My experience is that they get dirty very quickly here in SE London.

Yup they sure do, air around London is very dirty and still worse when close to a duel carriageway/ Motorway.

Bought one to replace the one in my partners car. Old one looked good as new so stuck it in the shed until next year .

The more I think about pollen filters, the more I realise what a con they are, considering the tiny amount of time spent in a car. This is an extract from Halford's absurd blurb....

What’s more, if you live in an urban or industrial area, without proper filtration you could be exposed to harmful toxic gases and air pollution that can cause headaches, nausea and fatigue. As professionals, we recommend that you replace your pollen air filter once a year - or every 10-15,000 miles. However, if where you park is surrounded by trees, or if you live in a heavily polluted area, it’s advised that you replace it more frequently.

.....looks as though I've saved myself the cost of at least sixteen pollen filters just for the family Focus!

The more I think about pollen filters, the more I realise what a con they are, considering the tiny amount of time spent in a car. This is an extract from Halford's absurd blurb....

The toxic gases bit looks pretty absurd but as I said upthread pollen is a serious allergen and is better off outside the car than in. Whilst there are many days I either don't use the car or only travel locally there are others, going on holiday for example, where I'm in it all day for a couple of days.

People who use a car for work will be in there for a chunk of most working days.

I'd also assume that dust trapped in the filter isn't clogging up the fan etc.

Indeed - as part of my (now former) career, I specified filtration systems for air handling plant of various sizes for buildings, and they won't filter out gases. Small particles of dust, dirt and pollen, yes, depending upon the grade/how fine the filter is. Vehicles will have similar filters.

More fancy systems akin to CATs are possible, but only in specialist industries and vehicles that need robust, long term fresh air such as the military. Certain equipment such as gas masks can proect against certain chemicals and gases, but only for quite brief periods before needing replacement and decontaimination.

It's the reason why modern cars have an air recirculation system, to avoid sucking in exhause fumes and other external pollutants and despositing into the cabin, as well as helping heat or cool it more rapidly when needed.

It is also why I make sure I vetnialte my car well when driving in less built-up areas with clearer roads, so the air inside the cabin is as good as I can get it when I have to switch to recirculated air in heavy traffic / in town.

If I recall, my car's maintenance schedule has my cabin filter being replaced every other service.

Indeed - as part of my (now former) career, I specified filtration systems for air handling plant of various sizes for buildings, and they won't filter out gases. Small particles of dust, dirt and pollen, yes, depending upon the grade/how fine the filter is. Vehicles will have similar filters.

I once worked in a building which had (IIRC) electrostatic filtering. Put in when the building was built, it was an enormous unit, with very secure entry. It would emit frequent, loud cracks and there was a smell of ozone. Are such systems still used?

I didn't come across that type of system during my career, though that doesn't mean they don't still exist today. Admitedly the overwhelming majority of my work involved air intake filtering - i.e. fresh air to buildings, labs, etc, rather than exhaust filtration of nasties from industrial and medical processes and equipment.

At a local industrial complex, they work with precious metals on a variety of processes, and they have 'ordinary' filters on the exhast ducts to trap those particles, wherby when full, the filter gets burned and the metals recovered, because the amount really adds up over time.

Other residual chemicals may not (all) be filtered, as I can often smell certain chemical smells (e.g. [I think] acetate) from the chimneys when the wind is occasionally blowing in the direction of my housing development.

Modern filtration via panel or bag filters can get down to very tiny particles, though obviously not to the atomic scale. I suspect some older methods may be still suited to certain industrial processes, but higher grade physical filters may now be used more often because they are cheaper over the long run.

My home vacuum cleaner has about 5 filters, including on the exhaust, which is of the hepa type. Cleaning and eventually replacing them is certainly not an easy or cheap job for me! I'd rather have the old style filter and bag combined.

I also should really check my car's cabin (pollen) filter to see whether it does get changed as per the maintenance schedule, as I'd bet good money that it's that sort of thing that some dealers will scrimp on to get more profits.