The PSI is based on a scale devised by the United States Environmental Protection Agency (EPA) to provide a way for broadcasts and newspapers to report air quality on a daily basis.
The index figures enable the public to determine whether the air pollution levels in a particular location are good, unhealthy, hazardous or worse.
The following PSI table is grouped by index values and descriptors, explaining the effects of the levels, according to Singapore's National Environment Agency (NEA).
Singapore has been regularly hit by smoke haze from forest fires in nearby Sumatra, Indonesia, brought over by wind.
These forest fires have been attributed to the slash-and-burn method favoured by several large plantation owners to clear their land, as opposed to a more expensive and inconvenient mechanical approach using excavators and bulldozers.
[10] In June 2013, severe haze hit Singapore, pushing the nation's PSI into Hazardous levels for the first time in its history.