For Egyptians, autumn brings not only the welcome cooling of the summer's ferocious heat, but also the less welcome arrival of an ominous black cloud that blankets the sky and turns the air stale with the smell of ash. The "sahab as-sawda" [black cloud], as it is known, is a toxic mix of car exhaust, industrial emissions, and the smoke given off by incinerated garbage and the refuse straw from the rice crop. Out of this lethal blend -- which the government, the UN and non-governmental organisations (NGOs) concur poses serious environmental and health dangers -- the burning of the rice crop is arguably the largest part of the problem. It accounts for nearly 42 percent of the black cloud, according to a 2003 Environment Ministry report published in al-Ahram Weekly newspaper. The burning of the dried rice stalks after reaping has become a common practice for Egyptian farmers who have no space to dispose of the straw refuse and cannot afford to have it moved elsewhere. Farmer Sameer Mehdi Ibrahim says that the ash from the stacks of burning straw also acts as an essential fertilizer. "The problem is that because the rice straw is good for our lands and we are unable to grow wheat, we grow the wheat on this straw. After we burn the straw, we grow the wheat. We spread the straw," he says. But while the annual rice crop burning makes economic sense for farmers, the UN says it is costly for Egypt as a whole. In fact, the UN estimates that the black cloud costs Egypt roughly 6 billion U.S. dollars in annual losses due to environmental and public health damage. Farmer Abdul Nabi Abdul Mo'ati points out that the rice burning does not take long, while industrial waste and garbage is burned around-the-clock. "Of course it causes pollution. But what about the burning of garbage and the pollution caused by the factories in Abu Zaabel for example? Don't these cause more pollution than the burning of straw thatin ten or fifteen minutes? Pollution already exists," Mo'ati says. Both the government and private industry have engaged in periodic attempts to recycle the straw refuse from the rice crop and to transform it into compost, as many other countries do. However, their efforts have yet to remove the shadow of the black cloud that chokes Egyptians every late October and early November. The World Health Organisation representative in Egypt, Dr. Zuhair Hallaj, says that the crop burning is, without a doubt, a health danger and an enormous cost to the economy. "Of course the continuation of increasing pollution rates, especially the suspended material in the atmosphere, is related in all international experiences to an increase in the rate of chronic respiratory diseases," he says. "Just try to predict what the cost of all this is to Egypt. Its impact on public health, hospitals, the consumption of medicines, a decrease in work and study time. You'll notice that the impact is massive," adds Hallaj. The black cloud has also been blamed for damaging Egypt's precious antiquities, a key source of income for the largely poor nation. Egyptian antiquity officials have said that the smog from the burnt straw is slowly eroding priceless monuments that have otherwise survived over the centuries, particularly in the south of the country. One company that has set up a project to try and recycle the chaff from the rice crop, is the Qader Factory for Advanced Industries, which is part of the Arab Organisation for Industrialisation -- an enterprise that is part state, part privately-owned. The adviser to the company's chairman says recycling the waste is a necessity. "We are calling on all investors and all concerned to help solve this problem and build plants all over the Arab Republic of Egypt because we have a very large quantity of refuse, so we can take advantage of the rice straw instead of it being something that pollutes the environment. We can use it and turn it into a source of national wealth," says Ibrahim Abul Zied. While the debate over how best to tackle the problem continues, the burning goes on this year, as so often before, leaving Egyptians in the capital and much of the countryside in the shadow of the infamous black cloud.