Population composition: The nature of the organisms being treated as they differ markedly in susceptibility varies the effectiveness of an agent. For example, younger cells are usually more readily destroyed than mature organisms and bacterial endospores are much more resistant to most antimicrobial agents than are vegetative forms. Mycobacterium tuberculosis for example is very much resistant to most of the antimicrobial agents than most other bacteria. Hence, the composition of population also plays a major role in the effectiveness of antimicrobial agents.

Concentration of an agent: Often, but not always, the more concentrated a chemical agent or intense a physical agent, the more rapidly microorganisms are destroyed. This is not always true and over a short range, a small increase in concentration leads to an exponential rise in effectiveness, but beyond a certain point, the increase may not raise the killing. Sometimes an agent is more effective at lower concentrations. For example, 70% ethanol is more effective than 95% because its activity is enhanced in the presence of water.

Duration of exposure: The more the exposure duration, the more organisms are killed. Hence, this also form a vital effectiveness criterion for the antimicrobial agents.

Temperature: An increase in temperature at which the agent acts also often enhances its activity. Frequently, a lower concentration of disinfectant or sterilizing agent can be used at a higher temperature.

Local environment: The local environment in which the population is being controlled also plays a major role that may offer protection or aid in its destruction. For example, because heat kills more readily at an acid pH, acid foods and beverages such as fruits and tomatoes are easier to pasteurize. The organic matter also can protect microorganisms against heating and chemical disinfectants. Hence, for example when city's water supply has a high content of organic matter, more chlorine must be added to disinfect the water.