Environmental activists presented the Thai government with an hourglass filled with dust on Thursday as part of a plea to tackle the hazardous levels of air pollution that have hung over the capital in recent weeks. The Pollution Control Department warned this month that the level of particulate matter smaller than 2.5 micrometres, or PM2.5 dust, in the city had hit unhealthy levels and asked children to stay indoors. Thailand’s Excise Department is now studying a possibility to introduce vehicle pollution tax on motorcycles in a new effort to improve air quality.