Yes, in some cases it can — and the honest answer is more useful to you than a flat no. Dense security fog can look like smoke to certain smoke detectors. Whether that’s a real risk on your premises comes down to the type of detector in the fogged zone, not the fog system itself. A competent installer plans around this. They don’t ignore it.
Why this happens
Most smoke detectors work in one of two ways. Optical detectors, sometimes called photoelectric, watch for light scattering across a beam inside the unit. Ionisation detectors work differently, sensing changes in a small electrical current instead.
Dense fog is made of fine particles suspended in the air. To an optical detector, that can look a lot like real smoke, because both scatter light in a similar way. That’s the mechanism, not a guess. It’s why optical and dual-sensor detectors are the ones most likely to react if they sit inside a heavily fogged zone.
The two standards involved, and why they’re separate
Security fog systems in the UK are built to EN 50131-8:2019. That standard covers fog devices as part of an intruder alarm system. It says nothing about fire detection, because that’s not what it’s for.
Fire alarm systems in UK commercial premises sit under a different standard entirely: BS 5839-1. Two separate standards, two separate jobs. Neither one tells you how a fog system and a fire detector will behave together in the same room. That’s a design and installation question, not a certification question.
What a competent installer actually does
- Checks detector type in the protected zone. If optical or dual-sensor detectors sit inside the area the fog will fill, that’s flagged before install, not after.
- Plans zoning with the fire system in mind. On larger or more complex sites, this can mean coordinating directly with whoever manages the fire alarm.
- Doesn’t treat it as a footnote. This is exactly the kind of detail a proper site survey exists to catch, rather than something to discover after the first activation.
None of this makes a fog system unsuitable. It makes detector type a genuine item on the installation checklist, the same way alarm panel compatibility is.
What happens if it does trigger a detector
It’s worth separating two different things: an unwanted alarm signal, and an actual fire risk. Security fog is a water and glycol mix, not combustion. There’s nothing burning, and nothing toxic in the air. If an optical detector did react to it, that’s a false alarm to manage, not a fire to fight.
That’s still worth avoiding, since an unwanted fire alarm means evacuating a building and, often, a call-out. It’s simply a different category of problem from an actual fire. Good zoning and the right detector choice are what prevent it from happening in the first place.
Is this a reason not to install one?
No. It’s a reason to have the conversation before, not after. Thousands of fog systems operate in UK retail, warehouse and cash-handling premises without this becoming a problem, because it’s designed around at the survey stage. The risk isn’t the fog system. It’s skipping the check.
Frequently asked questions
Does security fog set off fire alarms?
It can, in premises where optical or dual-sensor smoke detectors sit inside the fogged zone, since dense fog can scatter light in a way that resembles smoke to that detector type. A proper site survey identifies this before installation, not after.
Which type of smoke detector is most likely to be triggered by fog?
Optical, or photoelectric, detectors — and dual-sensor units that include an optical element. They detect light scattering, which dense fog can also cause. Ionisation detectors work on a different principle and are less consistently affected by this specific mechanism.
Does security fog meet a fire safety standard?
Security fog systems meet EN 50131-8:2019, the standard covering fog devices within an intruder alarm system. That’s a separate standard from BS 5839-1, which governs UK fire alarm systems. One doesn’t certify compatibility with the other.
What should my installer do to avoid this?
Check the detector type in any zone the fog will reach, and plan zoning around it during the site survey. On larger sites, that can mean a direct conversation with whoever manages the fire alarm system.
Is this a reason not to install a security fog system?
No. It’s a reason to raise it at survey stage, where it’s a straightforward thing to plan around, rather than something to find out the hard way after the fact.
Want a survey that checks this properly from the start? Get a quote and we’ll walk through detector type and zoning before anything’s fitted.
This is one of four questions covered in the full security fog standards & safety guide.
