Quoted for a professional air quality test after a damp patch - worth it, or the same money on a monitor and a dehumidifier?

I have had one done and I also own a hygrometer and a pin moisture meter, so I can compare directly. The report told me there were spores in the air, which is true of every building on earth, and the only genuinely useful line in it was a moisture reading behind the skirting that I could have taken myself with a meter that cost less than a takeaway. A spore count with no outdoor control sample taken the same day is close to meaningless, so if you do pay for one, that is the question to ask before booking. For a cold wall with a stain, what you actually want is a surface temperature reading and a relative humidity log, not a lab.

For the price of that quote I bought a cheap hygrometer for each room, a pin moisture meter and an infrared thermometer, and had change. The picture came together in a week: that corner sat markedly colder than the rest of the wall, humidity in the room climbed overnight, and the wardrobe against the cold wall was stopping any air moving across it. Pulling the wardrobe six inches off the wall and running the extractor properly did more than any report would have.

Meter first, dehumidifier second, test only if you end up arguing with someone about it. And if anyone in the household has asthma or keeps getting chest symptoms in that flat, that is a conversation with a doctor rather than with a testing company.

There is one case where I would pay without hesitation: if you are in or heading for a dispute with a landlord, a builder or an insurer. Then you are not buying information, you are buying a dated document from a named professional, and that is worth real money. For your own curiosity in your own flat, it is not.

I paid for a test and what I actually bought was a sales pitch, because the company doing the testing also did the remediation and the recommended remediation was, remarkably, extensive. My rule since then: never let the people who test be the people who fix. If you want a test, use an independent assessor who does not sell removal.