How mold fogging actually works

Why a fog rather than a spray

A spray is a delivery method for coating a surface you can see and point at. That is fine for the face of a wall and useless for the reason most mold problems exist, which is that the growth is on the other side of it.

The behavior that matters is particle size. Atomise a liquid finely enough and it stops behaving like falling droplets and starts behaving like a gas: it disperses to fill the volume it is released into, and it goes where air goes. A room is connected to its wall cavities by more paths than people expect, including outlet and switch boxes, plumbing and wiring penetrations, and gaps at the top plate. The fog follows all of them.

It is called dry because the particle is small enough that surfaces do not end up wet. That matters for what can be treated: electronics, soft furnishings and finished surfaces come through a treatment rather than being soaked by it.

What the two fogs do

They are different products doing different jobs, and running only one of them is a common shortcut.

The first is a sterilant. Its job is to kill what is alive right now, everywhere the fog reaches, including the growth nobody has seen because it is inside an assembly. This is the step that does the actual remediation.

The second, applied after the space has been vacuumed again, leaves a protective layer on the treated surfaces. Its job is different: to make regrowth start from nothing rather than from whatever survived. That second application is what a regrowth guarantee is written against, and a job with only the first fog has no equivalent.

Kill what is there, remove what is left, then make the surface harder to recolonize. Three jobs, and no single product does all three.

Why there are two vacuums as well

The first HEPA vacuum comes before any fog. Its purpose is unglamorous: remove loose spores, dust and household film so the fog meets the material rather than the layer sitting on top of it. Fogging a dusty surface treats the dust. Skipping this step is the most common reason a fogging job underperforms, and it is invisible in the finished result.

The second comes after the first fog, and it exists because dead spores are still spores. A killed colony left in place is still allergenic material in your building, and for anyone reacting to mold rather than merely objecting to it, that distinction is the whole point. Removing the remains is part of the job rather than an optional extra.

What it cannot do

Two categories, and an honest company names both before quoting.

The first is saturated material. The fog treats surfaces and cavities; it does not rescue drywall that has been sitting wet, insulation that has absorbed enough water to have stopped insulating, or framing with genuine structural rot. Killing a fungus does not give a joist back its strength. Those sections still come out, and they get identified at the inspection rather than discovered partway through the job.

The second is the source. The fog treats what is growing. It does nothing at all about the bathroom fan venting into the attic, the crawlspace with bare soil, the slab leak or the roof flashing that has been failing for two winters. Every one of those keeps producing water after a perfect treatment. This is why the work starts at an inspection rather than at the fogger, and it is the reason behind the line the company organizes itself around: what is not corrected at the source always comes back.

How you know it worked

Not because the contractor says so. Post-treatment air samples go to a laboratory that did not perform the remediation, and they are read against an outdoor control sample taken the same day, because an indoor spore count means nothing without the air outside to compare it to.

California does not require any of that. There is no state license for mold remediation and no mandated residential clearance standard, so a remediator issuing their own certificate of completion is doing everything the law asks. The reason to want somebody independent reading the result is that nobody else is going to.

Further reading: the full process page sets out all five steps in order, and the testing and clearance page covers what the laboratory report actually says.