The group was led by deputy leader Ongart Klampaiboon and Suchatchavee Suwansawas, the party’s policy chief for the Bangkok metropolitan area.
The PM2.5 level was higher than Thailand’s safe standard of 50 micrograms per cubic metre of air.
PM2.5 refers to dust particles that are 2.5 micrometres or smaller in diameter and can be easily inhaled. Long-term exposure to PM2.5 can be fatal.
The Democrats will make PM2.5 an urgent issue in Parliament with their Clean Air Act if they win the May 14 election, Suchatchavee said.