Cleaning, disinfection and sterilization

Surfaces and equipment carry germs from one person to the next. A door handle, a shared keyboard, a blood pressure cuff, a tattoo machine grip, a change table or a reused instrument can all transmit infection if they are not cleaned and, where needed, disinfected or sterilized. This lesson explains the difference between those three processes, how to choose the right one, how to use disinfectants safely and effectively, and how to organize environmental cleaning so that it actually happens.

Three different processes

Cleaning is the physical removal of dirt, organic material such as blood and body fluids, and many germs from a surface, using water, detergent and friction. Cleaning does not necessarily kill germs, but it removes most of them, and it is always the first step: disinfectants and sterilants cannot work properly through a layer of dirt or dried blood.

Disinfection uses a chemical or physical process to kill most disease-causing microorganisms on a cleaned surface, but not necessarily bacterial spores. Disinfectants are classed by how much they kill. Low-level disinfectants kill most vegetative bacteria, some fungi and enveloped viruses such as influenza. Intermediate-level disinfectants also kill tuberculosis bacteria and more resistant viruses. High-level disinfection kills all microorganisms except large numbers of bacterial spores and is used for some reusable medical devices.

Sterilization destroys all forms of microbial life, including spores. It is used for instruments that enter sterile body tissue or the bloodstream, such as surgical instruments and reusable piercing tools. Common methods are steam under pressure (autoclaving), dry heat and low-temperature chemical methods for heat-sensitive devices.

Sanitizing is a term used especially in food service for reducing germs on food contact surfaces to safe levels; sanitizers are approved for that use and are usually rinsed or allowed to air dry according to the label.

The Spaulding classification

Reusable equipment is classified by how it is used, following a system proposed by Earle Spaulding in the 1960s and still used in Canada and the United States. The more invasive the use, the higher the level of processing needed.

Processing reusable medical and dental devices, called reprocessing, is a specialized job. Facilities that reprocess critical or semi-critical devices must follow the manufacturer's instructions for use, validated processes and national standards, with monitoring, records and trained staff. Failures in reprocessing have caused outbreaks and mass notifications of patients who may have been exposed to hepatitis or HIV. Many smaller clinics and studios use single-use sterile items for critical uses to avoid the risk. If a device cannot be reprocessed according to the manufacturer's instructions with the equipment available, it should not be reused.

Choosing a disinfectant

In the United States, surface disinfectants are pesticides registered by the Environmental Protection Agency, and each carries an EPA registration number. The label states the organisms the product is registered to kill, the contact time, the dilution and the surfaces it can be used on, and under federal law the label is the legal instruction for use. The EPA publishes lists of registered products that meet its criteria for specific pathogens, such as List N for the virus that causes COVID-19, List G for norovirus and List K for C. difficile spores. Disinfectants and sterilants for medical devices are regulated by the Food and Drug Administration.

When choosing a product, consider:

• the organisms of concern (for example, a product effective against norovirus during a gastrointestinal outbreak, or a sporicidal product for C. difficile);
• the contact time: the length of time the surface must stay wet for the product to work, which can range from 30 seconds to 10 minutes;
• compatibility with the surfaces and equipment, according to the equipment manufacturer;
• safety for workers and occupants, including respiratory and skin irritation;
• whether the product is a one-step cleaner-disinfectant that can clean and disinfect lightly soiled surfaces in one go;
• ease of use, such as ready-to-use wipes or sprays versus concentrates that need dilution.

Contact time and log reduction

Disinfectants do not work instantly. The label's contact time is the time the surface must stay visibly wet. Wiping a surface and drying it immediately, or using so little product that it dries in ten seconds when the label says three minutes, may leave most germs alive. Ready-to-use wipes are convenient, but one wipe may only keep a small area wet; large surfaces may need several.

Disinfectant performance is described as a log reduction. Each log is a tenfold reduction in the number of live organisms. A 3-log reduction kills 99.9 percent; a 6-log reduction kills 99.9999 percent.

import math
# Log reduction from a starting count N0 to a surviving count N
for n0, n in [(1_000_000, 1000), (1_000_000, 1), (50_000, 50)]:
    logs = math.log10(n0 / n)
    killed = 1 - n / n0
    print(f"{n0:>9,} -> {n:>5,} survivors: {logs:.1f}-log reduction ({killed:.4%} killed)")

1,000,000 -> 1,000 survivors: 3.0-log reduction (99.9000% killed)
1,000,000 ->     1 survivors: 6.0-log reduction (99.9999% killed)
   50,000 ->    50 survivors: 3.0-log reduction (99.9000% killed)

Consider what a 99.9 percent kill means when the starting number is large: a million germs become a thousand survivors. That is why cleaning first matters. Removing most of the soil and organisms physically lowers the starting number, so the disinfectant has fewer survivors to deal with and is not inactivated by organic material.

Diluting bleach

Household bleach is sold at several strengths, commonly about 5.25 percent or 6 percent sodium hypochlorite, and concentrated products are higher. Public health agencies recommend different working strengths for different jobs. A common general disinfection strength is about 1,000 parts per million (ppm) of available chlorine; for blood spills and for norovirus or C. difficile, guidance often recommends about 5,000 ppm. Always follow the product label and your public health unit's guidance, because strengths and claims vary. One percent equals 10,000 ppm.

# Bleach dilution: how much stock bleach to make 1 L (1000 mL) of working solution
def ml_of_stock(stock_percent, target_ppm, final_ml=1000):
    stock_ppm = stock_percent * 10_000
    return target_ppm * final_ml / stock_ppm

for stock in (5.25, 6.0):
    for target in (1000, 5000):
        v = ml_of_stock(stock, target)
        print(f"{stock}% bleach to {target:>5} ppm: {v:5.1f} mL bleach + water to make 1000 mL")

5.25% bleach to  1000 ppm:  19.0 mL bleach + water to make 1000 mL
5.25% bleach to  5000 ppm:  95.2 mL bleach + water to make 1000 mL
6.0% bleach to  1000 ppm:  16.7 mL bleach + water to make 1000 mL
6.0% bleach to  5000 ppm:  83.3 mL bleach + water to make 1000 mL

Make diluted bleach fresh, usually daily, because it loses strength over time, especially in warm conditions and in light. Label the container with the contents, strength and date. Never mix bleach with ammonia, acids or other cleaners, which can release toxic chloramine or chlorine gas. Workers who prepare disinfectants need training under WHMIS in Canada or the Hazard Communication standard in the United States, access to safety data sheets and the protective equipment the label and data sheet call for, usually gloves and eye protection.

Environmental cleaning programs

In health care and long-term care, environmental services staff are front-line infection control workers. Good programs define what is cleaned, how, how often, with what product and by whom, and they check the results. Key principles include:

• clean from cleaner areas to dirtier areas and from high surfaces to low surfaces, so contamination is not spread;
• give special attention to high-touch surfaces, such as bed rails, call bells, light switches, door handles, taps, toilet flush handles and shared equipment;
• increase frequency during outbreaks and in areas with patients on additional precautions;
• use color-coded cloths and mops for different areas, such as bathrooms versus patient rooms, and change them often;
• use microfiber cloths where appropriate, which remove more organisms than cotton and do not bind quats;
• clean and disinfect shared equipment between users, with a clear rule on who is responsible for which items.

Checking cleaning quality is important because visual inspection misses a lot. Fluorescent markers placed on surfaces before cleaning, then checked with ultraviolet light, show whether surfaces were actually wiped. Adenosine triphosphate (ATP) meters measure organic residue. These tools are used for feedback and training rather than as a measure of germs directly.

Outside health care

Offices, schools, gyms and stores do not need hospital-level disinfection for routine cleaning. Regular cleaning with detergent, plus disinfection of high-touch surfaces at a frequency that matches use, is usually enough. During outbreaks of gastrointestinal illness or when public health advises, increase the frequency and use a product with a claim against the relevant organism. Avoid fogging or spraying disinfectant into the air or onto people; it exposes workers and occupants to chemicals and is not a substitute for cleaning. Child care settings follow public health requirements for cleaning toys, change tables and food areas.