Which goes to show you're far better off filtering at source while pollutants are at very high concentrations. Far better than trying to remove it when it's down to a few tens of µg / m⁻³.

A damn sight more efficient, and no doubt far cheaper too.

Which rather shows up filtering buses as a lovely bit of greenwashing to avoid mentioning lack of adequate UK regulation. Regulation requiring all polluters filter PM10 and PM2.5 at source -- chimney or tailpipe -- wouldn't be as much of a story, but would actually achieve something worthwhile.