Insufficient evidence from studies to use an hourly PSI

Today Online 3 Oct 15;

We thank Mr Tan Zhi Rui for his suggestions in “Hourly PSI readings would allow for better decision-making” (Sept 28). The National Environment Agency (NEA) is providing hourly, real-time haze information on our various platforms.

Since April last year, the 24-hour Pollutant Standards Index (PSI), three-hour PSI and one-hour PM2.5 concentrations have been published hourly, on the hour, on the NEA website, the haze microsite ( and our myENV mobile app.

The scale used to convert PM2.5 concentrations to the PSI has been derived based on health studies of exposure over a 24-hour period. This is why the NEA uses the 24-hour average concentrations to compute the PSI.

Although there have been recent studies of sub-daily or shorter PM2.5 exposure, the evidence from these studies is insufficient for the development of one-hour PSI based on exposure to PM2.5 for a one-hour period.

During periods of transboundary haze, the primary pollutant determining the PSI level is fine particulate matter, or PM2.5. Only the 24-hour PSI value has a corresponding Ministry of Health advisory because scientific and epidemiological studies of the health effects of exposure to particulate matter have been based on this duration of exposure.

The forecast of 24-hour PSI levels, which the NEA issues every evening during haze periods, and the corresponding health advisory can be used to plan ahead, such as for activities for the next day. For a guide to more immediate activities, the three-hour PSI and one-hour PM2.5 concentration levels can be used as indicative measures to make adjustments to daily activities.

For example, if the three-hour PSI and one-hour PM2.5 concentration levels are high, people may wish to postpone strenuous outdoor activities such as jogging.

The health impact of air pollution is related to the concentration levels of pollutants, the duration of exposure, as well as the individual’s health status and level of activity.

For more information, members of the public may visit our website ( and the haze microsite, follow us on Twitter (@NEAsg) and Facebook ( or download the myENV app.