The whole lot has to be spotlessly clean for it to work, in London they clean the seats on the trains and do the maintenance work there, but the actual tunnels get cleaned once in a blue moon. The whole thing needs to be polished and polish-able. London Underground isn't built like that.
A fair amount of the dirt is from the streets above, so tyres that wear to also wear the road surfaces, that sort of cruft gets drawn all the way down into the Underground.
As for actual pollution levels then this depends on where you are on the network. TfL don't measure enough to give a heatmap of pollution, updated hourly. Your statistic from the Guardian sounds about right, generally speaking though the particulates are at 10x the EU max levels.
In a communist style country you could have an army of people polishing the tube stations and the tunnels as part of some National Service. But, in a Western Democracy where everyone has to pay a four figure sum for rent + travel no matter how they try and mix it, this isn't going to happen.
They have tried to create magic sweeper trains to hoover everything up but it hasn't been a success. Really that is what is needed as well as the trains being built to 'hoover as they go', using the air-con to filter out all the cruft.
In eastern europe we have very effective ventilation - for number of reasons, one of them is that it was built much later, the other is that some of the station were built to serve as a nuclear shelter so they had to have very safe ventilation systems...