Many cleaning professionals have heard that a surface must be cleaned before applying a disinfectant. However, they might not know exactly why. But before explaining the reasons, it will help to identify the differences between cleaning and disinfecting.

According to the Centers for Disease Control and Prevention (CDC), here is how these terms are defined:
Cleaning removes germs, dirt, and impurities from surfaces or objects.
Disinfecting kills germs on surfaces or objects.

But these definitions do not tell us why cleaning must be done before a surface can be disinfected, often referred to as the "two-step" process.

The following explanations from leading health and safety sources can help explain why the two-step process is necessary.

The CDC explains it this way:

[Cleaning] does not necessarily kill germs, but by removing them, it lowers their numbers and the risk of spreading infection. . . .

[Disinfecting] does not necessarily clean dirty surfaces or remove germs, but by killing germs on a surface after cleaning, it can further lower the risk of spreading infection.

A publication on the U.S. Environmental Protection Agency website states:

Clean first before you disinfect. Germs can hide underneath dirt and other material on surfaces where they are not affected by the disinfectant. Dirt and organic material can also reduce the germ-killing ability of some disinfectants.

The Food Standards Agency (FSA), serving England, Wales, and Northern Ireland, claims:

Chemical disinfectants only work if surfaces have been thoroughly cleaned first to remove grease and other dirt.

From the book A Practical Guide to Decontamination in Healthcare: 

To ensure efficacy, cleaning is a necessary first step followed by disinfection.
Among the steps cleaning professionals can take to ensure efficacy are the following:

 • Use only microfiber cleaning cloths for cleaning, and change them frequently.

 • For larger areas, select an aqueous ozone cleaning system, which will prove both faster and effective.

 • While some pathogens can be removed using a one-step cleaner-disinfectant, use the two-step process to be sure.