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The Pollutants Standard Index (PSI), devised by the US Environment Protection Agency, is based on the levels of five pollutants in the atmosphere – ozone, nitrogen dioxide, sulphur dioxide, carbon monoxide and particulate matter.

The concentration of each is measured and converted to a number on a scale of 0 to 500. The highest of the five numbers sets the reading. The range 0 to 50 is classed as “good”, 51 to 100 is “moderate”, 101 to 200 is “unhealthy”, 201 to 300 is “very unhealthy”, more than 300 is “hazardous” and levels over 400 can be life-threatening to the ill and elderly and those who have respiratory problems such as asthma.

Malaysia uses a slight modification of this index called the Air Pollution Index. The scale is the same as the PSI up to 50, but then diverges to slightly higher numbers. For example, a PSI of 200 converts to about 257 on the Malaysian scale and 500 to about 577. Sarawak, where a state of emergency has been declared, recorded a maximum of 658 last Friday.