Fogging Q&A

Foggers are becoming an increasing part of our new Covid-19 business environment but there’s still a bit of mystery around what foggers are, what they’re for, how they work, how to use them and what fluids you should use them with.

So here we will try and clarify a few questions that people frequently ask us. 

What is fogging?

Fogging is an old and fairly low-tech technology that has been used for many years for agricultural, pesticidal and insecticidal purposes. Fogging involves the distribution of any suitable chemical formulation in very fine particles like a fog or a very fine mist, enabling a pervasive surface contact coating of all surfaces very quickly and efficiently.

How does a fogging machine work?

There are 2 distinctly different types of fogging machine. A “cold fogger” such as the Sterimax fogger works by pumping a liquid through a specially designed nozzle so it is emitted as a colourless fine mist (it's more like a fine mist than a fog, depsite being called a fogger). It is also called an Ultra Low Volume Fogger, ULV Fogger or Ultra Low Capacity Fogger – they do not spray large volumes of fluid and they do not spray at high pressure.

Because a cold fogger does not heat the liquid it does not change the chemical composition of the cleaning agent used and therefore any cleaning agent that can be used in a pump spray bottle can be used in a ULV Fogger.

The other type of fogger is a “hot fogger”, more correctly known as a thermal fogger. Thermal foggers work by heating the fogging liquid over a coil and vaporising it. This type of fogger will emit a fog that looks like a grey or white smoke. Thermal foggers are more commonly used in outdoor settings for large jobs such as mosquito spraying in tropical environments. They are also the type that you now see quite frequently on the TV news with people in hazmat suits spraying train stations and other large public spaces that looks like a scene from a zombie movie.

Don’t worry - our Sterimax foggers are cold foggers and they are a much tamer and more civilised piece of kit. They emit a colourless fine mist, not a scary grey smoke.

I've heard that the particles from foggers can suspend in the air inside a room for several hours and that this might be unpleasant or dangerous for people breathing it in. Is this true?

The Sterimax ULV Fogger produces a mist with micron sizes of at least 20 microns. A 20 micron particle in still air has a "settle velocity" of about 43 metres per hour so from a height of 8 feet the settle time is 3.5 minutes - and is much, much shorter for larger droplets. So air suspension of disinfectant chemical particles is not an issue or a threat with cold foggers, allowing rooms to be reopened very quickly.

Thermal foggers on the other hand vaporise the liquid into a true fog-like emission and can spray out particles as small as 0.5 microns in size. A 0.5 micron particle has a settle time from 8 feet of 41 hours. Clearly not the type of thing you can use indoors and open up to people quickly.

This is a thermal fogger. They will disperse fog particles as small as 0.5 microns.

The Sterimax fogger (below), on the other hand, is a cold fogger. It delivers a clear mist with particle sizes that have too much mass to suspend in the air for more than a few minutes. This type of machine should more accurately be called a "mister".

 What disinfectant products are suitable for spraying through Sterimax foggers for coronavirus disinfection purposes?

To kill coronavirus you will need an antiviral disinfectant liquid. The Sterimax fogger will spray any liquid through it whatsoever. (Do not be misled by some other companies selling foggers that say you must use a fluid specifically designed for their fogger. It is total nonsense and they are just trying to con you into buying their over-priced fluid). The best way to understand and decide upon your antiviral disinfectant choices is to read our dedicated Fogging Liquids Info page.

 Can a fogger replace the need for manual cleaning?

No. Fogging is additional to cleaning but it delivers a wider scope of additional disinfection that manual cleaning alone cannot achieve. Cleaning in this sense basically means spraying and wiping surfaces, particularly hard surfaces and especially frequently touched surfaces such as door handles, hand rails, remote controls,  toilet flush handles, light switches, work surfaces etc. That needs to keep happening.

However fogging will allow you to also disinfect all the places that are not possible to manually clean including soft surfaces (such as curtains, carpets, headboards, cushions, chairs and sofas) as well as “wide-area hard surfaces” that would take far too long to manually clean with spraying and wiping, such as walls, ceilings, cupboards, and doors, undersides of desks and other furniture. Foggers are also excellent at blasting those often darker, damper and less well ventilated bathroom and shower areas that bacteria and viruses like to inhabit.

How often should I fog?

You need to take into account and prioritise any cleaning guidelines and regulations that apply to your specific industry sector. However those guidelines will not include specific requirements on fogging because fogging is not a mandatory requirement in itself but is rather a tool that can be used to help you efficiently comply with guidelines.

With that in mind, and with full deferment to specific industry sector protocols, we would suggest as follows (to supplement cleaning, not instead of cleaning):

Hotels, guest houses and B&Bs:

  • Guest bedrooms: fogged in between departure and arrival.
  • Reception areas, dining areas, communal seating indoor and outdoor, corridors, lifts: Daily, or rather nightly.
  • Toilets: if closing toilets for 30 minutes after fogging is too inconvenient or unfeasible then fog once in the night time and use a spray and wipe cleaning schedule for the busier times.

Self-contained holiday rental properties: In between tenants.


  • End of each day for designated rooms as required (classrooms, staff rooms, reception areas, assembly areas etc)
  • Sports facilities and changing facilities - in between each user group

Leisure and sports facilities:

  • Gym equipment and gym areas: often – every 3 or 4 hours with mandatory user spray and wipe protocols after each use.
  • Changing rooms and communal shower & toilet areas – we’d suggest every 3 or 4 hours with more frequent spray and wipe on high touch areas such as handles etc.

Churches & Community Halls:

  • In between each separate group booking for community and church halls and then also at end of day.
  • For church services - in between services when there is more than one service on a single day. For a once daily service then an hour or two before the service is due to start.
  •  (note that colder spaces such a stone floors in churches might take longer for surfaces to completely dry)

Bars and restaurants:

  • In the evening after closing and with tables bare. Then wait until morning to re-lay tables for service. Toilets as per above for hotels.

How long does it take to fog a room?

Obviously that depends on the size of the room and the amount of clutter that needs to be surface-disinfected. But as a rule of thumb a 20m2 hotel room will take about 1 minute. A 50m2 open aspect dining room with minimal clutter will take about 2 minutes. These machines are extremely quick at doing what they do, and they do it very well.

Can the fogger be used outside?

Yes it will work outside and will surface-disinfect whatever you spray it at (outdoor seating areas are a good example) but obviously you need to take into account electrical safety, the impact of wind and the risk of spraying onto people or animals, which should never be done.

What PPE do I need to wear to use a fogger?

You should wear at least an FFP2 or FFP3 rated respiratory protective mask and safety goggles for protection against disinfectant chemicals, and also cover up exposed skin. For more specific PPE recommendations you should follow the manufacturer's Safety Data Sheet recommendations for the brand of disinfectant that you are using. It is a legal requirement for all manufacturers of such products to publish satefy data sheets. 

Please note that the surgical masks that may be included with your unit when you receive it are not suitable for prolonged use except as a makeshift interim protective measure in case you have not yet sourced more suitable respiratory protective equipment.

Please also refer to UK HSE's leaflet COSHH SR03 guidelines on the use of space sprayers (this section gratefully acknowledges the information provided by Mr C Park - thank you).

I have a hotel / holiday rental house and need to disinfect the bedrooms / whole property between guests. Do I fog first and then clean afterwards or vice versa?

Good question and the source of quite some debate. We can’t tell you definitively what to do but the most common school of thought is to fog first (which protects the cleaner), then come back to the area within 15 minutes or so and do the usual housekeeping clean. Then either fog again on the way out or spray and wipe just the usual hard surfaces on the way out with an antiviral spray and cloth. Clearly at the same time you need to be following any new industry cleaning and room preparation protocols for your specific sector of the hospitality industry.

 Can I open the windows while I am fogging?

Definitely. There is no evidence that a virus can come inside from outside so why not. Obviously you need to be aware of any wind coming in that might affect the fogging process.

How long do I need to wait after fogging to use the space again for customers or staff?

We recommend 30 minutes.

As long as you are using a suitable disinfecting fluid then the timelines of activity are as follows: 1 minute of surface contact will kill bacteria. 5 minutes of surface contact will kill any virus. After 10-12 minutes the sprayed area should be dry within an indoor space that is reasonably well ventilated and at normal indoor temperatures. A harder spray on to a slightly colder surface such as a laminate floor or a cold window will take a little longer to dry. Same for a (darker and more humid) bathroom or shower room.

Add a few minutes for margin and you can be re-opening the space with confidence after 30 minutes to your guests, customers or colleagues, free of viruses and bacteria.

In terms of respiratory risk, the smallest droplet size (20 microns) produced by the Sterimax ULV Fogger will have dropped to the ground from a height of 8 feet (2.5m) within 3.5 minutes, meaning no respiratory risk of people breathing in airborn droplets after you have re-opened.

After fogging can I keep working in a room to prepare it for new  customers/users within the 30 minute period?

Yes you can. After spraying you should leave the room for at least 10-15 minutes with the door closed to allow all the fog to drop out of the air and the surface disinfecting process to be completed. Then you can go back in and keep working, bearing in mind that some surfaces may still be very slightly damp to the touch.

Can I fog my kitchen and bar service areas and crockery / cutlery / drinks glasses?

If spraying with Hypochlorous Acid then yes no problem at all. If spraying with industrial virucidal disinfectant products then you need to crefgully read and understand the manufacturers guidelines on use with a food service environment.

Do I need to cover all electrical items and appliances before fogging?

In terms of the basic electricals you would find in a space such as a hotel room (such as TVs, lamps etc) the answer is No you do not need to cover them up but with the caveat that you shouldn't fog directly and up-close onto any electrical item. Rather you should just ensure directional light coverage onto the item. For TVs avoid fogging into the back of the unit where the all the holes and vents are located. The same goes for plug sockets. Ultimately you need to use your judgement and exercise some basic caution. Electrical items and liquids obviously don't go well together but a very light fogging that will evaporate within a few minutes should not do any damage. It is important however to avoid touching any electrical items until they are competely dry. For sensitive electronics such as laptops, routers and non-waterproof mobile phones then the answer is do not fog them. 

Do foggers set off smoke alarms?

No, but don't fog directly into a smoke alarm from close range as the moisture may damage its electrical circuits.

How reliable are these machines and what warranty do you offer?

The only moving part in our ULV Fogger is the electric motor which operates as an air blower. It has a maximum 20-minutes run time before you need to turn it off and let it cool (but you could fog between 250-300 m2 of space during that time so you may be unikely to ever use it for that long anyway).

The liquid flow mechanism contains no moving parts, with fluid moving through a series of tubes and small-aperture valves. So all in all it is a very robust and reliable machine. Not high tech at all. There are no circuit boards or other delicate electronics to go wrong.

The main problem you could have if you're not careful is a blockage of the flow valves with debris that you have inadvertently allowed to get inside the fluid reservoir. That will result in no mist coming out after you turn on the machine. (But first check that the flow control screws are open  - that's the most obvious cause of zero flow). The most common foreign debris that gets into the fluid reservoir in the reservoir is tiny pieces of J-cloth fibre or other cleaning cloth material - no, we are not sure how but it seems to happen with housekeeping staff occasionally.

If it happens we can send you a link to a youtube video explaining how to sort that out. It's very simple stuff. Worst case scenario is you might need new valves or tubes. We stock spare parts and can supply you with those.

We offer a 6 month repair or replace warranty in the event of the unit not working. The warranty is invalidated if the problem is found to be caused by any of the following:

1) Flow tube blockages as a result of physical debris ingress into the fluid reservoir (but that is extremely easy to repair with a DIY fix and spare part tubes and valves are available if required).

2) Burn out of motor from over-use due to excessive run-time.

3) Physical damage to either the unit or the power cable, or evidence of water or fluid exposure inside the motor unit.

We are committed to ensuring your long term satisfaction and delight with your ULV Fogger. Contact us in the very unlikely event that you have any operating problems. We pride ourselves on our personal customer service via either email or telephone. It is very likely that we'll be able to fix an issue over the phone by running through a very basic checklist and therefore save the hassles of a needless return.