Community medicine chapters
Quarantine, isolation, disinfection and sterilisation
The difference between isolation and quarantine, the definitions of sterilisation, disinfection and antisepsis, the physical and chemical methods with their uses, and the classic exam facts on autoclaving, hot air oven and chlorination.
- Isolation separates infected people during their infectious period; quarantine restricts healthy contacts for the longest incubation period.
- Sterilisation kills all microbes including spores; disinfection kills most pathogens but not necessarily spores; antiseptics are disinfectants safe for living tissue.
- Autoclave: moist heat at 121°C, 15 lb/in² (psi), 15–20 minutes; hot air oven: dry heat at 160°C for 2 hours; chlorination needs 0.5 mg/L free chlorine after 1 hour contact.
On this page
Overview
Two groups of control measures stop infection spreading:
- Separating people: isolation and quarantine.
- Destroying microbes on objects, surfaces, skin and water: disinfection and sterilisation.
Isolation and quarantine
| Feature | Isolation | Quarantine |
|---|---|---|
| Applies to | Infected (sick) people or carriers | Healthy contacts who have been exposed |
| Duration | The period of communicability | The longest incubation period of the disease, counted from the last exposure |
| Aim | Stop spread from known cases | Stop spread if a contact turns out to be infected |
- Absolute (complete) quarantine: full restriction of movement.
- Modified quarantine: selective restriction (for example excluding children from school).
- Segregation: separating some people (for example susceptible children) from others.
- Surveillance: close supervision of contacts without restricting movement, to detect disease early.
Isolation is for the sick; quarantine is for healthy contacts, for the longest incubation period.
International quarantine today is governed by the International Health Regulations (2005); for example, travellers from yellow fever areas may need a vaccination certificate on entering Pakistan.
Definitions
- Sterilisation: killing or removing all forms of microbial life, including bacterial spores.
- Disinfection: killing or removing pathogens on objects, but not necessarily spores.
- Antiseptic: a disinfectant that can be safely applied to living tissue (skin, wounds).
- Asepsis: preventing contamination of sterile sites.
- Decontamination (cleaning): removing dirt and organic matter; an essential first step, because organic matter protects microbes.
Types of disinfection
| Type | When |
|---|---|
| Concurrent | During the illness: disinfecting the patient's discharges, linen and articles immediately |
| Terminal | After the patient has left (recovered, moved or died): cleaning the room and contents |
| Precurrent (prophylactic) | Before any infection: for example chlorinating water, pasteurising milk, hand washing |
Terminal disinfection is now mostly thorough cleaning and airing; concurrent disinfection is the more important.
Physical methods
| Method | Conditions | Uses |
|---|---|---|
| Autoclave (moist heat under pressure) | 121°C, 15 lb/in², 15–20 minutes | Most surgical instruments, linen, dressings, culture media; the most reliable method. Kills spores |
| Hot air oven (dry heat) | 160°C for 2 hours (or 170°C for 1 hour; 180°C for 30 minutes) | Glassware, glass syringes, powders, oils, sharp instruments |
| Incineration | High-temperature burning | Infected waste, dressings, pathological waste, animal carcasses |
| Flaming / red heat | Direct flame | Inoculating loops, needle tips in the lab |
| Boiling | 100°C for 10–30 minutes | Disinfects but does not reliably kill spores |
| Pasteurisation | Holder: 63–66°C for 30 minutes; flash (HTST): 72°C for 15–20 seconds | Milk |
| Radiation | Gamma rays (ionising) | Single-use plastics, syringes, catheters (industrial) |
| Ultraviolet light | Surface and air | Operating theatres, laboratories |
| Filtration | Membrane filters | Heat-sensitive fluids (sera, some drugs) |
Moist heat is more effective than dry heat because it coagulates proteins at lower temperatures and penetrates better. That is why the autoclave works at 121°C while the hot air oven needs 160°C.
The effectiveness of an autoclave is checked with spores of Geobacillus stearothermophilus (biological indicators) and autoclave tape (chemical indicators).
Chemical methods
| Agent | Uses and notes |
|---|---|
| Chlorine compounds (bleaching powder, sodium hypochlorite) | Water, surfaces, blood spills (1% hypochlorite), faeces; inactivated by organic matter |
| Alcohols (70% ethanol or isopropanol) | Skin antisepsis, hand rub, thermometers; do not kill spores |
| Glutaraldehyde (2%) | High-level disinfectant for heat-sensitive instruments (endoscopes); sterilant with long contact |
| Formaldehyde | Fumigation of rooms; preserving specimens |
| Ethylene oxide (gas) | Sterilises heat-sensitive equipment (plastics, heart-lung machines) |
| Phenols (cresol, Lysol) | Faeces, floors; toxic to skin |
| Iodine, povidone-iodine | Skin antisepsis before surgery |
| Chlorhexidine | Skin and hand antisepsis, mouthwash, umbilical cord care |
| Hydrogen peroxide | Wounds; plasma sterilisation |
Spaulding classification of items
| Item | Contact | Requirement |
|---|---|---|
| Critical | Enters sterile tissue or blood (surgical instruments, needles) | Sterilisation |
| Semi-critical | Contacts mucous membranes (endoscopes) | High-level disinfection |
| Non-critical | Contacts intact skin (BP cuff, stethoscope) | Low-level disinfection or cleaning |
Chlorination of water
How it works: chlorine forms hypochlorous acid, which kills bacteria and viruses.
Principles (WHO conditions):
- Water should be clear (low turbidity).
- Chlorine demand must be met first.
- Contact period: at least 1 hour.
- Free residual chlorine of at least 0.5 mg/L at the end of the contact period.
- pH below about 8.
Breakpoint chlorination: adding chlorine until all chlorine demand is met; chlorine added beyond this appears as free residual chlorine.
Horrocks' apparatus is used to find the dose of bleaching powder needed to disinfect water in wells.
Chlorine at usual doses does not reliably kill cysts (amoeba, Giardia), Cryptosporidium, or hepatitis A virus without higher doses or longer contact.
Expect: isolation vs quarantine and its duration, sterilisation vs disinfection vs antisepsis, concurrent vs terminal disinfection, autoclave and hot air oven settings and uses, incineration for infected waste, which agents kill spores, the Spaulding classification, and the conditions for chlorination of water.
Quick revision
- Isolation: infected; quarantine: healthy contacts for the longest incubation period.
- Sterilisation kills spores; disinfection usually does not; antiseptic for skin.
- Concurrent: during illness; terminal: after the patient leaves.
- Autoclave 121°C, 15 psi, 15–20 min. Hot air oven 160°C, 2 hours.
- Glutaraldehyde and ethylene oxide for heat-sensitive items.
- Chlorination: 0.5 mg/L free residual chlorine after 1 hour.
Written by The House Job from Excel Community Medicine and Park's Textbook of Preventive and Social Medicine (named in the UHS PGET syllabus), with water chlorination following WHO guidance.
FAQ
What is the difference between isolation and quarantine?
Isolation separates infected people during their infectious period; quarantine restricts healthy contacts for the longest incubation period of the disease.
What are the standard autoclave conditions?
121°C at 15 pounds per square inch for 15 to 20 minutes.
What is the difference between sterilisation and disinfection?
Sterilisation destroys all microbes including spores; disinfection destroys pathogens but not necessarily spores.
What level of free chlorine should remain after chlorinating water?
At least 0.5 mg/L after a contact period of one hour.