WHAT'S NEW - Fog, Haze Machines and COVID

Fog, Haze Machines and COVID

by Sean Derrig

There has been interest in whether fog and haze generators might help or hinder the spread of COVID-19. Rather surprisingly there seem to be no rigorous, peer-reviewed studies or trials into this specific question so to answer the question we need to start with basic scientific principles and go from there. 

CLAIM: the oil / haze particles are sterile

TRUE. Most professional machines take pharmaceutical grade white oil or a water / glycol mix and heat it to a high enough temperature to evaporate it, forming a mist. The temperatures are such that as the mist comes out of the nozzle it will be sterile in and of itself will not be adding to any infectious burden in the air. 

Note that we’re not talking about water foggers / crackers that don’t use heat to vaporise the liquid in this document, just the thermal ones. 

CLAIM: the oil / haze particles will not support virus growth

TRUE. Viruses can only reproduce in specific target cells found in their specific host(s). They can’t utilise any old carbon source (like an oil) to multiply like bacteria can. 

CLAIM: fog created by these units is identical to that stipulated in “ISO14644 – Cleanrooms and Associated Controlled Environments”

TRUE BUT IRRELEVANT. The standard cited refers to fog machines when used to test filtration systems in cleanrooms etc - and many units used on productions produce droplets of the size range required in ISO14644. But this does not mean the machine itself is providing disinfection. The standard claim does not speak to that. 

CLAIM: the fog particles ‘block’ respiratory droplets so they fall more quickly

SORT OF TRUE - BUT IRRELEVANT. It sounds plausible that droplets in the air might slow down respiratory droplets and help them fall to the ground. This question gets insanely complicated very, very quickly so we’re going to ignore pesky stuff like Stokes’ Law, the Wells curve, surface tension and phase dynamics. 

Let’s keep it simple and think about the physics. Let’s assume everything’s a sphere (just like physicists do) and think about a moving respiratory droplet colliding with a (for all practical purposes) stationary fog droplet. The sort of fog droplets you might encounter in a production are about 0.2 microns across  - they need to be that small in order to stay in the air almost indefinitely. Respiratory droplets come in a range of sizes - but let’s think about ones small enough to linger in the air for a little bit. 

• If a 1μm respiratory droplet collides with a 0.2μm fog particle, its weight increases by 0.8115%

• If a 5μm respiratory droplet collides with a 0.2μm fog particle, its weight increases by 0.0065%

• If a 10μm respiratory droplet collides with a 0.2μm fog particle, its weight increases by 0.0008%

If you recall from school the volume of a sphere is 4/3rds πr3 and it’s that ‘cubed’ bit that does it. Maths is a cruel mistress. It’s a bit like how much more pizza do you actually get in a 14” rather than 12”. 

So yes, if we ignore a lot of potentially confounding factors it could be that if fog droplets do coalesce with respiratory droplets it may them slow down – but any effect will be way too small to make a difference. 

CLAIM: it’s been scientifically proven that fog liquid kills germs.

FALSE. There is a paper doing the rounds (Lester, Kaye, Robertson, & Dunklin, 1950) which some are claiming ‘proves’ the chemicals used in fog kill bacteria and viruses. If we actually read the paper (all of it, not just the abstract) it doesn’t contain the sort of detail that excites microbiologists nowadays (like how good is it at killing Our Microbial Overlords) plus it makes it explicit that the effectiveness of triethylene glycol vapour rapidly loses its effect in humid conditions. So any effect is only really seen in dry air. And mist isn’t really very dry, is it…

In fact it states “the concentration of glycol vapor in the environment, relative humidity, temperature, and rate of ventilation all affect the attainment of a satisfactory disinfection rate”. Rather problematically it does not define ‘satisfactory’ but in fairness the paper is 70 years old. 

It was also aimed at truly airborne nasties rather than those ensconced in respiratory droplets. Work in this field petered out quite shortly after that. And don’t forget we don’t really need to ‘disinfect the air’ – droplets that might carry the virus fall under gravity; the virus doesn’t fly about like some demented mosquito. 

So, is this a forgotten intervention from the ‘50s that might save the day?

No. ‘fraid not. But there’s more. The regulation of biocides in the UK and EU is comprehensive. You can only make biocidal claims (such as ‘this kills germs’) if the active ingredient is ‘supported’ by something called the Biocidal Products Regulation (EU 528/2012). So let’s look at the chemicals you find in chemical fogging products:

• Pharmaceutical grade white oil (CAS 8042-47-5) NOT SUPPORTED

• Propylene glycol (CAS 57-55-6) NOT SUPPORTED• Triethylene glycol (CAS 112-27-6) NOT SUPPORTED

• Glycerol (CAS 56-81-5) NOT SUPPORTED

What does this mean? It means that you cannot make germ-killing claims of any of these products – because no one has actually proved they kill germs. It could be that this was an amazing breakthrough that was forgotten about – but that doth butter no parsnips with the Health and Safety Executive.

 Any supplier making biocidal claims for these products is breaking the law. Probably not maliciously, but such claims are illegal under the BPR. That’s the law, plain and simple. 

VERDICT: There is no hard scientific data showing any COVID risk or benefit associated with thermal fogging devices used in the industry. And don’t hold your breath for any either. 

But if we look at the basic science of these things thermal stage foggers really aren’t anything to worry about – nor are they an intervention that will add anything to your disinfection strategies either. 



Lester, W., Kaye, S., Robertson, O. H., & Dunklin, E. W. (1950, July). Factors of Importance in the Use of Triethylene Glycol Vapor for Aeriel Disinfection. Am J Public Health Nations Health, 40(7), 813-820. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1528959/