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 dow

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.