Using a roof-mounted filtration system, a Bluestar bus in Southampton, UK, is now cleaning the air as it drives. The bus, operated by Go-Ahead Group, is fitted with a air filter that representatives of the company say will capture fine particles as it moves. After being ‘cleaned’, the air is blown out of the back of the bus, making the air behind the bus cleaner than that in front of it. This prototype is part of Southampton’s response to the World Health Organization’s warning that the city is “at its limit of unsafe air pollution”.