Dwell Time

Many of ALLCLIR's benefits relate to the grossly underappreciated concept of "dwell time" — alias "contact time", "wet time", "exposure time". Dwell time is listed on each disinfectant's label per government requirements. The US EPA, for instance, requires the disinfectant manufacturer to perform original microbiological testing using EPA-approved methods to tie the general microbial kill claim (e.g. 99.9999%) of the disinfectant manufacturer to a particular microbe and concentration of disinfectant and to a minimum “contact time”. The contact time is the minimum duration the disinfectant must remain “visibly wet” (i.e. to the healthy naked eye, which for a clear liquid means about 10 microns deep) on a hard surface to achieve the kill claim with respect to the particular microbe and disinfectant concentration.

These contact times range from 15 seconds to 10 minutes and sometimes even longer!

Why is so much time required to really do the job?

Well, the bad guys we are addressing with disinfectants — e.g. SARS-CoV-2 (COVID-19), SARS-CoV-1, MERS-CoV, other coronaviruses, Methicillin-resistant Staphylococcus aureus (MRSA), c. difficile, to name just a few — don’t die right away like in the movies, they die hard.

Now, if the contact time is not observed, the user of the disinfectant cannot legally claim they have disinfected the surface in question. As required by the US Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), product labels state, "it is a violation of Federal law to use this product in a manner inconsistent with its labeling."

Think about that.

The concept of contact time is necessary but fraught with challenges. Keeping a surface visibly wet long enough can be especially difficult when using disinfectants that require a long contact time such as ten minutes. Under some conditions, e.g. high temperatures and low humidity, the challenge is just as difficult when the labeled contact time is as short as three or four minutes. Disinfectants with high alcohol content evaporate very quickly and thus are most problematic vis-à-vis contact time.

If the disinfectant does dry on the surface before the contact time is reached, label instructions typically insist on re-application to ensure the contact time requirement is met.

Truth is, the contact time requirement is rarely observed.

Which is to say, disinfection to the intended kill percentage is rarely confirmed and probably rarely occurs.

Clearly that needs to change — and fast.


The ALLCLIR sensor immediately detects when the presence of a disinfectant — whether chlorine-, thymol-, hydrogen peroxide-, or quaternary ammonium-based — is sufficient for proper disinfection to ensue.

The ALLCLIR sensor changes color immediately when initial wetness with the disinfectant has been achieved on the surface. Although for true disinfection the disinfectant should remain visibly wet on the surface for anywhere from 15 seconds to 10 minutes or more, depending on the product's labeled contact time, ALLCLIR confirms the necessary condition of initial wetness. Moreover, ALLCLIR heuristically allows the operator to optimize the application process to avoid the costs of over-spraying — which include damage to the surfaces, environment and perhaps people. Without that very obvious feedback, the operator must rely on visible wetness only, a reliance compromised by dependence on distance, angle, lighting, eyesight and, yes, diligence.

The ALLCLIR sensor providesa measurable proving the surface was wetted with disinfectant. The sensor remains in its triggered state permanently — signaling "ALLCLIR" to cleaners, management, auditors and customers alike. You can then dispose of the sensor or file it as a permanent record of disinfection.

This validation, training and auditing tool does not replace the concept of contact time but heuristically honors and reinforces it, resulting in a much greater frequency of true disinfection — and thus a much lower frequency of infections and a markedly greater confidence among staff and customers.

As you will surmise, the ol' spray-and-wipe technique simply does not work to disinfect.

There are four main reasons disinfectants do not immediately achieve their labeled kill percentage (e.g. 99.99%) but rather require anywhere a significant duration to do so:

  • Death is based on chemical reactions, and every chemical reaction takes time, especially when the molecules we are trying to disrupt, denature or inactivate — namely cell wall lipids, cell proteins, RNA, DNA, energy-producing enzymes — are very complicated molecules.
  • Moreover, we are trying to effect huge numbers of such changes in order to achieve a certain kill percentage (e.g. 99.9% or more). Which is to say, we are very much talking in terms of thermodynamics — and that means big, macroscopic chunks of time must be involved. (* See details at bottom.)
  • It also means liquid water must be present, as a solvent, to facilitate both the chemistry and the thermodynamics.
  • Furthermore, we are constrained to use disinfectants that are not too strong, not too fast acting, as to be too dangerous to us or destructive to the material we are disinfecting.

* The effectiveness of a given disinfectant depends on the concentration of disinfectant, contact time, temperature, turbidity, particulate concentration, and specific microorganisms. Because the concentration of microorganisms varies widely, microorganism concentrations are typically expressed in what are called log units, which is simply a way to compact a very wide range of data so it fits nicely within a far more convenient and thus heuristic purview. If concentrations are expressed in #/ml, then the log concentration is equal to the base 10 logarithm of the actual concentration (Table 1).

Table 1. Log units as used to define microorganism concentrations.

Log units

Concentration (#/ml)



























The effect of disinfectant concentration and contact time on the mortality of typical microorganisms is presented in Figure 1. For many microorganisms, the rate of kill is a straight line on a semi-log plot. Therefore, it takes the same time to reduce the concentration of microorganisms from 1,000,000 to 100,000/ml as from 100 to 10/ml.

Figure 1. Effect of disinfectant concentration and contact time on mortality of typical microorganisms.

Note a reduction in 1 log unit is equal to a 90% reduction in concentration and a reduction in 2 log units is equal to a 99.0% reduction in concentration. Common goals in this respect are 99.99% and 99.9999% reductions, which are referred to as “4 log kill” and “6 log kill”, respectively. As the concentration of disinfectant is increased, the rate of kill is increased. Therefore, to achieve a specific final (low) concentration of microorganisms, a high disinfectant concentration at a short contact time or a low concentration at a long contact time may be used.