Wednesday, April 19, 2006

Coal and Dust.

In 2004, 6,000 people died in China as a result of coal mining accidents, which has added another black mark to the Chinese coal mining industry. Burning coal is furthermore a major source of arsenic poisoning in rural communities. China is both the world's largest producer and consumer of coal, and its production has increased year on year for the past 25 years to an annual output of almost 1.7 billion tonnes. While in the U.S. only a small fraction of coal is consumed domestically, perhaps 1%, in China over 50% of the energy for urban households is supplied by burning coal in stoves and small coal fired boilers. 75% of China's primary energy is produced from coal, in contrast say to the U.K. which uses less than 20%, but around 40% natural gas and 32% petroleum (including 26% used for transportation; most of the other 6% for industry) instead. The loss of 6,000 lives in a single year has prompted the Chinese government to announce the closure of 7,000 coal mines across the entire country; however, these are the smallest mines and so coal production should be overall little affected. Nonetheless, since it is the small mines that are less well regulated and operate with less stringently imposed safety rules, a significant improvement to the industry's safety record is to be expected.
At a ratio of 6,000 deaths/1.7 billion tonnes of coal, which amounts to a fatality rate of four deaths per million tonnes of coal produced, China has the worst safety record in the world. By means of comparison, the United States produces one billion tonnes of coal per year, which is an output similar to that of China, but at a death toll of 50 miners, giving a fatality rate of 0.04 per million tonnes of coal, or over a hundred times better, as a simple statistic, which it is not to those directly involved or to their families. The consequences of China's industrialisation programme, fuelled as it is by coal, are unpleasant but predictable. Air-borne pollution and smogs blight major cities, which is a problem compounded by the rising number of cars and other road vehicles. As a result, there are unprecedented levels of pulmonary (lung) disease.
However, even more rural regions such as Guizhou Province are far from immune to the effects of burning coal, where over 3,000 of its population are documented to be suffering from severe arsenic poisoning. The principal cause of this epidemic is thought to be the consumption of chili peppers that have been dried over fires fuelled by coal containing up to 35,000 parts per million (i.e. 3.5%!) of arsenic, from which they absorb an average of 500 parts per million of their own weight of arsenic. This is not the sole problem, since more than 10 million people in Guizhou Province are showing symptoms of fluorosis: an accumulation of excess fluoride in the teeth and bones. The excess fluoride arises from eating corn that has been dried over burning briquettes fabricated from coal containing high levels of fluoride bound with clay that also has a high fluoride content. A double-whammy.
The practice of burning coal in poorly ventilated homes produces high ambient levels of PAH (Polycyclic Aromatic Hydrocarbons), which are known carcinogens (cancer causing agents) and are believed to contribute to the elevated incidence of esophageal and lung cancers in parts of China. I note, however, that the Chinese tend to be heavy smokers, and it is perhaps the combined burden of these two sources of PAH that causes the problem. Incidences of mercury and selenium poisoning have also been attributed to the domestic burning of coal that is contaminated by these elements.
While this is an essentially localised "dust" problem, I find my thinking being led onto one that is not: namely, the long-range transport of mineral dust, principally from North Africa (meaning the Sahara Desert and regions north of there), into the global atmosphere. Measurements made at sampling stations over the Earth indicate that atmospheric dust can be considered as an almost homogeneous component of the atmosphere, something like a trace gas.
Soil dust is a main provider of airborne particles ubiquitously to the atmosphere, as attested to by the sighting of dust-plumes, which are one of the most prominent and commonly visible features imaged by satellites. It is thought that dust may play a fundamental role in many biogeochemical processes, though the details remain as yet speculative. Dust particles that are transported over large distances usually have a mean particle diameter of less than 10 microns (one micron is one thousandth of a millimeter, hence 10 microns is one hundredth of a millimeter. For comparison, a human hair is on average 70 microns thick). Larger particles tend to settle-out of the atmosphere before they can travel very far.
Particles of less than 2.5 microns in size have been established by the Environmental Protection Agency (EPA) as constituting a human health hazard, because they are small enough to be inhaled into the deep lung. Since dust particles from North Africa can be detected as far away as Florida, concerns have been raised regarding its potential health hazard, although clearly this kind of dust has been with humankind throughout the course of our evolution, and it is the truly anthropogenic aerosols that are a greater cause for concern, e.g. that from combustion processes, including running motorised vehicles, and sulphate particles which arise at least partly through human activities, though they are mostly an entirely natural phenomenon.
It might be argued that because of climate change and the increased desertification of central regions of Africa, e.g. the inexorable advance of the Sahara Desert that I have noted in previous postings, the atmospheric dust load is increasing and so there is a potential health issue thus connected with climate change. It is further plausible that there may be an interplay, in which increasing dust generation through global warming induced climate change participates in a feedback loop where the mobilised dust causes "climate-forcing", e.g. to cool the earth's surface by reflecting sunlight away from the earth and back into space.
In truth, these are all complex issues that cannot realistically be addressed in isolation of one another. The global experiment that we are all part of will need to run its full course before we may perceive a valid answer to any of our currently vexed questions. As the English writer Sir Osbert Sitwell put it: "But what is dust, save time's most lethal weapon...?"

No comments: