The 1-hr PM2.5 concentration is the average concentration of PM2.5, in micrograms per cubic metre (µg/m3), measured in the air over the past hour. The National Environment Agency in Singapore reports PM2.5 concentrations for five locations across the country. During the haze episodes, the NEA recommends to refer to 1-hour PM2.5 concentration readings which are a better indicator of the current outdoor air pollution for immediate activities. For example going out for a run or a walk. On the other hand, the Pollutant Standards Index (PSI) values are based on the last 24-h data consequently PSI readings are more useful when planning for activities in the coming days or events during which participants will be outdoors for an extended period of time.
The NEA calculates classifies the concentrations of 1-hr PM2.5 in four categories as follows: "normal", "elevated", "high" and "very high" and for each category provides health recommendations.