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Scientists search for clear answers on smog
Publication Date : 26-02-2014
Links between premature deaths and pollution are still being studied
Among the world's best- known mega-cities, Beijing and Shanghai must be the only ones where house prices are still climbing rapidly, despite severe air pollution that will take years to solve.
But for people unhappy at becoming human vacuum cleaners in a polluted metropolis, and who are equally unwilling to forgo an urban lifestyle, which is the better choice - the capital city in the north, or the southern international trade center at the end of the Yangtze River?
During the past week, Beijing has been covered with a thick layer of smog that is forecast to persist until Thursday at least.
The smog, covering 1.43 million square kilometres mainly in northern China, prompted the Beijing government to issue an orange alert - the second-highest warning - and urge people not to leave their homes.
A study in 2013 by researchers at Tsinghua University identified more than 1,300 microbes in the atmosphere over Beijing. Although most of them may pose no direct threat to human health, the number still makes for alarming reading.
Zhuang Guoshun, director of the Center for Atmospheric Chemistry Study at Fudan University, said Beijing is covered by smog for an average of 60 per cent of the year, while the figure for Shanghai is 30 to 50 per cent.
However, research studies have found that higher air pollution levels do not necessarily mean higher risks of premature death or other health hazards.
One of the scientists' main concerns is the level of PM2.5 - particulate matter of 2.5 microns or smaller, which can penetrate deep into the lungs and even enter the bloodstream.
Guangzhou and Shanghai in the south of China have slightly higher relative risks to mortality from the effects of PM2.5 than Xi'an and Beijing in the north, even though the levels of PM2.5 in the northern cities were roughly twice those in the south, according to a study published in September 2013, led by Pan Xiaochuan, a professor at Peking University's School of Public Health.
Meanwhile, a recent study by the Center for Environmental Risk and Damage Assessment at the Chinese Academy for Environmental Planning concluded that even within the Beijing-Tianjin-Hebei cluster, Handan and Hengshui - two cities in Hebei province that recorded the highest number of preventable deaths during the heavy smog of January 2013 - did not have the highest levels of PM2.5 among the 12 cities in their region during the 2013 incident.
An apparent anomaly
Environmental experts and epidemiologists said there are many different reasons for the apparent anomaly, including the duration of exposure to the pollutants, local habits and customs, and economic factors.
The components and toxicity of airborne pollutants in different cities may also vary because the differing sources of pollution, according to Pan's study. He said most of the major pollutants in Xi'an are the result of coal consumption, while Beijing, Shanghai and Guangzhou have much more complex sources of pollution because of the rapidly growing number of automobiles on their roads.
Experts said the degree of risk from particulate matter, even among those of similar size, can vary widely, depending on their composition.
"Studies overseas have concluded that particulate matters formed by burning oil are more hazardous than those formed by burning coal," said Kan Haidong, a professor at Fudan University's School of Public Health.
Zhuang said scientists are aware of 200 to 300 chemicals in the mixture of fine particles known as PM2.5. Among them, the most hazardous are organic compounds containing polycyclic aromatic hydrocarbons, a group of compounds thought to be carcinogenic.
He said the concentration of organic compounds in PM2.5 is between 20 per cent and 30 per cent. However, that figure can rise to more than 70 per cent in PM1 - ultrafine particulate matter with a diameter of less than 1 micron - and scientists believe that the increased concentration means PM1 poses a greater risk to human health than PM2.5.
"Although Beijing is more frequently enveloped by smog than Shanghai, the PM2.5 levels in the two cities are similar during bad weather. But when it comes to the more-hazardous PM1, the level in Shanghai is higher than in Beijing, which is why visibility in Shanghai is usually poorer than in Beijing during smoggy conditions," said Zhuang, who has worked in the field of atmospheric chemistry for 30 years.
Extremely high temperatures can also increase the links between airborne pollution and premature death. In southern cities where mean daily temperatures are much higher than in the north, residents are more likely to have an increased exposure to pollution because they have a greater exposure to high temperatures, according to a study led by Kan's team in 2011, which focused on eight cities across the country.
Those differences lead to wide fluctuations in the Exposure-Response Function, a key factor used to measure the health risk posed by airborne pollution in different regions. It is calculated by measuring the rise in the number of premature deaths associated with a 10-microgram-per-cubic-metre increase in the concentration of pollutants.
The relationship between mortality rates and exposure to particulate matter ranging from low to high concentrations suggests that the exposure-response curve often flattens out at higher concentrations, according to a 2009 study led by Arden Pope, a professor of economics at Brigham Young University who is also one of the world's foremost experts in environmental science. The Pope's conclusion has been verified by studies in China and a number of other countries.
The exposure-response function in the southern city of Guangzhou is more than seven times higher than in Xi'an, but the average concentration of PM2.5 in Xi'an is more than twice that in Guangzhou, said Wang Jinnan, vice-president of the Chinese Academy for Environmental Planning, quoting findings from an unpublished 2012 report conducted by the Environmental Protection Ministry.
Moreover, a 2012 study conducted by researchers from China and Australia, discovered that in Brisbane, the risk of premature death as a result of airborne pollution is approximately 7.5 times higher than in Beijing, even though the Chinese capital is about 7.6 times more polluted.
The exposure-response function is important because it's a determining factor in the calculation of premature deaths per region as a result of airborne pollution. The factor used can make a vast difference to the results of scientific studies.
A 2010 study, jointly conducted by the World Bank, the World Health Organization and the Chinese Academy for Environmental Planning, concluded that in China, 350,000 to 500,000 people die prematurely every year as a result of outdoor air pollution.
However, the Global Burden of Disease Study 2010, a survey conducted by scientists across the globe, came to the conclusion that air pollution was a major factor in the premature deaths of approximately 1.2 million people in China in that year.
The authors of the studies attributed the wide fluctuation in their findings to a number of factors, such as the pollutants targeted; PM2.5, PM10 or PM1. Also, the global burden report included the rural population, while the joint study did not.
But they all admitted that their findings may be inaccurate because the calculations for the exposure-response functions they employed were based on overseas studies rather than being generated by cohort studies - observations conducted by following a group of people over a given time scale - in China.
Fudan University's Kan said the accurate exposure-response function of any given region should be determined by a thorough cohort study into the effects of air pollution on the health of the local population, a process that could take 12 years, or even longer.
However, cohort studies are expensive and time-consuming, so instead of conducting their own studies, Chinese researchers have to rely on results from the two most-famous cohort studies in the US. One, conducted by Harvard University in the 1970s, included more than 8,000 people in six cities, while the other, conducted by the American Cancer Society from 1982 to 1998, involved approximately 500,000 people.
"To get the most-accurate number of premature deaths caused by air pollution in China, a domestic cohort study covering a large-enough sample of the population and a long life span is a precondition," said Kan.