Hourly PSI has relevance to urban Singapore

CHONG LEE MING Today Online 6 Oct 15;

I refer to the National Environment Agency’s reply, “Insufficient evidence from studies to use an hourly PSI” (Oct 3), to Mr Tan Zhi Rui’s letter, “Hourly PSI readings would allow for better decision-making” (Sept 28).

While I note its explanation that “scientific and epidemiological studies of the health effects of exposure to particulate matter” have been based on a 24-hour duration, some factors unique to Singapore are worth considering.

First, many large countries have temperate continental weather patterns, in which the weather tends to change slowly, while Singapore has a tropical coastal weather pattern, where the weather changes quickly.

In larger countries, a 24-hour Pollutant Standards Index (PSI) would therefore be fairly accurate in measuring a person’s exposure to haze.

In Singapore, due to quick changes in wind direction, the haze condition 24 hours ago might differ substantially from the condition now. In this situation, the 24-hour PSI is only a mathematical average.

Second, Singapore is densely built-up, with a good public transport system. Hence, except for some activities such as construction, it is not necessary for most people to stay outdoors for a long time.

For example, one needs only to spend 30 minutes outdoors at most before boarding a bus.

Third, Singapore is an affluent country; a significant proportion of households have air conditioners, public transport is air-conditioned and there are many air-conditioned shopping centres.

Kudos also to grassroots organisations for opening up air-conditioned facilities to residents needing refuge from the haze. Most people here would therefore not have to be exposed to the haze for a long time, much less a 24-hour period.

During hazy conditions, individuals’ decisions tend to be short-term in nature, such as whether to go outdoors, stay indoors or be in air-conditioned places. Nevertheless, most people want to minimise their exposure to the haze.

An hourly PSI would help them achieve this. Moreover, for people with health conditions and who are more sensitive to the haze, a 24-hour or three-hour PSI might be misleading.

While the 24-hour or three-hour PSI may appear acceptable due to a lower PSI earlier, the haze condition at the moment might pose an unnecessary risk to them.

Therefore, while the 24-hour PSI may be useful for macro-level decision-making — for example, whether to shut schools and stop other activities such as construction — an hourly PSI has relevance to Singapore’s urban environment at the micro level.

Individuals can then make informed decisions on how to carry out their activities.