There is a plethora of devices aimed at very narrow markets. If you aren’t in that market you will never use one of those devices.
Every now and again you may be asked to measure or control some phenomenon that is outside your expertise and may require considerable research and development. Isn’t it nice to discover that there are many niche products that meet that and only that need?
I once developed the controls for a sprinkler system in an aircraft hangar. Because the fire-quenching foam is corrosive, the customer didn’t want lightning activity in the area to trigger a false alarm and unnecessarily damage expensive aircraft parts. At the time we had to buy an expensive stand-alone product, but today there is a chip just for the purpose- the AS3935 Franklin Lightning Sensor IC .
Natural gas can have water vapour mixed in with it. When pressurised, the water vapour condenses and rusts the metal container. This is considered a BAD thing and so I was detailed with creating a dryer technique to heat the gas and get rid of the water. It would have been convenient to measure the relative humidity but at the time specialized ICs were not available and the available sensors were not that accurate.
Now there seem to be a few, all of which include a temperature sensor. There are some Honeywell offerings that are available both in analog and digital outputs. Servoflo carries a series of devices for measuring humidity. Silicon Labs produce a range of humidity sensors some of which have quite good accuracy specifications. Sensirion is yet another provider of humidity sensors.
Associated with relative humidity is the dew point, and yes there is a sensor for that, also available from Servoflo.
Want to measure dust? I never have, but just in case there is the Amphenol Telaire Dust Sensor . Whilst we are yakking about air quality if you