For the uninitiated, air purifiers work by pulling air through a series of filters—typically a HEPA filter, which captures 99.97% of particles as small as 0.3 microns (think: pollen, dust, and ...

Particulate pollution on the Tube can be up to 30 times higher than the levels in roadside air, depending on the line. This is due to the lack of fresh air, trains’ braking systems, and old ...