1.  Site of treatment - harsh chemicals or extreme physical factors such as heat
     can damage tissues so they are not used on humans, animals or fragile objects
     that are sensitive such as plastics.  Milder antiseptic and sanitizing agents can
     be used to reduce transient microbial populations on tissues.

Medical instruments that will contact tissues and can tolerate such treatment should be sterilized prior to and immediately after use and pre-sterilized single-use equipment should be disposed of properly and subsequently sterilized following use.

2.  Relative susceptibility of microorganisms - The figure below shows the relative
     susceptibility of microbes and microbial structures to antimicrobial agents:


High-level germicides kill all pathogens, including endospores.

Intermediate-level germicides kill fungal spores, vegetative bacterial cells, cysts and inactivate viruses, but cannot kill endospores.

Low-level germicides kill vegetative bacterial cells, protozoa, fungi and inactivate some viruses, but are ineffective against endospores, cysts, very resistant pathogens such as mycobacteria and most viruses.
 

3.  Environmental conditions


Temperature and pH both affect how well antimicrobial agents work.  Warm disinfectants generally work more quickly than cold ones (graph above) and this activity is also enhanced by a more acidic pH.

Other chemical disinfectants, such as bleach (sodium hypochlorite) work better at lower pH.

Excess accumulations of organic matter such as blood, urine, vomit, feces and the polysaccharide layers of biofilms can lessen the effectiveness of heat and radiation and can also inactivate some chemical disinfectants.  Thorough cleaning of the surface prior to application of the disinfecting agent increases it ability to penetrate, come in contact with microbes and reduce their populations.