Advice also applies to mold. A lot of people worry about mold in their house. It's actually quite straightforward to determine if this is actually a problem: Home Depot sells "test kits" for a few bucks that are essentially petri dishes. Buy two of these, put one inside where you are worried and then the other outside and wait 3-4 days. If they look radically different, then send the inside one in for actual analysis, which is an additional ~$40 USD. Then and only then do you need to action and by that point you know exactly what the problem is so you don't have to pay some "expert" to sell you some massively expensive mitigation strategy that you probably don't need.

Is it common to have an invisible mould problem? Lots of people are in the situation where there is really visible mould but the problem is getting their landlord to fix it, without getting into allegations over drying clothes inside etc.

There's a lot of fear over things like black mold in the states. It's extremely rare but it can be life threatening, and it's one of those things that people see and get nervous about when they see that for instance their HVAC vent has mold spots, that they are breathing toxic air. 9 times out of 10, it's totally fine. This method mentioned above gives homeowners the peace of mind that they aren't poisoning their families accidentally for an extremely affordable price. The alternative of calling mold remediation experts in is going to be extremely pricey, and those people cannot be trusted to be upfront on whether your mold problem is 1. actually a problem and 2. actually dangerous - because they make money on selling expensive remediation solutions.

I would expect that mold can build up ie in ventilation where it remains unseen unless it literally clogs he whole system. You can't see the spores an often there is no strong foul smell.

honesty depending where you are such allegations might be very baseless & meaningless for many reasons like

- high base humidity of the place (e.g. the city where I live has a yearly avg. humidity of 70%, but specific to my apartment and ignoring the dry seasons over 80% is the norm (also for context not tropical but central EU, it's stuff like 20C+85% humidity). So airing out your room might increase air humidity...

- in small apartments it's the quite often norm that the side effect of taking a show can temporary rise humidity quite a bit, even if you ventilate properly. Most bathrooms in small appartments are just not well designed wrt. this (context I'm not speaking about long hot showers, but short normal warm showers).

- in small bed rooms night sweat can rise humidity by quite a lot, mostly if you are slightly sick but anyway

- just basic flowers can raise humidity, too

excluding dry areas IMHO for most no large apartments the landlord has forsaken any right to claim it's your fault if they don't provide reasonable measurements against humidity (even if it's just a half way decent (noise wise) air humidifier. Reason: Just standard normal expected usage will cause to high humidity level even if you do air out the apartment twice a day (which depending on weather conditions you might not even be able to do)

Sadly that isn't necessary the local laws/regulations POV.

Our local library system offers these to borrow for six-week spans (or whatever the length of the testing is). It’s a one-and-done deal and you’re good for as long as you stay in your home. Batteries included.