Exhaust fumes belched by millions of cars mixed with the hypertoxic emissions of the annual burning of rice stubble in rural areas of the Nile Delta are a prime cause, along with the city's ever-expanding population.
"We have here 540 micrograms per cubic metre of PM10 (polluting particles), which is three times higher than the official limit, and 10 times the limit of the World Health Organisation," meteorologist Magdi Abdel Wahab told AFP.
The thick "lead blanket" settles every year over this Nile city, triggering serious health concerns for its 16 million residents.
Emissions of nitrogen dioxide and carbon monoxide gases, mainly from the city's traffic, are mixed with the PM10 particles to create a potentially lethal cocktail, experts say.
The WHO has recommended lowering levels of the particles, which can cause a range of problems from bronchitis to foetal deformities, to 20 micrograms per cubic metre per year.
"Visits to my clinic have increased in recent weeks by 50 percent, and that is due to the pollution," pneumologist Assem al-Issawy told AFP.
The black cloud started appearing in Cairo skies in 1999 to the dismay of the city's residents who already struggle to deal with surging pollution on a par with Beijing and Mexico City.
Since then, it returns faithfully every September and lasts until early winter, intensifying air pollution which kills up to 5,000 people every year, according to medical sources.
The authorities lay much of the blame with farmers in the Nile Delta who traditionally burn rice stubble to enrich the land for the next harvest.
On top of factory and traffic pollution, the burden is too much for Cairo's environment.
A Franco-Italian team of researchers says that pollution in Egypt's capital is three times greater than in Beijing, another city whose environment is struggling to keep pace with breakneck economic development.
"That gives you an idea of how bad pollution is in Cairo," says French expert Stephane Alfaro.
A law has been passed banning the burning of rice stubble and those who ignore it face the threat of fines. The government also has a plan to set up several stubble recycling plants in the Delta.
One state-run plant has been set up to do just that at El-Qorein.
"From 300,000 tonnes of rice waste, we produce 160,000 tonnes of fertiliser. It's a good start," said plant director Ibrhaim Gayed.
However, the project faces problems including the lack of facilities to compress the waste so it can be transported to the plant and farmers' complaints that the waste is not taken away for a long time.
"Only 20 percent of the 3.5 million tonnes of rice waste will be recycled this year," says Hamza Abdel Hakim, an engineer with the Arab Organisation for Industrialisation.
Secretary of State for the Environment Maged George Elias says rather optimistically that the black cloud will disappear by 2010.
But even if the authorities manage to rein in the agricultural pollution, a fight on a pharaonic scale will still be required to tame the city's exploding population and ageing cars.