Contact Time (CT) is how long it takes a sanitizer to kill a specific pathogen.
Contact time is a parameter oftenly used when talking about RWI (recreational water illnesses), those which people can get sick from when coming in contact with water from pools, hot tubs/spas, splash pads, rivers and lakes, if the water is contaminated with pathogens, or in other words the water is poorly sanitized.
The amount of time it takes for a certain concentration of chlorine (or other sanitizer) to kill a given pathogen is known as the contact time. These times are published online and are well known from lab studies. CT equals the concentration of chlorine (in ppm) multiplied by the minutes it takes to kill the pathogen.
It's important to know that these contact times often depend on pH, and temperature, as well as chlorine concentration. In simple terms, if the CT value for a given germ is 10, that means it takes 1 ppm of free chlorine during 10 minutes to be able kill it. Or it takes 10 ppm of chlorine just for 1 minute to kill it. Here are some contact times published by the CDC: