Clear the air on pollutant levels

Andy Ho, Straits Times 19 Jun 10;

IN THE past half year, scientists have determined that inhaling fine air pollutant particles can actually kill people with heart conditions.

Last December, a US Environmental Protection Agency (EPA) review of the best studies found this relationship to be established beyond reasonable doubt. Last month, the American Heart Association (AHA) issued a statement reiterating this 'causal relationship', saying in regard to all available data: 'At present, no credible alternative explanation exists.'

Particulate matter is a mixture of tiny particles and liquid droplets. While larger particles are coughed or sneezed out, tinier particles penetrate the lungs, where serious damage ensues. The tiniest particles also pass through lung tissue into blood and thence circulate throughout the body. In the blood, they can cause harm to the inner lining of small blood arteries. This then triggers off various cellular processes that eventuate in heart damage.

The AHA concluded that short-term exposure of 'over a few hours to weeks' to PM2.5 can trigger fatal heart attacks, heart failure and heart rhythm problems. PM2.5 are particulate matter with diameters of under 2.5 micrometres. Also, 'longer-term exposure (eg, a few years)' to PM2.5 'reduces life expectancy... by several months to a few years'.

Notice that although they are inhaled into the lungs first, the novel finding is that PM2.5 cause heart problems. Found in car emissions, among other things, PM2.5 are 30 times tinier than a human hair. So they penetrate the lungs deeper than PM10 can. The latter, particles under 10 micrometres in diameter, are referred to in the daily air quality report posted on the National Environment Agency's (NEA) website. Coming from dusty roads and dusty industrial sites, among other sources, PM10 are about seven times smaller than a human hair.

Although PM2.5 levels are already being monitored daily throughout Singapore, they are not divulged publicly. Only the annual average PM2.5 level is offered in the NEA yearly report. The agency says there is 'no internationally harmonised protocol' for reporting air quality.

Many developing countries cannot afford the very expensive equipment to monitor PM2.5 levels but the 11 air monitoing stations in Singapore are already equipped to do so. The NEA points out that Britain and France report only PM10. But Taiwan already reports PM2.5.

In 2008 and last year, the annual average PM2.5 levels here exceeded the safe thresholds set by both the EPA and the World Health Organisation. This is a matter of concern since the AHA has opined that there is, in fact, no 'discernible 'safe' threshold' for both short- and long-term PM2.5 exposure. For this reason, it recommended public efforts to lower PM2.5 'below even' official safe levels.

Compared to PM10, the smaller PM2.5 can not only penetrate deeper into the lungs but also carry more concentrated toxic substances, including transition metals and endotoxins. The reason is that the smaller a sphere's diameter, the larger its surface area-to-volume ratio. This means that a higher concentration of toxic substances is absorbed on PM2.5 surfaces than PM10. So the former carry more toxic substances into our bodies than the latter.

A particular class of chemicals that PM2.5 carry is called polycyclic aromatic hydrocarbons (PAHs). Formed when fossil fuels and other biomass are burnt, PAHs are known to cause genetic damage. Studies the EPA also reviewed suggest that PAHs probably can cause lung cancer.

Diesel and petrol engines are major sources of PM2.5, with forest fires yet another. (The Indonesian forest fires usually occur from July to September.) A third source is photochemical reactions that occur among gaseous pollutants emitted by power plants and factories.

The AHA said PM2.5 exposure was a 'modifiable factor'. Since traffic is a main source of PM2.5, one should avoid non-mandatory trips, rush hour travel and also roads with traffic jams. One should stay indoors, close the windows and run some air filtering system. One should refrain, too, from strenuous outdoor exertion during high PM2.5 periods. Finally, try to live as far away from traffic as possible.

In many cities, PM2.5 levels tend to peak twice a day, once during the morning rush hour and the other during the evening commute home. The latter peak lasts longer as PM2.5 is a pollutant with the longest lifespan in the air. Thus PM2.5 levels can build up in a neighbourhood over the work-week.

It is not only adults with pre-existing heart problems who should avoid situations when they are likely to absorb high PM2.5 levels. Children, too, should take care, for they tend to spend more time outdoors. Because they are smaller in size, their exposure means higher doses of PM2.5 per kilogram of body weight. But without timely information about daily PM2.5 levels here, at-risk groups cannot know when to modify their behaviour accordingly.

As to whether Singaporeans should be concerned about PM2.5, the Health Ministry said: 'The PSI (which uses PM10 rather than PM2.5) cut-offs are internationally accepted standards... Individuals may choose to avoid vigorous outdoor activities at PSI levels less than 100 if... still concerned.'

Better still, perhaps daily PM2.5 levels should be provided to the public.