SINGAPORE - Even after the dust from this year's great haze settles, a more insidious matter will remain: the issue of fine particles.
Whether in hazy periods or in normal times, one of the main culprits driving Singapore's air quality down is fine particulate matter less than 2.5 microns in size, or PM2.5 for short.
In a haze episode, the PM2.5 lingering in Singapore's air comes from burning plant matter; in non-haze periods, it is from diesel engines and industry. PM2.5, more than larger particles, can have long-term health effects.
When Prime Minister Lee Hsien Loong said last month that Sars is infectious but haze is not, and so Sars can kill while haze will not, he was right - mostly. These fine particles are not some virus passed from one person to the next, causing them to fall very ill.
But that does not mean they have no effect on your health.
PM2.5 is fine enough to enter the bloodstream, and being exposed to PM2.5 increases the risk of death in general and from heart and lung illnesses - and for every 10-microgram increase in annual PM2.5 concentrations, the added risk of dying from overall causes, cardiopulmonary disease and lung cancer goes up 4 per cent, 6 per cent and 8 per cent respectively over a 16-year period.
Newer studies suggest that fine and very fine particles can also reach the brain. The cognitive performance in people exposed to such particles is lower than in those less exposed. The smaller particles may also increase the risk of neurodegenerative diseases like Alzheimer's and Parkinson's.
Worse, they can be a vehicle for other toxic substances, ferrying these into the bloodstream and across the protective bloodbrain barrier into the brain.
But PM2.5 doesn't directly infect people like the Sars virus. Instead, your experience will vary depending on who you are and how long you were exposed.
Most studies and health advisories are based on annual PM2.5, a measure of long-term exposure; the World Health Organisation's short-term PM2.5 guidelines are based on studies of short-term PM10 exposure.
The most vulnerable include young children whose bodies are still developing; the elderly; pregnant women; and those with chronic heart and lung diseases.
Singapore should pay close attention to fine particulate matter - its air quality on this front could be much better. Since 2002, the annual PM2.5 has ranged from 16 to 23 micrograms per cubic metre - double its long-range targets.
In non-haze years since 2005, the 24-hour PM2.5 has been 30 to 42 micrograms per cubic metre. In contrast, the annual level in Australia's Sydney is seven micrograms per cubic metre, and in Los Angeles, US, it is 14.8.
Some of these fine particles are emitted by vehicles or industry, others are formed by chemical reactions in the atmosphere.
To its credit, Singapore has tightened its vehicle emissions standards. By 2014, new diesel vehicles must meet the Euro V diesel standards, which will have particulate matter emissions standards of a maximum 0.005 g per km.
The National Environment Agency (NEA) has stepped up its booking of smoky vehicles, even installing video cameras in some of the vans to catch them in the act. And off-road diesel engines used in construction must also meet emissions standards.
Will this cost more? It depends.
There are some 38,000 old diesel commercial vehicles on the roads, belching smoke. Any incentive scheme to switch to the new Euro standard would conflict directly with a different measure announced in this year's Budget, which allows commercial vehicle owners to extend their COEs for five years to manage rising costs.
But SMRT, for instance, upgraded some of its buses to Euro V five years ago, while SMRT and ComfortDelGro cab companies have also made the upgrade. So consumers may already have accepted these direct costs.
In any case, residents will have to decide what price they are willing to pay for cleaner air - if commercial vehicle owners use the best available technology, this will be a one-off cost for at least some years.
Should the country go further, imposing more stringent guidelines or publishing more data? It depends on what's practical and for whom. The World Health Organisation says that as each person's exposure and experience may vary, each country should do what it needs to achieve the lowest PM2.5 concentration it can.
Even the United States doesn't always meet its own standards. An American Heart Association review found that "at the end of 2008, 211 US counties (or portions of counties) were in non-attainment of the 2006 daily PM2.5 national ambient air quality standards".
More stringent guidelines, then, would be useless if they were not achieved.
However, PM2.5 and its hazards could be better communicated. While the Pollutant Standards Index does include PM2.5 as a subset of PM10, or all particles smaller than 10 microns in size, the fraction of PM2.5 within PM10 may vary. Two PM10 readings may both be "100 micrograms per cubic metre" and produce the same PSI value. But one may be 90 per cent PM2.5, while the other 20 per cent PM2.5 - with very different health effects.
During the haze, NEA put out the 24-hour average PM2.5 readings in micrograms per cubic metre every hour. Last year, it began giving the 24-hour PM2.5 readings thrice a day. Previously, it gave just the annual average.
During the worst of the haze last month, there were calls for the PSI to be changed to include PM2.5, but the PSI can't be changed to do this in the middle of a crisis. It would be too confusing.
Instead, the NEA added more information on PM 2.5, and issued its health advisories based on the more unhealthy of the 24-hour PM2.5 reading and PSI values.
In future, it should go further and either colour-code the PM2.5 reading, or convert it into an individual PSI value, using the same sort of charts used for other pollutants.
In the long term, researchers are keen to work with the NEA to study in more detail how particulate matter is formed and dispersed in Singapore's tropical urban context, and where it goes.
The bottom line? PM2.5 matters - even after the haze is over.
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