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Florida dew points sit above 70°F for roughly half the year. Air conditioning removes some moisture as a side effect of cooling, but on mild days the system barely runs and the house quietly climbs. Mold needs sustained surface moisture, and sustained indoor humidity above roughly 60 percent supplies it. What to look for:

• Capacity sized to the space, then some. An undersized unit runs constantly and still loses. Manufacturer coverage figures assume a moderate climate, so a Florida house needs more capacity than the box suggests.
• A continuous drain option. A bucket you empty by hand gets emptied for two weeks and then stops. A gravity drain to a laundry sink or a built-in condensate pump is what makes it survive month three.
• A real humidistat, not just a dial. You want to set a target near 50 percent and have the unit hold it, rather than guessing between low, medium and high.
• Auto restart after a power cut. Florida loses power. A unit that stays off until somebody notices is a unit that was off for a week.

A true HEPA filter captures at least 99.97 percent of particles at 0.3 microns, the size range that is hardest to catch. Mold spores are generally larger than that, so a genuine HEPA unit handles them well. Two things to watch:

• Avoid HEPA-type and HEPA-like. Those phrases exist precisely because the filter does not meet the standard.
• Match the rating to the room. A purifier rated for a bedroom does very little in an open-plan great room. Air changes per hour matter more than the marketing square footage.
• Budget for filters, not just the unit. A loaded filter stops working. If the replacement cadence is not realistic for you, pick a different unit.
• Skip ozone generators. Anything marketed as producing ozone or activated oxygen to treat mold is not something I would run in an occupied home.

A purifier cleans air passing through it. It does nothing about growth inside a wall cavity, and a purifier running in a room with an active moisture source is treating a symptom.

• A pinless moisture meter for scanning drywall and masonry without leaving holes. Readings are relative and are influenced by density, salts and nearby metal, so treat a high reading as a place to look closer rather than a verdict.
• A pin meter if you want a firmer number in wood, framing, baseboards or subfloor. Pins read electrical resistance between two points and are less easily fooled, at the cost of two small holes.
• A logging hygrometer in each problem area. This is the one most people skip and the one that teaches you the most. A spot reading tells you the humidity right now. A log tells you the house sits at 68 percent every night at 3 a.m., which is the fact that actually explains the mold.
• An infrared camera for scanning the surfaces where condensation forms: supply registers, exterior wall corners, and the wall behind an air handler. It reads a whole surface at once, which is the part a single-point thermometer cannot do.

One caveat on the camera, and it is the reason I mention it last. It reads surface temperature, not moisture. A cold patch is a place worth investigating, which is genuinely useful, but thermal images get read as showing mold constantly. They do not show mold, and no infrared camera at any price does.

Home test kits that grow whatever lands on a petri dish are the most common purchase I talk people out of. Viable mold spores are present in every building and in the outdoor air, so a settle plate will always grow something. Without a calibrated volume of air, an outdoor reference sample taken at the same time, and laboratory identification, the result cannot tell you whether your indoor levels are unusual. A plate that grows colonies proves the plate was exposed to air.

Nothing on this page substitutes for finding the moisture source. If a room keeps reading high after a dehumidifier is running correctly, or growth returns after cleaning, the source has not been found yet, and that is an assessment question rather than a shopping question.