sterilization: Definition, Synonyms from Answers.com

Voluntary sterilization: the clinical possibilities

Sterilization refers to the procedure performed to stop fertility permanently, in either the male or the female. It is the most reliable type of contraception; consequently it is the most common form of contraception in couples over the age of 35 in the UK who have completed their families.

Fertilization occurs when a man's sperm reaches and joins an egg released from the ovary of a woman. The released egg is picked up by one of the two Fallopian tubes, which transports it to the uterus. Fertilization usually occurs in the Fallopian tube; if fertilized, the egg then implants into the wall of the uterus to establish a pregnancy. Sterilization interrupts this process permanently, either by preventing the release of sperm or by stopping fertilization by blocking the Fallopian tube.

Female sterilization may be achieved by a number of techniques, over a hundred having been described since Lungren performed what is thought to be the first tubal sterilization in 1880. Operations on women are commonly called tubal ligation, as the procedures aim to occlude or ‘ligate’ (tie) the Fallopian tubes. The majority of operations described involved cutting the Fallopian tube and oversewing the cut end so that it was hidden. These operations required an incision either on the abdominal wall or through the top of the vagina to gain access to the tubes. The early operations are thought to have had a failure rate of around 1 in 200.

Sterilization can be performed at the time of Caesarean section, which allows easy access to the Fallopian tubes. This is seldom done now, as the failure rate is higher than with other techniques applied after the womb has returned to its non-pregnant state.

Modern surgical techniques have been developed and now include laparoscopic (telescopic) procedures to occlude the Fallopian tubes. These may use diathermy (burning) of the tubes to damage them and seal them. Alternatively, one of two common types of device are used to block the tubes. The first type involves placing small rubber bands over a pinched-up portion of each tube. The second type involves placing a crushing clip across them.

If a sterilization procedure fails there is an increased risk of the pregnancy implanting outside the womb. This is known as an ectopic pregnancy, which most commonly occurs in the Fallopian tube and can sometimes, if the pregnancy grows, result in the tube rupturing and causing internal bleeding.

Male sterilization is achieved by the operation of vasectomy: cutting or ligating, on both sides, the vas deferens, which transports sperm from the testicles to the penis. The major advantage to a couple of a vasectomy is that it is a much simpler operation than female sterilization, due to the easy access to the vas within the scrotum. Vasectomy can easily be performed as an outpatient operation under local anaesthetic. After the operation the man must wait for around twelve weeks before the sperm count falls to sterile levels. Normally doctors ask for two sperm samples to check the operation has worked before alternative contraception can be abandoned.

Men and women requesting sterilization are usually seen at least twice by doctors prior to being sterilized, as the operation is designed to be permanent. Although reversal may be possible, sterilization should not be performed if either of the partners is unsure. It is also important that the doctor excludes any medical reason why the operation should not be performed. Women who have been taking the contraceptive pill because of painful periods are warned that these may recur if the Pill is stopped.

Current research is investigating reversible forms of contraception such as foam plugs for the Fallopian tubes or for the vas deferens.

Eugenical sterilization as a solution to social problems

Eugenicists in the US were particularly active in lobbying for the passage of state eugenical sterilization laws in the 1920s and 30s. Similar movements occurred in Canada, Britain, and Germany, but no laws were passed. Scandinavian countries — Denmark, Norway, Sweden, and Finland — passed laws in the 1930s. In the US, Harry H. Laughlin, of the Eugenics Record Office in Cold Spring Harbor, had drawn up a ‘Model Sterilization Law’ that served as a prototype for local use at home or abroad. Although the earliest such laws had been passed before World War I, the majority were established in the inter-war period. Eugenical sterilization laws were considered quite different from the punitive sterilization laws that existed in many states and countries in the nineteenth century. Although both types were compulsory, eugenical sterilization was aimed at prevention of future problems rather than serving as a punishment for past ones.

Eugenical sterilization was always couched in medical and scientific terms, and was justified as a means of saving the taxpayer money. In most countries it was aimed specifically at those individuals in mental or penal institutions who, from family pedigree analysis, were considered likely to give birth to socially defective offspring. In the US, sterilization could be ordered only after a patient had been examined by a eugenics committee, usually composed of a lawyer or family member representing the individual; a judge; and a doctor or other eugenic ‘expert’. Lobbyists often included members of the local state American Eugenics Society and a network of progressive enthusiasts who thought this was the way to cure social problems at their roots — it was the prime example of ‘social efficiency’.

Between 1907 (when the first law was passed, in Indiana) and 1941, over 38 000 eugenical sterilizations were performed in the US, with California in the lead. By the early 1960s the number had risen to over 60 000. The most famous sterilization case was that of Carrie Buck in Virginia, where a test case was set up to determine whether the law which had been passed was constitutional. Buck v. Bell was tried in 1925 in the Virginia Circuit Court. When the lower court ruled in favour of the law, an appeal was sent to the Supreme Court of the US, where Justice Oliver Wendell Holmes wrote the majority report. In upholding the lower court ruling Holmes made his oft-repeated assertion: ‘Three generations of imbeciles are enough.’ More to the legal point, Holmes claimed that ‘The principle that sustains compulsory vaccination is broad enough to cover cutting the Fallopian tubes.’

Laughlin's model law served also as a basis for a similar law in Germany, passed by the National Socialist government in 1933. For this effort, as well as for his enthusiastic support of Nazi eugenics programmes, Laughlin was awarded an honorary doctorate from Heidelberg University in 1936 — three years after the Nazis had come to power, and two years after their sweeping sterilization act had gone into effect.

While it would not be historically accurate to claim that state sterilization laws were repealed en masse after World War II and the disclosure of Nazi eugenic excesses, the general trend in 1945-55 was a decline in their application, increasing challenges to their constitutionality, and repeal of laws in some states — but it was not, for example, until 1977 that New Jersey outlawed the compulsory sterilization of the ‘developmentally disabled’. More telling, perhaps, than legislative changes, is the history of the actual application of sterilization laws in the post-war era — where they were not repealed, there was a dramatic decrease in the number of sterilizations reported. This decline was partly due to changing attitudes about the actual genetic effectiveness of sterilization — the acceptance that feeblemindedness might not have such a hereditary basis as had been supposed.

Influenced by their proximity to the Nazi experience, the Scandinavian countries repealed their laws after World War II. However, related issues of sterilization and control of reproduction in other guises continued, and gained considerable momentum after 1950, with respect to world population growth. Funded by the Rockefeller Foundation, pilot population control programmes were initiated in India, Puerto Rico, and other Third World countries; by 1960 these had been taken over and expanded by the US government funded bodies. Some post-war proponents of population control had been involved in eugenics in the 30s and 40s, and they used comparable arguments to those advanced earlier for eugenical sterilization — the differential birthrate of non-Western, non-European/American peoples. Now on a global scale, population control was, in the words of historian and critic Allan Chase ‘aimed at the gonads of the poor’.

— Linda Cardozo, Philip Toozs-Hobson, Garland E. Allen

Bibliography

Allen, G. E. 1986. The eugenics record office at Cold Spring Harbor: an essay in institutional history. Osiris 2 (2nd Series), 224-64.
Chase, A. 1977. The legacy of Malthus. Alfred A. Knopf, New York.
Reilly, P. 1992. The surgical solution: a history of involuntary sterilization in the United States. Johns Hopkins University Press, Baltimore

See also castration; eugenics; vasectomy.

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Sterilization (or sterilisation, see spelling differences) refers to any process that effectively kills or eliminates transmissible agents (such as fungi, bacteria, viruses, spore forms, etc.) from a surface, equipment, article of food or medication, or biological culture medium.^[1]^[2] Sterilization does not, however, remove prions. Sterilization can be achieved through application of heat, chemicals, irradiation, high pressure or filtration.

Applications

Foods

The first application of sterilization was thorough cooking to affect the partial heat sterilization of foods and water. Cultures that practice heat sterilization of food and water have longer life expectancy and lower rates of disability. Canning of foods by heat sterilization was an extension of the same principle. Ingestion of contaminated food and water remains a leading cause of illness and death in the developing world, particularly for children.

Medicine and surgery

In general, surgical instruments and medications that enter an already sterile part of the body (such as the blood, or beneath the skin) must have a high sterility assurance level. Examples of such instruments include scalpels, hypodermic needles and artificial pacemakers. This is also essential in the manufacture of parenteral pharmaceuticals.

Heat sterilization of medical instruments is known to have been used in Ancient Rome, but it mostly disappeared throughout the Middle Ages resulting in significant increases in disability and death following surgical procedures.

Preparation of injectable medications and intravenous solutions for fluid replacement therapy requires not only a high sterility assurance level, but well-designed containers to prevent entry of adventitious agents after initial sterilization.

Compare sterilisation to sanitization which reduces the number of viable organisms to an acceptable level. An example of the latter is Pasteurization.

Heat sterilization

Steam sterilization utensils

Front-loading autoclaves

A widely-used method for heat sterilization is the autoclave, sometimes called a converter. Autoclaves commonly use steam heated to 121–134 °C (250–273 °F). To achieve sterility, a holding time of at least 15 minutes at 121 °C (250 °F) or 3 minutes at 134 °C (273 °F) is required. Additional sterilizing time is usually required for liquids and instruments packed in layers of cloth, as they may take longer to reach the required temperature (unnecessary in machines that grind the contents prior to sterilization). Following sterilization, liquids in a pressurized autoclave must be cooled slowly to avoid boiling over when the pressure is released. Modern converters operate around this problem by gradually depressing the sterilization chamber and allowing liquids to evaporate under a negative pressure, while cooling the contents.

Proper autoclave treatment will inactivate all fungi, bacteria, viruses and also bacterial spores, which can be quite resistant. It will not necessarily eliminate all prions.

For prion elimination, various recommendations state 121–132 °C (250–270 °F) for 60 minutes or 134 °C (273 °F) for at least 18 minutes. The prion that causes the disease scrapie (strain 263K) is inactivated relatively quickly by such sterilization procedures; however, other strains of scrapie, as well as strains of CJD and BSE are more resistant. Using mice as test animals, one experiment showed that heating BSE positive brain tissue at 134–138 °C (273–280 °F) for 18 minutes resulted in only a 2.5 log decrease in prion infectivity. (The initial BSE concentration in the tissue was relatively low). For a significant margin of safety, cleaning should reduce infectivity by 4 logs, and the sterilization method should reduce it a further 5 logs.

To ensure the autoclaving process was able to cause sterilization, most autoclaves have meters and charts that record or display pertinent information such as temperature and pressure as a function of time. Indicator tape is often placed on packages of products prior to autoclaving. A chemical in the tape will change color when the appropriate conditions have been met. Some types of packaging have built-in indicators on them.

Biological indicators ("bioindicators") can also be used to independently confirm autoclave performance. Simple bioindicator devices are commercially available based on microbial spores. Most contain spores of the heat resistant microbe Geobacillus stearothermophilus (formerly Bacillus stearothermophilus), among the toughest organisms for an autoclave to destroy. Typically these devices have a self-contained liquid growth medium and a growth indicator. After autoclaving an internal glass ampule is shattered, releasing the spores into the growth medium. The vial is then incubated (typically at 56 °C (133 °F)) for 24 hours. If the autoclave destroyed the spores, the medium will remain its original color. If autoclaving was unsuccessful the B. sterothermophilus will metabolize during incubation, causing a color change during the incubation.

For effective sterilization, steam needs to penetrate the autoclave load uniformly, so an autoclave must not be overcrowded, and the lids of bottles and containers must be left ajar. Alternatively steam penetration can be achieved by shredding the waste in some Autoclave models that also render the end product unrecognizable. During the initial heating of the chamber, residual air must be removed. Indicators should be placed in the most difficult places for the steam to reach to ensure that steam actually penetrates there.

For autoclaving, as for all disinfection or sterilization methods, cleaning is critical. Extraneous biological matter or grime may shield organisms from the property intended to kill them, whether it physical or chemical. Cleaning can also remove a large number of organisms. Proper cleaning can be achieved by physical scrubbing. This should be done with detergent and warm water to get the best results. Cleaning instruments or utensils with organic matter, cool water must be used because warm or hot water may cause organic debris to coagulate. Treatment with ultrasound or pulsed air can also be used to remove debris.

Food

Food utensils

Dishwashers often only use hot tap water or heat the water to between 49 and 60 °C (120 and 140 °F), and thus provide temperatures that could promote bacterial growth. That is to say, they do not effectively sterilize utensils. Some dishwashers do actually heat water up to 74 °C (165 °F) or higher; those often are specifically described as having sterilization modes of some sort, but this is not a substitute for autoclaving.

Note that dishwashers remove food traces from the utensils by a combination of mechanical action (the action of water hitting the plates and cutlery) and the action of detergents and enzymes on fats and proteins. This removal of food particles thus removes one of the factors required for bacterial growth (food), it clearly explains why items with cracks and crevices should either be washed by hand or disposed of: if the water cannot get to the area needing cleaning, the warm, moist, dark conditions in the dishwasher can actually promote bacterial growth.

Bathing

Bathing and washing are not hot enough to sterilize bacteria without scalding the skin. Most hot tap water is between 43 and 49 °C (109 and 120 °F), though some people set theirs as high as 55 °C (131 °F). Humans begin to find water painful at 41 to 42 °C (106 to 108 °F), which to many bacteria is just starting to get warm enough for them to grow quickly; they will grow faster, rather than be killed at temperatures up to 55 °C (131 °F) or more.

Other methods

Other heat methods include flaming, incineration, boiling, tindalization, and using dry heat.

Flaming is done to loops and straight-wires in microbiology labs. Leaving the loop in the flame of a Bunsen burner or alcohol lamp until it glows red ensures that any infectious agent gets inactivated. This is commonly used for small metal or glass objects, but not for large objects (see Incineration below). However, during the initial heating infectious material may be "sprayed" from the wire surface before it is killed, contaminating nearby surfaces and objects. Therefore, special heaters have been developed that surround the inoculating loop with a heated cage, ensuring that such sprayed material does not further contaminate the area. Another problem is that gas flames may leave residues on the object, e.g. carbon, if the object is not heated enough.

A variation on flaming is to dip the object in 70% ethanol (or a higher concentration) and merely touch the object briefly to the Bunsen burner flame, but not hold it in the gas flame. The ethanol will ignite and burn off in a few seconds. 70% ethanol kills many, but not all, bacteria and viruses, and has the advantage that it leaves less residue than a gas flame. This method works well for the glass "hockey stick"-shaped bacteria spreaders.

Incineration will also burn any organism to ash. It is used to sanitize medical and other biohazardous waste before it is discarded with non-hazardous waste.

Boiling in water for fifteen minutes will kill most vegetative bacteria and inactivate viruses, but boiling is ineffective against prions and many bacterial and fungal spores; therefore boiling is unsuitable for sterilization. However, since boiling does kill most vegetative microbes and viruses, it is useful for reducing viable levels if no better method is available. Boiling is a simple process, and is an option available to most people, requiring only water, enough heat, and a container that can withstand the heat; however, boiling can be hazardous and cumbersome.

Tindalization^[3] /Tyndallization^[4] named after John Tyndall is a lengthy process designed to reduce the level of activity of sporulating bacteria that are left by a simple boiling water method. The process involves boiling for a period (typically 20 minutes) at atmospheric pressure, cooling, incubating for a day, boiling, cooling, incubating for a day, boiling, cooling, incubating for a day, and finally boiling again. The three incubation periods are to allow heat-resistant spores surviving the previous boiling period to germinate to form the heat-sensitive vegetative (growing) stage, which can be killed by the next boiling step. This is effective because many spores are stimulated to grow by the heat shock. The procedure only works for media that can support bacterial growth - it will not sterilize plain water. Tindalization/tyndallization is ineffective against prions.

Dry heat can be used to sterilize items, but as the heat takes much longer to be transferred to the organism, both the time and the temperature must usually be increased, unless forced ventilation of the hot air is used. The standard setting for a hot air oven is at least two hours at 160 °C (320 °F). A rapid method heats air to 190 °C (374 °F) for 6 minutes for unwrapped objects and 12 minutes for wrapped objects.^[5]^[6] Dry heat has the advantage that it can be used on powders and other heat-stable items that are adversely affected by steam (for instance, it does not cause rusting of steel objects).

Prions can be inactivated by immersion in sodium hydroxide (NaOH 0.09N) for two hours plus one hour autoclaving (121 °C/250 °F). Several investigators have shown complete (>7.4 logs) inactivation with this combined treatment. However, sodium hydroxide may corrode surgical instruments, especially at the elevated temperatures of the autoclave.

Glass bead sterilizer, once a common sterilization method employed in dental offices as well as biologic laboratories,^[7] is not approved by the U.S. Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) to be used as inter-patients sterilizer since 1997.^[8] Still it is popular in European as well as Israeli dental practice although there are no current evidence-based guidelines for using this sterilizer.^[7]

Chemical sterilization

Chemicals are also used for sterilization. Although heating provides the most reliable way to rid objects of all transmissible agents, it is not always appropriate, because it will damage heat-sensitive materials such as biological materials, fiber optics, electronics, and many plastics. Low temperature gas sterilizers function by exposing the articles to be sterilized to high concentrations (typically 5 - 10% v/v) of very reactive gases (alkylating agents such as ethylene oxide, and oxidizing agents such as hydrogen peroxide and ozone). Liquid sterilants and high disinfectants typically include oxidizing agents such as hydrogen peroxide and peracetic acid and aldehydes such as glutaraldehyde and more recently o-phthalaldehyde. While the use of gas and liquid chemical sterilants/high level disinfectants avoids the problem of heat damage, users must ensure that article to be sterilized is chemically compatible with the sterilant being used. The manufacturer of the article can provide specific information regarding compatible sterilants. In addition, the use of chemical sterilants poses new challenges for workplace safety. The chemicals used as sterilants are designed to destroy a wide range of pathogens and typically the same properties that make them good sterilants makes them harmful to humans. Employers have a duty to ensure a safe work environment (Occupational Safety and Health Act of 1970, section 5 for United States) and work practices, engineering controls and monitoring should be employed appropriately.

Ethylene Oxide

Ethylene oxide (EO or EtO) gas is commonly used to sterilize objects sensitive to temperatures greater than 60 °C such as plastics, optics and electrics. Ethylene oxide treatment is generally carried out between 30 °C and 60 °C with relative humidity above 30% and a gas concentration between 200 and 800 mg/L for at least three hours. Ethylene oxide penetrates well, moving through paper, cloth, and some plastic films and is highly effective. Ethylene oxide sterilizers are used to process sensitive instruments which cannot be adequately sterilized by other methods. EtO can kill all known viruses, bacteria and fungi, including bacterial spores and is satisfactory for most medical materials, even with repeated use. However, it is highly flammable, and requires a longer time to sterilize than any heat treatment. The process also requires a period of post-sterilization aeration to remove toxic residues. Ethylene oxide is the most common sterilization method, used for over 70% of total sterilizations, and for 50% of all disposable medical devices.

The two most important ethylene oxide sterilization methods are: (1) the gas chamber method and (2) the micro-dose method. To benefit from economies of scale, EtO has traditionally been delivered by flooding a large chamber with a combination of EtO and other gases used as dilutants (usually CFCs or carbon dioxide). This method has drawbacks inherent to the use of large amounts of sterilant being released into a large space, including air contamination produced by CFCs and/or large amounts of EtO residuals, flammability and storage issues calling for special handling and storage, operator exposure risk and training costs

Ethylene oxide is still widely used by medical device manufacturers for larger scale sterilization (e.g. by the pallet), but while still used, EtO is becoming less popular in hospitals. Since Eto is explosive from its lower explosive limit of 3% all the way to 100%, EtO was traditionally supplied with an inert carrier gas such as a CFC or halogenated hydrocarbon. The use of CFCs as the carrier gas was banned because of concerns of ozone depletion [2] and halogenated hydrocarbons are being replaced by so-called 100% EtO systems because of the much greater cost of the blends. In hospitals, most EtO sterilizers use single use cartridges (e.g. 3M's Steri-Vac line[3], or Steris Corporation's Stericert sterilizers[4]) because of the convenience and ease of use compared to the former plumbed gas cylinders of EtO blends. Another 100% method is the so-called micro-dose sterilization method, developed in the late 1950s, using a specially designed bag to eliminate the need to flood a larger chamber with EtO. This method is also known as gas diffusion sterilization, or bag sterilization. This method minimizes the use of gas.^[9]

Another reason for the decrease in use of EtO are the well known health effects. In addition to being a primary irritant, EtO is now classified by the IARC as a known human carcinogen.^[10] The US OSHA has set the permissible exposure limit (PEL) at 1 ppm calculated as an eight hour time weighted average (TWA) [29 CFR 1910.1047] and 5 ppm as a 15 minute TWA. The NIOSH Immediately dangerous to life and health limit for EtO is 800 ppm.^[11] The odor threshold is around 500 ppm^[12] and so EtO is imperceptible until concentrations well above the OSHA PEL. Therefore, OSHA recommends that some kind of continuous gas monitoring system be used to protect workers using EtO for sterilization.^[13] While the hazards of EtO are generally well known, it should be noted that all chemical sterilants are designed to kill a broad spectrum of organisms, by exposing them to high concentrations of reactive chemicals. Therefore, it is no surprise that all the common chemical gas sterilants are toxic and adequate protective measures must be taken to protect workers using these materials.

Spore testing

Bacillus subtilis, a very resistant organism, is used as a rapid biological indicator for EO sterilizers. If sterilization fails, incubation at 37 °C causes a fluorescent change within four hours, which is read by an auto-reader. After 96 hours, a visible color change occurs. Fluorescence is emitted if a particular (EO resistant) enzyme is present, which means that spores are still active. The color change indicates a pH shift due to bacterial metabolism. The rapid results mean that the objects treated can be quarantined until the test results are available.

Ozone

Ozone is used in industrial settings to sterilize water and air, as well as a disinfectant for surfaces. It has the benefit of being able to oxidize most organic matter. On the other hand, it is a toxic and unstable gas that must be produced on-site, so it is not practical to use in many settings.

Ozone offers many advantages as a sterilant gas; ozone is a very efficient sterilant because of its strong oxidizing properties (E = 2.076 vs SHE, CRC Handbook of Chemistry and Physics, 76th Ed, 1995-1996) capable of destroying a wide range of pathogens, including prions^[14] without the need for handling hazardous chemicals since the ozone is generated within the sterilizer from medical grade oxygen. In 2005 a Canadian company called TSO3 Inc^[15] received FDA clearance to sell an ozone sterilizer for use in healthcare. The high reactivity of ozone means that waste ozone can be destroyed by passing over a simple catalyst that reverts it back to oxygen and also means that the cycle time is relatively short (about 4.5 hours for TSO3's model 125L). The downside of using ozone is that the gas is very reactive and very hazardous. The NIOSH immediately dangerous to life and health limit for ozone is 5 ppm, much 160 times smaller than the 800 ppm IDLH for ethylene oxide.Documentation for Immediately Dangerous to Life or Health Concentrations (IDLH): NIOSH Chemical Listing and Documentation of Revised IDLH Values (as of 3/1/95) and OSHA has set the PEL for ozone at 0.1 ppm calculated as an eight hour time weighted average (29 CFR 1910.1000, Table Z-1). The Canadian Center for Occupation Health and Safety provides an excellent summary of the health effects of exposure to ozone.^[16] The sterilant gas manufacturers include many safety features in their products but prudent practice is to provide continuous monitoring to below the OSHA PEL to provide a rapid warning in the even of a leak and monitors for determining workplace exposure to ozone are commercially available.

Bleach

Chlorine bleach is another accepted liquid sterilizing agent. Household bleach consists of 5.25% sodium hypochlorite. It is usually diluted to 1/10 immediately before use; however to kill Mycobacterium tuberculosis it should be diluted only 1/5, and 1/2.5 (1 part bleach and 1.5 parts water) to inactivate prions. The dilution factor must take into account the volume of any liquid waste that it is being used to sterilize.^[17] Bleach will kill many organisms immediately, but for full sterilization it should be allowed to react for 20 minutes. Bleach will kill many, but not all spores. It is highly corrosive and may corrode even stainless steel surgical instruments.

Bleach decomposes over time when exposed to air, so fresh solutions should be made daily.^[18]

Glutaraldehyde and Formaldehyde

Glutaraldehyde and formaldehyde solutions (also used as fixatives) are accepted liquid sterilizing agents, provided that the immersion time is sufficiently long. To kill all spores in a clear liquid can take up to 12 hours with glutaraldehyde and even longer with formaldehyde. The presence of solid particles may lengthen the required period or render the treatment ineffective. Sterilization of blocks of tissue can take much longer, due to the time required for the fixative to penetrate. Glutaraldehyde and formaldehyde are volatile, and toxic by both skin contact and inhalation. Glutaraldehyde has a short shelf life (<2 weeks), and is expensive. Formaldehyde is less expensive and has a much longer shelf life if some methanol is added to inhibit polymerization to paraformaldehyde, but is much more volatile. Formaldehyde is also used as a gaseous sterilizing agent; in this case, it is prepared on-site by depolymerization of solid paraformaldehyde. Many vaccines, such as the original Salk polio vaccine, are sterilized with formaldehyde.

Phthalaldehyde

Ortho-phthalaldehyde (OPA) is a chemical sterilizing agent that received Food and Drug Administration (FDA) clearance in late 1999. Typically used in a 0.55% solution, OPA shows better myco-bactericidal activity than glutaraldehyde. It also is effective against glutaraldehyde-resistant spores. OPA has superior stability, is less volatile, and does not irritate skin or eyes, and it acts more quickly than glutaraldehyde. On the other hand, it is more expensive, and will stain proteins (including skin) gray in color.

Hydrogen Peroxide

Hydrogen peroxide is another chemical sterilizing agent. It is relatively non-toxic when diluted to low concentrations, such as the familiar 3% retail solutions although hydrogen peroxide is a dangerous oxidizer at high concentrations (> 10% w/w). Hydrogen peroxide is strong oxidant and these oxidizing properties allow it to destroy a wide range of pathogens and it is used to sterilize heat or temperature sensitive articles such as rigid endoscopes. In medical sterilization hydrogen peroxide is used at higher concentrations, ranging from around 35% up to 90%. The biggest advantage of hydrogen peroxide as a sterilant is the short cycle time. Whereas the cycle time for ethylene oxide (discussed above) may be 10 to 15 hours, the use of very high concentrations of hydrogen peroxide allows much shorter cycle times. Some hydrogen peroxide modern sterilizers, such as the Sterrad NX have a cycle time as short as 28 minutes.

Hydrogen peroxide sterilizers have their drawbacks. Since hydrogen peroxide is a strong oxidant, there are material compatibility issues and users should consult the manufacturer of the article to be sterilized to ensure that it is compatible with this method of sterilization. Paper products cannot be sterilized in the Sterrad system because of a process called cellulostics, in which the hydrogen peroxide would be completely absorbed by the paper product. The penetrating ability of hydrogen peroxide to not as good as ethylene oxide and so there are limitations on the length and diameter of lumens that can be effectively sterilized and guidance is available from the sterilizer manufacturers.

While hydrogen peroxide offers significant advantages in terms of throughput, as with all sterilant gases, sterility is achieved through the use of high concentrations of reactive gases. Hydrogen peroxide is primary irritant and the contact of the liquid solution with skin will cause bleaching or ulceration depending on the concentration and contact time. The vapor is also hazardous with the target organs being the eyes and respiratory system. Even short term exposures can be hazardous and NIOSH has set the Immediately Dangerous to Life and Health Level (IDLH) at 75 ppm.^[11] less than one tenth the IDLH for ethylene oxide (800 ppm). Prolonged exposure to even low ppm concentrations can cause permanent lung damage and consequently OSHA has set the permissible exposure limit to 1.0 ppm, calculated as an 8 hour time weighted average (29 CFR 1910.1000 Table Z-1). Employers thus have a legal duty to ensure that their personnel are not exposed to concentrations exceeding this PEL. Even though the sterilizer manufacturers go to great lengths to make their products safe through careful design and incorporation of many safety features, workplace exposures of hydrogen peroxide from gas sterilizers are documented in the FDA MAUDE database.^[19] When using any type of gas sterilizer, prudent work practices will include good ventilation (10 air exchanges per hour), a continuous gas monitor for hydrogen peroxide as well as good work practices and training. Further information about the health effects of hydrogen peroxide and good work practices is available from OSHA^[20] and the ATSDR.^[21]

Hydrogen peroxide can also be mixed with formic acid as needed in the Endoclens device for sterilization of endoscopes. This device has two independent asynchronous bays, and cleans (in warm detergent with pulsed air), sterilizes and dries endoscopes automatically in 30 minutes. Studies with synthetic soil with bacterial spores showed the effectiveness of this device.

Dry sterilization process

Dry sterilization process (DSP) uses hydrogen peroxide at a concentration of 30-35% under low pressure conditions. This process achieves bacterial reduction of 10^-6...10^-8. The complete process cycle time is just 6 seconds, and the surface temperature is increased only 10-15 °C (18 to 27 °F). Originally designed for the sterilization of plastic bottles in the beverage industry, because of the high germ reduction and the slight temperature increase the dry sterilization process is also useful for medical and pharmaceutical applications.

Peracetic acid

Peracetic acid (0.2%) is used to sterilize instruments in the Steris system.

Prions

Prions are highly resistant to chemical sterilization. Treatment with aldehydes (e.g., formaldehyde) have actually been shown to increase prion resistance. Hydrogen peroxide (3%) for one hour was shown to be ineffective, providing less than 3 logs (10⁻³) reduction in contamination. Iodine, formaldehyde, glutaraldehyde and peracetic acid also fail this test (one hour treatment). Only chlorine, a phenolic compound, guanidinium thiocyanate, and sodium hydroxide (NaOH) reduce prion levels by more than 4 logs. Chlorine and NaOH are the most consistent agents for prions. Chlorine is too corrosive to use on certain objects. Sodium hydroxide has had many studies showing its effectiveness.

Silver

Silver ions and silver compounds show a toxic effect on some bacteria, viruses, algae and fungi, typical for heavy metals like lead or mercury, but without the high toxicity to humans that is normally associated with these other metals. Its germicidal effects kill many microbial organisms in vitro, but testing and standardization of silver products is yet difficult.^[22]

Hippocrates, the father of modern medicine, wrote that silver had beneficial healing and anti-disease properties^{[cite this quote]}, and the Phoenicians used to store water, wine, and vinegar in silver bottles to prevent spoiling. In the early 1900s people would put silver dollars in milk bottles to prolong the milk's freshness.^[23] The exact process of silver's germicidal effect is still not well understood. One of the explanations is the oligodynamic effect, which accounts for the effect on microorganisms but not on virii.

Silver compounds were used to prevent infection in World War I before the advent of antibiotics. Silver nitrate solution was a standard of care but was largely replaced by silver sulfadiazine cream (SSD Cream),^[24] which was generally the "standard of care" for the antibacterial and antibiotic treatment of serious burns until the late 1990s.^[25] Now, other options, such as silver-coated dressings (activated silver dressings), are used in addition to SSD cream. However, the evidence for the use of such silver-treated dressings is mixed and although the evidence on if they are effective is promising, it is marred by the poor quality of the trials used to assess these products.^[26] Consequently a major systematic review by the Cochrane Collaboration found insufficient evidence to recommend the use of silver-treated dressings to treat infected wounds.^[26]

The widespread use of silver went out of fashion with the development of antibiotics. However, recently there has been renewed interest in silver as a broad-spectrum antimicrobial. In particular, silver is being used with alginate, a naturally occurring biopolymer derived from seaweed, in a range of products designed to prevent infections as part of wound management procedures, particularly applicable to burn victims.^[27] In 2007, AGC Flat Glass Europe introduced the first antibacterial glass to fight hospital-caught infection: it is covered with a thin layer of silver.^[28] In addition, Samsung has introduced washing machines with a final rinse containing silver ions to provide several days of antibacterial protection in the clothes.^[29] Kohler has introduced a line of toilet seats that have silver ions embedded to kill germs. A company called Thomson Research Associates has begun treating products with Ultra Fresh, an anti-microbial technology involving "proprietary nano-technology to produce the ultra-fine silver particles essential to ease of application and long-term protection."^[30] The U.S. Food and Drug Administration (FDA) has recently approved an endotracheal breathing tube with a fine coat of silver for use in mechanical ventilation, after studies found it reduced the risk of ventilator-associated pneumonia.^[31]

It has long been known that antibacterial action of silver is enhanced by the presence of an electric field. Applying a few volts of electricity across silver electrodes drastically enhances the rate that bacteria in solution are killed. It was found recently that the antibacterial action of silver electrodes is greatly improved if the electrodes are covered with silver nanorods.^[32] Note that enhanced antibacterial properties of nanoparticles compared to bulk material is not limited to silver, but has also been demonstrated on other materials such as ZnO^[33]

Radiation Sterilization

Methods of sterilization exist using radiation such as electron beams, X-rays, gamma rays, or subatomic particles.^[34]

Gamma rays are very penetrating and are commonly used for sterilization of disposable medical equipment, such as syringes, needles, cannulas and IV sets. Gamma radiation requires bulky shielding for the safety of the operators; they also require storage of a radioisotope (usually Cobalt-60), which continuously emits gamma rays (it cannot be turned off, and therefore always presents a hazard in the area of the facility).
Electron beam processing is also commonly used for medical device sterilization. Electron beams use an on-off technology and provide a much higher dosing rate than gamma or x-rays. Due to the higher dose rate, less exposure time is needed and thereby any potential degradation to polymers is reduced. A limitation is that electron beams are less penetrating than either gamma or x-rays.
X-rays, High-energy X-rays (bremsstrahlung) are a form of ionizing energy allowing to irradiate large packages and pallet loads of medical devices. Their penetration is sufficient to treat multiple pallet loads of low-density packages with very good dose uniformity ratios. X-ray sterilization is an electricity based process not requiring chemical nor radio-active material. High energy and high power X-rays are generated by an X-ray machine that can be turned off for servicing and when not in use.
Ultraviolet light irradiation (UV, from a germicidal lamp) is useful only for sterilization of surfaces and some transparent objects. Many objects that are transparent to visible light absorb UV. UV irradiation is routinely used to sterilize the interiors of biological safety cabinets between uses, but is ineffective in shaded areas, including areas under dirt (which may become polymerized after prolonged irradiation, so that it is very difficult to remove). It also damages many plastics, such as polystyrene foam.
Further information: Ultraviolet Germicidal Irradiation
Subatomic particles may be more or less penetrating, and may be generated by a radioisotope or a device, depending upon the type of particle.

Irradiation with X-rays or gamma rays does not make materials radioactive. Irradiation with particles may make materials radioactive, depending upon the type of particles and their energy, and the type of target material: neutrons and very high-energy particles can make materials radioactive, but have good penetration, whereas lower energy particles (other than neutrons) cannot make materials radioactive, but have poorer penetration.

Irradiation is used by the United States Postal Service to sterilize mail in the Washington, DC area. Some foods (e.g. spices, ground meats) are irradiated for sterilization (see food irradiation).

Sterile filtration

Clear liquids that would be damaged by heat, irradiation or chemical sterilization can be sterilized by mechanical filtration. This method is commonly used for sensitive pharmaceuticals and protein solutions in biological research. A filter with pore size 0.2 µm will effectively remove bacteria. If viruses must also be removed, a much smaller pore size around 20 nm is needed. Solutions filter slowly through membranes with smaller pore diameters. Prions are not removed by filtration. The filtration equipment and the filters themselves may be purchased as pre-sterilized disposable units in sealed packaging, or must be sterilized by the user, generally by autoclaving at a temperature that does not damage the fragile filter membranes. To ensure sterility, the filtration system must be tested to ensure that the membranes have not been punctured prior to or during use.

To ensure the best results, pharmaceutical sterile filtration is performed in a room with highly filtered air (HEPA filtration) or in a laminar flow cabinet or "flowbox", a device which produces a laminar stream of HEPA filtered air.

References

Notes

^ WHO Glossary
^ UCLA Dept. Epidemiology: Definitions
^ Mesquita, J. A. M.; Teixeira, M.A. and Brandao, S. C. C. (1998). "Tindalization of goats' milk in glass bottles". J. Anim. Sci. /J. Dairy Sci. Vol. 76, Suppl. 1 / Vol. 81, Suppl. 1/: 21. http://www.asas.org/jas/98meet/98df.pdf. Retrieved 2007-03-06.
^ Thiel, Theresa (1999). "http://www.umsl.edu/~microbes/pdf/tyndallization.pdf" (pdf). Science in the Real World. http://www.umsl.edu/~microbes/pdf/tyndallization.pdf. Retrieved 2007-03-06.
^ [1]
^ Dental Volume 1 - Dentist training manual for military dentists
^ ^a ^b Zadik Y, Peretz A (Apr 2008). "The effectiveness of glass bead sterilizer in the dental practice". J Isr Dent Assoc 25 (2): 36–9. PMID 18780544.
^ http://www.CDC.gov/OralHealth/InfectionControl/faq/bead.htm 2008-09-11
^ Micro-dose sterilization method
^ http://monographs.iarc.fr/ENG/Monographs/vol60/volume60.pdf
^ ^a ^b http://www.cdc.gov/niosh/idlh/intridl4.html
^ http://www.atsdr.cdc.gov/MHMI/mmg137.html
^ http://www.osha.gov/SLTC/etools/hospital/central/central.html
^ http://www.tso3.com/en/news-events/news-tso3-gets-promising-223.php
^ http://www.tso3.com
^ http://www.ccohs.ca/oshanswers/chemicals/chem_profiles/ozone/basic_ozo.html
^ Beth Israel Deaconess Medical Center Biosafety Manual (2004 edition)
^ Office of Health and Safety (2007). Biosafety in Microbiological and Biomedical Laboratories (BMBL) (5th ed.). Centers for Disease Control and Prevention. http://www.cdc.gov/OD/ohs/biosfty/bmbl5/bmbl5toc.htm.
^ http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfMAUDE/search.CFM
^ http://www.osha.gov/SLTC/healthguidelines/hydrogenperoxide/recognition.html
^ http://www.atsdr.cdc.gov/MHMI/mmg174.html
^ Chopra I (April 2007). "The increasing use of silver-based products as antimicrobial agents: a useful development or a cause for concern?". The Journal of antimicrobial chemotherapy 59 (4): 587–90. doi:10.1093/jac/dkm006. PMID 17307768.
^ "Antibacterial effects of silver". http://www.saltlakemetals.com/Silver_Antibacterial.htm.
^ Chang TW, Weinstein L (December 1975). "Prevention of herpes keratoconjunctivitis in rabbits by silver sulfadiazine". Antimicrob. Agents Chemother. 8 (6): 677–8. PMID 1211919. PMC 429446. http://aac.asm.org/cgi/pmidlookup?view=long&pmid=1211919.
^ Atiyeh BS, Costagliola M, Hayek SN, Dibo SA (March 2007). "Effect of silver on burn wound infection control and healing: review of the literature". Burns : journal of the International Society for Burn Injuries 33 (2): 139–48. doi:10.1016/j.burns.2006.06.010. PMID 17137719.
^ ^a ^b Lo SF, Hayter M, Chang CJ, Hu WY, Lee LL (August 2008). "A systematic review of silver-releasing dressings in the management of infected chronic wounds". Journal of clinical nursing 17 (15): 1973–85. doi:10.1111/j.1365-2702.2007.02264.x. PMID 18705778.
^ Hermans MH (December 2006). "Silver-containing dressings and the need for evidence". The American journal of nursing 106 (12): 60–8; quiz 68–9. PMID 17133010.
^ "AGC Flat Glass Europe launches world’s first antibacterial glass". 2007-09-04. http://www.agc-flatglass.eu/AGC+Flat+Glass+Europe/English/Homepage/News/Press+room/Press-Detail-Page/page.aspx/979?pressitemid=1031.
^ "Samsung laundry featuring SilverCare Technology". Samsung. http://web.archive.org/web/20060531115914/http://www.samsung.com/PressCenter/PressRelease/PressRelease.asp?seq=20060213_0000233684. Retrieved 2007-08-06.
^ ""Ultra-Fresh technology is based on the power of silver to fight bacteria"". Express Textile. http://www.expresstextile.com/20050731/perspectives01.shtml.
^ "FDA Clears Silver-Coated Breathing Tube For Marketing". 2007-11-08. http://www.fda.gov/bbs/topics/NEWS/2007/NEW01741.html. Retrieved 2007-11-11.
^ O. Akhavan and E. Ghaderi "Enhancement of antibacterial properties of Ag nanorods by electric field" Sci. Technol. Adv. Mater. 10 (2009) 015003 free download
^ N. Padmavathy et al. "Enhanced bioactivity of ZnO nanoparticles—an antimicrobial study" Sci. Technol. Adv. Mater. 9 (2007) 035004 free download
^ Trends in Radiation Sterilization of Health Care Products, IAEA, Vienna,24 September 2008

General references

Ninemeier J. Central Service Technical Manual (6th ed.). International Association of Healthcare Central Service Materiel Management. http://cbspd.net/cstechpub.htm.
Control of microbes
Raju GK, Cooney CL (1993). "Media and air sterilization". in Stephanopoulos G. Biotechnology, 2E, Vol. 3, Bioprocessing. Weinheim: Wiley-VCH. pp. 157–84. ISBN 3-527-28313-7.

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