
• Not reusing serving utensils that have been licked.
• Proper storage of food so as to prevent contamination by vermin.
• Refrigeration of foods (and avoidance of specific foods in environments where refrigeration is or was not feasible).
• Labeling food to indicate when it was produced (or, as food manufacturers prefer, to indicate its "best before" date).
• Proper disposal of uneaten food and packaging.

Personal service hygiene

Personal service hygiene pertains to the practices related to the care and use of instruments used in the administration of personal care services to people:

Personal hygiene practices include:

• Sterilization of instruments used by service providers including hairdressers, aestheticians, and other service providers.
• Sterilization by [autoclave] of instruments used in body piercing and tattoo marking.
• Cleaning hands.

History of hygienic practices

Elaborate codes of hygiene can be found in several Hindu texts, such as the Manusmriti and the Vishnu Purana.^[36] Bathing is one of the five Nitya karmas (daily duties) in Hinduism, and not performing it leads to sin, according to some scriptures. These codes were based on the notion of ritual purity and were not informed by an understanding of the causes of diseases and their means of transmission. However, some of the ritual-purity codes did improve hygiene, from an epidemiological point of view, perhaps by accident, or because certain practices acquired ritual status on account of an empirical correlation with good health.

Regular bathing was a hallmark of Roman civilization.^[37] Elaborate baths were constructed in urban areas to serve the public, who typically demanded the infrastructure to maintain personal cleanliness. The complexes usually consisted of large, swimming pool-like baths, smaller cold and hot pools, saunas, and spa-like facilities where individuals could be depilated, oiled, and massaged. Water was constantly changed by an aqueduct-fed flow. Bathing outside of urban centers involved smaller, less elaborate bathing facilities, or simply the use of clean bodies of water. Roman cities also had large sewers, such as Rome's Cloaca Maxima, into which public and private latrines drained. Romans didn't have demand-flush toilets but did have some toilets with a continuous flow of water under them. (Similar toilets are seen in Acre Prison in the film Exodus.)

Until the late 19th Century, only the elite in Western cities typically possessed indoor facilities for relieving bodily functions. The poorer majority used communal facilities built above cesspools in backyards and courtyards. This changed after Dr. John Snow discovered that cholera was transmitted by the fecal contamination of water. Though it took decades for his findings to gain wide acceptance, governments and sanitary reformers were eventually convinced of the health benefits of using sewers to keep human waste from contaminating water. This encouraged the widespread adoption of both the flush toilet and the moral imperative that bathrooms should be indoors and as private as possible.^[38]

Islamic hygienical jurisprudence

Since the 7th century, Islam has always placed a strong emphasis on hygiene. Other than the need to be ritually clean in time for the daily prayer (Arabic: Salat) through Wudu and Ghusl, there are a large number of other hygiene-related rules governing the lives of Muslims. Other issues include the Islamic dietary laws. In general, the Qur'an advises Muslims to uphold high standards of physical hygiene and to be ritually clean whenever possible.

Hygiene in Ancient Europe

Contrary to popular belief^[39] and although the Early Christian leaders condemned bathing as unspiritual,^[40] bathing and sanitation were not lost in Europe with the collapse of the Roman Empire.^[41][42] Soapmaking first became an established trade during the so-called "Dark Ages". The Romans used scented oils (mostly from Egypt), among other alternatives.

Bathing did not fall out of fashion in Europe until shortly after the Renaissance, replaced by the heavy use of sweat-bathing and perfume, as it was thought in Europe that water could carry disease into the body through the skin. (Water, in fact, does carry disease, but more often if it is drunk than if one bathes in it; and water only carries disease if it is contaminated by pathogens.) Medieval church authorities believed that public bathing created an environment open to immorality and disease. Roman Catholic Church officials even banned public bathing in an unsuccessful effort to halt syphilis epidemics from sweeping Europe.^[43] Modern sanitation was not widely adopted until the 19th and 20th centuries. According to medieval historian Lynn Thorndike, people in Medieval Europe probably bathed more than people did in the 19th century.^[44]

Academic resources

• International Journal of Hygiene and Environmental Health, ISSN: 1438-4639, Elsevier

See also

References

External links

• US Centre for Disease Control and Prevention
• European Centre for Disease Control and Prevention
• Water Sanitation and Hygiene
• The International Scientific Forum on Home Hygiene
• Center for Hygiene and Health in the home and community, Simmons College, Boston, USA
• Hygiene Centre, London School of Hygiene and Tropical Medicine
• Water Supply and Sanitation Collaborative Council
• IRC International water and Sanitation Centre
• A virtual exhibition on the history of bathing
• Alliance for Prudent Use of Antibiotics on hand washing
• Centers for Disease Control on hand hygiene in healthcare settings
• Home Economics Archive: Tradition, Research, History (HEARTH)
  An e-book collection of over 1,000 books on home economics spanning 1850 to 1950, created by Cornell University's Mann Library. Includes several hundred works on hygiene in this period, itemized in a specific bibliography.