Although the dust has settled and pollutants have been partially dispersed by wind, the great sandstorm that hit Shanghai and much of the Yangtze River Delta will long be remembered. Many people's eyes are still red, throats sore and memories raw.

People struggle to make their way home amid the strong sandstorm that hit Northwest China's Gansu Province Thursday, April 28, 2011. [CFP]

After the sandstorm swept through last week, the city's air pollution index reached the ceiling level of 500 on Monday, despite rain the previous night that washed down some of the airborne pollutants and reduced their concentration.

For days the city was enveloped in a cloud of yellowish dust and from afar its blurred skyline silhouette looked like a mirage behind a thick veil of sand. Shanghai experienced similarly strong sandstorms in 2007 and 2010. But this time the filthy air has left a worse taste in people's mouths.

Local meteorologists squarely blamed the worst-ever air quality on the confluence of air currents from North China that carried the pollutants all the way south and a sandstorm that drifted offshore and then was pushed back inland due to a change in wind direction.

Shanghai was at the epicenter of an inexorable meteorological process which man cannot influence, weather forecasters argued, seeking to deflect criticism that they hadn't done enough to alert citizens to the looming sandstorm and its health consequences.

For years the desertification that besets North China has been used to justify foot dragging on proactive environmental protection, especially at the local level.

While our weathermen may well throw up their hands and say that they are powerless before the formidable power of sandstorms -- they indeed are -- doing little to get the public ready for a health menace can only be equated with inaction. Besides, there were abundant signs of an incoming sandstorm since late April when a swathe of North China was choking on the dusty air.

Grimmer fact

Nevertheless, sharp questions about the flawed pre-warning system, coupled with years of popular frustration over weather forecasters' inaccuracy, should not overshadow concerns about an even grimmer fact that often goes unnoticed.

Among all those sounding the alarm about the invisible dangers of exposure to dirty air during sandstorm season, Zhuang Guoshun's research findings will send a chill down people's spines.

Since 2000 the professor in Fudan University's Department of Environmental Science and Engineering has been gathering data about the content of sandstorm particles from samples he collects at 12 monitoring sites around the country.

Apart from the dirt blown in from the arid Loess Plateau and the steppes of the Inner Mongolian Autonomous Region, samples tested contained higher than usual levels of heavy metals such as arsenic and lead, as well as selenium, which is toxic in large amounts.

Heavy metals adhere to the lungs after inhalation and take years to be excreted, contributing to a higher incidence of lung cancer.

During the last major sandstorm in March 2010, the proportions of arsenic, lead and selenium in Shanghai's air were 2.4, 0.8, and 5.5 times the normal readings.

Flashback to April 2000, when Beijing was gripped by a severe sandstorm. The density of these airborne particles jumped twenty-seven fold, seven fold and thirty-two fold, respectively.

"They can only come from burning coal," Zhuang said.

That assertion reveals the complex content and evolution of potentially deadly sandstorms that are often simplistically understood as a moving dune.

In fact, Zhuang's work has afforded rare glimpses into a public health problem and the scant official measures that are taken to highlight it - let alone tackle it.

The routes of sandstorms overlap ominously with the locations of many coal-producing regions, including the Inner Mongolia Autonomous Region and Shanxi and Henan provinces.

So, whenever wind comes down from the north, it carries with it not only sand but also smutty coal powder, which increases over long distances and turns the swirl of dust into the sandstorm as we know it.

Unfortunately, measurement of the coal dust particles, classified as PM2.5 substances as they are no bigger than 2.5 microns in diameter, is not part of the official surveying system - and thus not known by the public. The official system only measures larger PM10 particles with a diameter of 2.5 to 10 microns.

On top of the heavy metals fallout, the flying grit is an excellent vehicle for pollutants such as nitric oxide and sulfur dioxide. After a series of natural chemical reactions, the mixture becomes even more toxic in the form of acid rain.

Holistic approach

Notwithstanding the gravity of sandstorms, officials and environmental experts are mostly pessimistic about the prospects of keeping the scourge at bay.

"We cannot hope to fix the problem once and for all just by employing local environmental schemes like planting trees. So long as the northern deserts continue to exist and expand, not much can be done to resist the onslaught of sandstorms," said Zhang Xiaoye, deputy director of the Chinese Academy of Meteorological Sciences.

Sandstorms, which move at an altitude of more than a thousand meters, cannot be intercepted by trees that are at most a dozen meters tall, he said.

"So far the efforts to alleviate sandstorms have been largely local, with little success," said Shu Jiong, a professor with the East China Normal University.

A holistic approach to fighting sandstorms, which some scholars see as lacking in the minds of policy makers, calls not for making futile attempts to shrink the deserts but improving China's energy efficiency and in particular its treatment of heavy metal refuse emanating from burning coal.

For instance, localities in the path of sandstorms can cushion the impact by cutting their industrial emissions, controlling the number of construction sites and making city streets clean. These measures will make it harder for the sandstorm to absorb local pollutants and intensify its magnitude, said Shu.

Still, trees do offer a reprieve from sandstorms, though the extent to which they help is rather limited, contrary to popular belief.

In the latest sandstorm, Huangpu District recorded the cleanest air in the city and pollution was around half the level in other districts. This is because its air quality monitoring spot is in a public park, where lush vegetation has sequestered a fair portion of dirt from percolating to the ground.

For Shanghai to grapple with sandstorms, another imperative, albeit a bitter pill, is to freeze its vehicle numbers, Shu noted.

Aside from health hazards, car exhaust fumes offset the meaningful work done on other fronts, like tree-planting campaigns, according to Shu.

Obviously, there is a lot more to the issue than meets the eye. Right now the city is abuzz with talk of upgrading the patchy system of sandstorm forecasts and early warnings, but that's not enough. After all, the seminal research on coal dust and heavy metals by experts like Zhuang is virtually unknown to the public. People are still dangerously ignorant of sandstorms, of their sources and evolution, and hence, they are likely to make misinformed decisions about how to battle them.

It's sad that only one year after the World Expo, when consecutive "blue sky" days were reported, we are thrust back into the cruel reality that breathing clean air can be a luxury.

The sandstorms that haunt China may just be a seasonal event, but their growing frequency and magnitude expose our underbelly: How poorly we are prepared for Mother Nature's fury at our addiction to growth at the cost of everything else.