How Disinfection Contact Time is Determined – And Why It is Critical to Keep the Surface Visibly Wet

Anytime you pick up a disinfectant and read the instructions for use, you will see a contact time. This is the proven time a disinfectant product is effective against bacteria and viruses. Usually, the label instructions say something like this: “Spray the surface. Allow surface to remain visibly wet for 2 minutes.” The words “visibly wet” have created some debate about proper use; but simply put, disinfecting products must remain on a surface for the listed contact time.  After the contact time has been reached, you can wipe away excess liquid with a clean cloth.

To understand why, it is important to know how products are tested for disinfection.

The Environmental Protection Agency (EPA) requires that disinfectant products are tested using standardized methods. For these tests, a sample of bacteria or virus is prepared, and the disinfectant spray is applied for the listed contact time. However, when disinfectant spray is applied in these tests, it is pooled on the surface rather than applied and allowed to dry. This means a sample with bacteria may be fully submerged in disinfectant and allowed to sit for the manufacturer recommended contact time. This amount of liquid disinfectant is in excess of how most people use disinfectants; but is in fact the basis of the approved contact times and explains why the surface must be visibly wet long enough to disinfect.

Standard Usage of Disinfectants May Not Meet Contact Times

Without proper training, people use disinfectants the same way they use cleaners: they spray a surface and then immediately wipe it with a cloth. When doing this, however, the surface dries within seconds – the disinfectant hasn’t been in contact with the surface long enough to disinfect. The best practice for disinfection is to spray the surface sufficiently to make it visibly wet and reapply disinfectant if the surface starts to dry early, and then wipe any excess liquid it with a clean cloth once required contact time has been reached. Keeping surfaces wet long enough may pose practical challenges if you are using a disinfectant with a long contact time.

Our Real-World Study Shows How Difficult it Is to Keep a Surface Visibly Wet for 10 Minutes

We wanted to understand how long it would take a custodian to disinfect the desk surfaces in a classroom using disinfectant products with different contact times. Eleven custodians at the Waterford K-12 School participated in the study and each were asked to spray the desks in a classroom and keep the desks visibly wet long enough to meet the label contact time before wiping the desk with a microfiber cloth. They used three different products: a 30-second contact time ready-to-use (RTU) spray, a 2-minute contact time RTU spray, and a concentrated product that when diluted has a 10-min contact time. We measured how long it took the custodians to disinfect the classroom desks and how much disinfectant liquid they used. Based on these two factors we determined how much it cost to disinfect the classroom.

We found that products with contact times greater than 2 minutes required the custodians to re-apply disinfectant to keep the surface wet for the full contact time. Overall, the RTU sprays with contact times of 2 minutes or less:

  • Were 8.7 minutes faster than the concentrated quat disinfectant
  • Used half as much liquid to disinfect
  • Were more than $1 cheaper per classroom to use than the concentrated quat disinfectant

Save Money Disinfecting with RTU Sprays with Fast Contact Times

Custodian labor is one of the largest parts of a cleaning operation budget. When disinfection is required, meeting the contact time for disinfection is necessary to ensure surfaces are truly disinfected. Failing to reach the contact time with a single application can lead to stress on custodians, a rushed process or even leave pathogens alive on surfaces. It is commonly assumed that RTU spray products are more expensive to use than concentrated products. However, this may only be true if the dilutable product has a shorter than 2-minute contact time, which is often not the case. In this study, we demonstrated that the shorter contact time product may often be the most affordable while ensuring disinfection is done properly.

To read more, see the research summary with full details.

Or to learn more about the value of RTUs, please watch this webinar I presented for ISSA.