What is the fine-grained topic of the following text: The 28 cities in and around the smog-prone Beijing-Hebei-Tianjin region were ordered in August to draw up plans to thin traffic and reduce industrial activity from October this year to March next year, part of efforts to slash concentrations of hazardous airborne particulate matter that is less than 2.5 microns (PM2.5) by at least 15 per cent year-on-year. Only return the topic.
Air pollution reduction plans in the Beijing-Hebei-Tianjin region