The National Environment Agency (NEA) on Saturday said it uses a standard set of methods and equipment based on the United States' Environmental Protection Agency (EPA) regulations.

Handheld particle meters might be inaccurate due to moisture in the air, which affects light-scattering meters, said NEA deputy chief executive Joseph Hui, who was responding to questions about the Pollutant Standards Index (PSI) and its accuracy.

Some members of the public have posted their own readings attained after they conducted their own informal measurements using such meters, which recorded readings higher than the official PM2.5 count.

"The only way we can compare is to have these instruments carry out their measurements right next to ours," said Mr Hui.

The PSI is derived from measurements at 11 ambient air monitoring stations, and uses five key pollutants including PM10, or all particles smaller than 10 microns, reflecting the pollutant with the highest concentration.