The newest of the portable agents and the one most often misunderstood, because the colour convention people learned does not include it. A mist unit holds de ionised water and forces it through a nozzle that produces a very fine spray: the mist cools the fire and the steam it generates displaces oxygen locally, and because the droplets are small and the water is de ionised, the stream does not conduct in the way a jet does.
A white fire extinguisher label, and what it means
The body of a UK extinguisher is red and a colour PANEL identifies the agent: white for water mist, alongside the older set of red for water, cream for foam, blue for powder, black for carbon dioxide and yellow for wet chemical. A white panel is therefore not a different class of fire, it is a different agent, and the classes it covers are printed beside it as ratings. That distinction matters because the colour tells you what is inside and only the rating tells you what it may be used on, which is the same trap people fall into with powder.
Where a water mist unit replaces two extinguishers
The practical attraction is coverage. A mist unit with the right rating deals with class A solids, class B liquids and, in the higher rated models, class F cooking oils, and because it is dielectrically tested it may be used near live electrical equipment at the distance stated on the label. In a small office, a shop or a village hall that can mean one unit on the wall instead of a water extinguisher plus a carbon dioxide unit, which is fewer services to pay for and one less decision for somebody to get wrong in an emergency. It leaves almost no residue, which is why heritage buildings, server rooms and kitchens have adopted it, and it uses a small volume of water, which limits the damage below.
What it cannot do, and the tests that matter
It is not for metal fires, which need a specialist powder, and it is not a substitute for a fixed system in a high hazard space. Beyond that the honest answer is that models differ far more than in other agent families, so the decision is made on two printed statements rather than on the category. The FIRE RATINGS, which give the tested performance on each class and are the only comparable measure between units. And the DIELECTRIC test statement, which says the unit was tested at a stated voltage and gives the minimum distance from live equipment, because a mist unit without that statement is a water extinguisher with a fine nozzle. Check both on the label and ask for the certification, and remember that a kitchen still needs its blanket and, where there is a fryer, a wet chemical unit.
Questions people ask about water mist extinguishers
Is a mist unit dearer to service?
The service regime is the same as any other portable extinguisher: a monthly visual check by somebody in the building, an annual basic service by a competent technician, and an extended service at the interval the standard sets. The purchase price is usually higher than a water extinguisher and the arithmetic that matters is how many units it replaces, because one unit serviced annually is cheaper than two.
Can it be used on a chip pan?
Only if the label carries an F rating, and even then a fire blanket is the better first response in a kitchen and a wet chemical extinguisher is what the risk assessment should reach for a fryer. The reason to be careful is that cooking oil reignites above its auto ignition temperature, so the agent has to cool as well as smother, and the F rating on the label is the evidence that the unit was tested to do it.
Does it replace a CO2 unit next to a server rack?
Often yes for the portable duty, provided the dielectric statement covers the distance you would actually use it at, and it has the advantage of leaving no residue and of not displacing the oxygen in a small room the way a carbon dioxide discharge does. What it does not replace is a fixed suppression system where the risk assessment has called for one, and it does not remove the first step, which is isolating the supply where it is safe to do so.