A cloud of ultra-fine dust particles known as PM2.5 has returned to Bangkok due to cooler temperatures, with air quality indicators across the capital registering unhealthy levels of the harmful micro pollutant.

According to the Pollution Control Department (PCD), the concentration of PM2.5 pollutants in Bang Phlat reached 53 microgrammes per cubic metre (µg/m³) yesterday -- exceeding the PCD's safe threshold of 50 µg/m³.

The PCD said PM2.5 levels are likely to increase in the next couple of days across the city, and warned residents about the health risks posed by the micro pollutant.

Officials said a high pressure system above China -- which has lowered temperatures in the northern part of Thailand -- is causing dust to accumulate as the air stagnates.

While Bangkok has not registered unhealthy levels of PM2.5 pollutants in the past several months, earlier in the year PM2.5 levels soared beyond unhealthy levels, prompting officials to adopt emergency measures -- including spraying water from high-rise buildings.