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Spray; Fog; Mist – Explained!

May 6, 2021

Disinfectants are chemicals that can destroy microorganism. While they can ensure our environment is free from germs, it is ill-advised to overuse these chemicals, as they can often do more harm than good when used inappropriately. Therefore, it is essential to maximise disinfectant’s efficacies and practise responsible use of chemicals.

Spray nozzle mechanism

One of the most widely adopted methods to apply disinfectants is through spraying/fogging machines. Pathogens may be present in the air and on surfaces; using spray/fog technologies can uniformly disperse the disinfectant to ensure good contact time with the air and exposed surfaces. In fact, airborne and surface disinfection by dry fogging has been used widely in healthcare industry,[1] and even demonstrated to efficiently inactivate SARS-CoV-2, the virus responsible for COVID-19.[2]

There are two broad types of spray nozzles: (1) high pressure hydraulic nozzles; and (2) air atomizing nozzles. High pressure hydraulic operates at pressures between 20-100 Bars and generates droplet sizes of between 15-100 microns. Increasing the pressure will decrease droplet size and increase liquid flow rate of the device. One shortcoming of this mechanism is the need for an auxiliary fan to further distribute the droplets generated.

Air atomizing nozzles rely on a different mechanism. These nozzles mix fluid and air to form a fine mist or fog. The presence of compressed air helps further propel the droplets several meters away from the nozzle and rapidly filling a room with fog/mist. This is the preferred nozzle technology for spraying/fogging disinfection machines.

Do droplet sizes matter?

Yes! In fact, droplet size can directly affect surface contact behaviour and spray distribution. The following briefly summarises the characteristics of different droplet sizes:[3]

  1. Spray (Large droplets, >100 micron): Limited airborne residence time with minimal spray distribution. Effective surface coverage only in the direct line of sight of sprayer.
  2. Mist (medium droplets, 40-100 micron): Low air residence time, but good surface coverage. Strong air currents required to further distribute disinfectant effectively.
  3. Wet fog (small droplets, 10-40 micron): Excellent surface and air coverage. Spray distribution can be easily achieved by air movement.
  4. Dry fog (very small droplets, sub 10 micron): Excellent air coverage and acceptable surface disinfection efficacies. Long residence time and require ventilation to reduce inhalation risk.

It is vital to use the correct spray technologies for specific applications. For example, if spot disinfection is required, traditional spray bottles with large droplets may be most appropriate. However, if a large area needs uniform air and surface disinfection, spray bottles will result in ineffective disinfection efficacy, leading to an overuse of chemical. In this scenario, an atomizing nozzle generating wet/dry fog is most efficient.

Furthermore, damage and material incompatibility caused by wetting of surfaces must be considered. Large droplets and mist are often not recommended when electrical equipment are present. In these situations, wet and dry fog will reduce surface wetting and may be more suitable to enhance material compatibility.

Final Remarks:

  • Spraying and fogging machines are great tools for air and surface disinfection. However, these are not universal solutions, and physical removal of dirt and debris is crucial in maintaining a clean and safe environment.
  • Responsible use of chemicals can be achieved by maximising disinfectants’ coverage. This can be controlled by choosing the appropriate aerosolizing mechanism and droplet size.
  • Additional empirical research is needed to establish concrete data in the field of disinfection via spraying/fogging mechanisms. Other factors (i.e. chemicals used, evaporation rates, electrostatic properties, adhesiveness etc) can affect disinfection efficacies and have not been considered in this insight article.

[1] Nasr G. G. et al., Int. J. Multiphys., 2012 (6), 149-66

[2] Schinköthe J., Scheinemann H. A. et al., Appl. Environ. Microbiol., 2021, 87:e02019-20

[3] Zytynski I., Spraying for Covid19, Spray People Group, 2020

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