Now we live out in a rural area and its very noticeable how cleaner windows and paintwork are - the same also applies to our lungs!

You know a part of me did think you were about to say you had been monitoring the air in my garden, and given Facebook, Snowden, Russia, NSA and various Acts of Parliament I would not have been that surprised by one more person watching me :-)

Your sensors are going to break. Depending on the environment and the mode of operation of the sensor, you'll see between a one month and one year lifespan.

Why? Exactly because of the thing you want to measure: dust, oil particulates, water vapour and chemical reactions will quickly deteriorate the accuracy of the sensors. To properly protect them you will need at least a moisture trap and some kind of air pump to get a constant flow of air through your sensor. You most likely also want to filter out large particulates (if you're interested in sub 2.5 micrometer measurements).

That said, you can [1] get quite accurate readings from the sensors within their specified operation environment (sub 80% Relative Humidity).

[1] https://www.mdpi.com/2073-4433/10/2/41/pdf-vor