Disinfectant efficacy: Getting the temperature right

BY DR TIM SANDLE  |  20 MARCH 2023

 

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Apart from a few exceptions, disinfection processes are considerably slower at low temperatures than at higher temperatures. This carries consequences for the use of disinfectants, especially in cold rooms where ‘cold-loving’ microbes present a contamination risk (1). To address this, either concentrations or contact times need to be increased, or an alternative disinfectant agent used. In this article, we take a look at the science.

Influences on disinfectant efficacy

 

There are several factors that influence disinfectant efficacy in addition to microbial types and population. These include the physical conditions of the chemical reaction such as temperature, pH, dosage of disinfectant, mechanical action, humidity, water hardness and contact time of the reaction. These are inter-related factors, in that each is dependent upon the other. For example (and of relevance to this article), the contact time for a disinfectant differs according to the temperature of the liquid or the surface at the time of application.

 

This is in keeping with the 1959 publication by Herbert Sinner, the German chemical engineer at Henkel, who illustrated the cleaning / disinfection process as a circle with four sectors representing temperature, chemistry, mechanical action and time.

 

Sinner's Circle is still used when describing cleaning processes and how varying the size of some sectors will affect the other sectors.

 

Figure 1: Sinner’s Cleaning Circle, where each area of the circle needs to be optimal in relation to the other areas.

The effect of temperature

 

 

Figure 2: Disinfecting a cleanroom cold store

 

 

The reaction rate in disinfectants (and detergent) is temperature-dependent without exception. To model this, the temperature coefficient (Q10) provides the clearest assessment. As a general ‘rule of thumb’ Q10 shows that the rate of a reaction increases for every 10-degree rise in the temperature, and conversely the level at which the corresponding rate at which the reaction decreases. The expression of Q10 is given by the Van’t Hoff equation (which relates the change in the equilibrium constant, Keq, of a chemical reaction to the change in temperature, T) (2). Hence, Q10 can be defined as the ratio between the rate of a process at two temperatures separated by 10°C. Establishing what this ratio is will vary according to the disinfectant.

 

A note of caution - while the activity of most disinfectants increase as the temperature rises, there are exceptions. It also stands that an upper temperature will be reached, which will cause the disinfectant to degrade. This will weaken the germicidal activity (one study placed this at around the 40oC mark for a range of chemicals) (3). A further concern relates to a high temperature causing the evaporation of the main active ingredient, where the active ingredient is lost too fast and therefore the antimicrobial effect is not achieved (4).

However, the bigger challenge is with colder temperatures. Under cold conditions (10oC or cooler) many disinfectants lose up to half their efficacy compared with standard room temperature conditions down to 1 or 1oC (5).  Quaternary ammonium compounds, aldehydes, organic acids and phenolics are among the classes of disinfectants most likely to be affected by temperature alterations.. Let's consider a quaternary ammonium compound - cold temperatures have a significant effect on the denaturation speed caused by the quat-based disinfectant. After this, at temperatures below zero, most disinfectant solutions cause a film to form on the surface when an aqueous application solution is used. Efficacy is significantly limited under these conditions.

 

Often a quat-based disinfectant will need to have four times the concentration or four times the contact time to achieve the same microcidal effect on bacteria and fungi at 4oC, compared to the use of the same disinfectant applied to the same surface type and target microorganisms 20-25oC.  Therefore, to compensate for the loss of efficacy under colder conditions, the dosage or contact time of the disinfectant needs to be increased. However, there will come a point when efficacy is lost altogether as the temperature decreases. Depending upon the required operational temperature and disinfectant types, it might be that the biocidal agent ceases to function.

 

Other types of disinfectants, such as hydrogen peroxide-based ones, are less effected by temperature extremes and remain effective at lower temperatures. As well as disinfectants with active oxygen compounds, alcohols are generally not affected at lower temperatures.

Why does this matter in pharmaceutical and healthcare environments?

 

The reason Q10 matters is because disinfectant standards (including the European Norms) require disinfectant activity to be assessed at a temperature of 20oC ±1oC. This means the given concentration for the recommended contact time will be for what needs to be applied under standard ‘room temperature’ conditions. However, the contact times will theoretically be faster or slower at different temperatures. While certain items of equipment will be functioning at temperatures above 30oC, cleanrooms will not (not least given that the conditions will become impossible to work in and cleanroom suits would no longer function as an effective filter). However, cold rooms are commonplace in pharmaceutical facilities.

 

While the risk of microbial contamination is somewhat lower, the presence of psychrotolerant (having the characteristic to grow near 0 °C but with their optima lying in mesophilic range) and psychrophilic organisms, provides a continuing challenge (6). This challenge requires regular cleaning and disinfection and under these circumstances, the Q10 factor is likely to apply, leading to the disinfectant contact times needing to be increased.

 

The issue of disinfectant compatibility and cold temperatures can be further complicated by the activities of different agents and different surface materials.

 

 

Moving forwards: Cold room disinfection

 

In order to use disinfectants effectively, facilities operating cold areas need to understand how their disinfectants are affected and what adjustments are required. Adjustments include overcoming any limitation due to low temperatures by adjusting disinfectant concentrations or using longer contact times. If either of these two options cannot be undertaken, then the final recourse is with the selection of a new disinfectant. Alternatively, some facilities have attempted to mix disinfectants with antifreeze with variables results (7).

 

 

Summary

 

The temperature at which a disinfectant is used is an important factor for when it is used outside ambient conditions. Additional efficacy studies may be required, or advice sought from disinfectant manufacturers in order to optimise disinfectant use in cold areas. This is especially important when optimising contact time and disinfectant concentration.

 

 

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References

 

  1. Sandle, T. and Skinner, K. Study of psychrophilic and psychrotolerant micro-organisms isolated in cold rooms used for pharmaceutical processing, J Appl Microbiol. 2013 114(4):1166-74
  2. Elias M, Wieczorek G, Rosenne S, Tawfik DS. The universality of enzymatic rate-temperature dependency. Trends Biochem Sci. 2014;39:1–7
  3. Gélinas P, Goulet J, Tastayre GM, Picard GA. Effect of Temperature and Contact Time on the Activity of Eight Disinfectants - A Classification. J Food Prot. 1984 Nov;47(11):841-847. doi: 10.4315/0362-028X-47.11.841
  4. Berardi A, Perinelli DR., Merchant HA, et al. Hand sanitisers amid CoVID-19: A critical review of alcohol-based products on the market and formulation approaches to respond to increasing demand. Int J Pharm.2020;584. pmid:32461194
  5. Kramer A und Assadian O (Hrsg.) Wallhäußers Praxis der Sterilisation, Desinfektion, Antiseptik und Konservierun 2008, S. 169
  6. Hoover, R.B. and E.V. Pikuta. Psychrophilic and Psychrotolerant Microbial Extremophiles in Polar Environments Polar Microbiol, 2010, 115-116
  7. Davison, C.E. Benson, A.F. Zeigler, R.J. Eckroade. Evaluation of disinfectants with the addition of antifreezing compounds against nonpathogenic H7N2 avian influenza virus Avian Dis., 43 (1999), pp. 533–537

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