If air quality was such a concern why is he not addressing the appalling quality of air on the tube?
Air quality on the underground doesn't effect everyone all day and night..
And as has been said a couple of times they are doing something about it, the problem is that it's not something that can be dealt with quickly or easily as they're trying to adapt infrastructure from 100 years ago.
As has been posted they've done things like change the braking materials on the trains, IIRC they've changed the materials the wheels and tracks are made from (reduces rate of wear thus particulates), and they've been doing things like slowly doing a deep clean whilst also maintaining a normal cleaning routine, not to mention things like the ban on smoking which massively improved air quality on it.
About the only other thing they could potentially do is new ventilation shafts and more active ventilation but that then requires major engineering works and very careful planning because they need to take into account everything from the size of the tube tunnels and the equipment that goes through it, to any new fire risks, and noise that might be transmitted via additional vents to the surface.
I think that is being worked on, but it's very slow and something that you're unlikely to see (I suspect some of the station refurbishments have probably had an element of consideration for airflow).
The tube in the UK is very much a victim of it's age and being the first real one, as it was recognised not long after it was built that there were issues with ventilation etc and even now they're still learning and applying lessons to newer tube networks around the world.
Basically, they can care about the air overground at the same time as underground, and you're very unlikely to be aware of most of the stuff they're doing to the underground.
