The 24-hour PSI rolling averages even out the peaks and troughs of hourly readings and provide a useful indicator of air quality over a period of time.
Individuals are advised to use three-hour PSI or one-hour PM2.5 concentration levels to decide on whether they should pursue immediate outdoor activities during hazy weather.
Three-hour PSI updates are more easily accessible but less valuable as a decision-making tool.
When past three-hour PSI levels are rising consecutively, a current three-hour PSI value of 92 may mean that the most recent hour's PSI has exceeded 100.
Without real-time PSI information, one has to estimate the latest hour's PSI by considering the current three-hour PSI and the trend of past three-hour PSI readings. Sometimes it may be easier to gauge air quality just by looking outside.
An unexpected jump in the number of hot spots and/or sudden strong unfavourable winds from one hour to the next may cause the PSI reading to spike.
For example, PSI readings last Saturday jumped from 126 at 2pm to 142 at 3pm.
Half-hourly PSI updates in real time, including the time the readings were taken, are more meaningful for individuals, schools and organisations to make decisions on immediate activities.
Including the time of the readings completes the information at a glance without the need to check the time separately. This helps especially if a system glitch delays the PSI update beyond half an hour.
The half-hourly readings eliminate the guesswork and allows organisers and companies to take timely measures for those who plan to stay outdoors to reduce prolonged or frequent exposure to harmful haze particles.
Tan Lay Hoon (Ms)