Science Fair Project Encyclopedia
Disinfection The destruction of pathogenic and other kinds of microorganisms by physical or chemical means
Disinfectants are chemical substances used to kill viruses and microbes (germs), such as bacteria and fungi. The ideal disinfectant would offer complete sterilization, without harming other forms of life, be inexpensive, and non-corrosive. Unfortunately ideal disinfectants do not exist. Many disinfectants are only able to partially sterilize. The most resistant pathogens are bacteria spores but some viruses and bacteria are also highly resistant to many disinfectants.
All disinfectants are also, by their very nature, potentially harmful (even toxic) to humans or animals. They should be treated with appropriate care. Most come with safety instructions printed on the packaging, which should be read in full before using the disinfectant. Most modern household disinfectants contain Bitrex, an exceptionally bitter substance designed to discourage ingestion, as an added safety measure. Those that are used in people's homes should never be mixed with other cleaning products as chemical reactions can occur.They are frequently used in hospitals, dental surgeries, kitchens and bathrooms to kill infectious organisms.
The choice of the disinfectant to be used depends on the particular situation. Some disinfectants have a wide spectrum (kill nearly all microorganisms). (In the UK there was a long running advert for Domestos bleach in which is was claimed that "Domestos kills all known germs Dead!") Others kill a smaller range of disease-causing organisms but are preferred for other properties (they may not be corrosive, and relatively non-toxic to humans).
A note on terminology
Disinfectants sterilize surfaces, medical equipment and other man-made objects. Antiseptics disinfect skin. Antibiotics either kill or interfere with the life cycle of bacteria inside the body. Substances which kill bacteria are said to have a bactericidal effect, while those which interfere with cell growth and reproduction are said to be bacteriostatic. Disinfectants and antiseptics are bactericidal (some disinfectants are becteriostatic at low concentrations): antibiotics can be either bactericidal or bacteriostatic.
Sanitation refers to killing 99+ % of germs in applicable situations. Sanitizers are compounds that sanitize.
- Chlorine - Used to disinfect swimming pools, and is added in small quantities to drinking water to reduce waterborne diseases.
- Chlorine dioxide - Used as an advanced disinfectant for drinking water to reduce waterborne diseases. In certain parts of the world, it has largely replaced chlorine because it forms fewer byproducts.
- Sodium chlorite , sodium chlorate , and potassium chlorate have little disinfection effect but are used as precursors for generating chlorine dioxide
- Alcohol - Usually ethanol or isopropanol - Wiped over benches and skin and allowed to evaporate for quick disinfection.
- Hydrogen peroxide- Used in hospitals to disinfect surfaces. It is sometimes mixed with colloidal silver. It is often preferred because it causes far fewer allergic reactions than alternative disinfectants. Also used in the food packaging industry to disinfect foil containers.A 3% solution is also used as an antiseptic. When hydrogen peroxide come into contact with the catalase enzyme in cells it is broken down into water and oxygen. It is the oxygen that kills bacteria. However, as recent studies have show hydrogen peroxide to be toxic to growing cells as well as bacteria, its use as an antiseptic is not longer recommended.
- Iodine - Usually dissolved in an organic solvent or as Lugol's iodine solution. It is used in the poultry industry. It is added to the birds' drinking water. Iodine is rapidly neutralised by the presence of organic material, so surfaces must be cleaned prior to disinfection. Tincture of iodine has also been used as an antiseptic for skin cuts and scrapes.
- Ozone - a gas that can be added to water for sanitation.
- Phenol and other phenolics - The active ingredient in most bottles of "household disinfectant". It is also to found in some mouthwashes and in disinfectant soap and handwashes. Phenol is probably the oldest disinfectant (used by Lister) and was called carbolic acid in the early days of antiseptics. Phenol is rather corrosive to the skin and sometimes toxic to sensitive people, so the somewhat less corrosive substitute phenolic
o-phenylphenol is often used as part of a disinfectant formula. Hexachlorophene is a phenolic which was once used as an germicidal additive to some household products but was banned due to suspected harmful effects.
- Potassium permanganate - used to disinfect aquariums.
- Quaternary ammonium salts (quats) are a large group of related compounds. Some have been used as a low level disinfectant. They are effective against bacteria, but not against spores or viruses. Quats are biocides which also kill algae and are used as an additive in large-scale industrial water systems to minimize undesired biological growth.
- Hypochlorites - Sodium hypochlorite, often in the form of common household bleach, is used in the home to disinfect drains, and toilets. A dilute form is used under the brand name milton to disinfect baby bottles. Other hypochlorites such as calcium hypochlorite are also used, especially as a swimming pool additive. Hypochlorite gives off free chlorine and it is the chlorine that is the true disinfectant. Hypobromite solutions are also sometimes used.
- Parvo-Virucide - a total biocidal agent (inactivates viruses, bacteria, spores, fungi) used mainly in animal contact areas such as kennels, catteries, veterinary surgeries etc. It can be used in clean and dirty areas contaminated with high levels of organic matter such as urine and faeces without loss of biocidal activity, whereas most disinfectants are severely challenged by organic matter and their effectiveness much reduced.
Parvo-Virucide was originally manufactured to specifically inactivate the virus CANINE PARVOVIRUS, which is a relatively new usually fatal disease affecting puppies. Parvo-virucide was evaluated by the Central Veterinary Laboratory of the U.K. Ministry of Agriculture (now DEFRA) who tested the disinfectant under severe conditions and discovered that "Parvo-Virucide disinfectant inactivated canine parvovirus at a dilution of 1:200 using the UK yeast method".
- Virkon - A wide-spectrum disinfectant used in labs. It kills bacteria, viruses, and fungi. It is used as a 1% solution in water, and keeps for 1 week once it is made up. It is expensive, but very effective, its pink colour fades as it is used up so it is possible to see at a glance if it is still fresh.
In addition to these methods ultraviolet light can be used for disinfecting water.
Relative effectiveness of disinfectants
One way to compare disinfectants is to compare how well they do against a known disinfectant and rate them accordingly. Phenol is the standard, and the corresponding rating system is called the "Phenol coefficient ". The disinfectant to be tested is compared with phenol on a standard microbe ( usually Salmonella typhi or Staphylococcus aureus). Disinfectants that are more effective than phenol have a coefficient > 1. Those that are less effective have a
coefficient < 1.
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