The effectiveness of antimicrobial agents is affected by following factors.
Population size. A larger population size calls for much time for the microorganisms to die while a smaller population size requires little time for the microorganisms to die.
Population composition. The effectiveness of an antimicrobial agent varies with the nature of organisms to be treated because microorganisms have different sensitivity.
Concentration or intensity of the agent. A more concentrated chemical agent or an intense physical agent accelerates the rate at which microorganisms are killed.
Treatment duration. When microorganisms are subjected to a microbial agent for a longer time, more of them are killed.
Temperature. An increase in temperature accelerates the activity of a chemical agent hence more organisms are destroyed.