With more than a million cars spewing exhaust fumes into the air...
Egypt - With more than a million cars spewing exhaust fumes into the air each day, the Pyramids and pollution have become the twin hallmarks of this ancient city in the Arab world's most populous country.
But some scientists say there is light on the horizon.
Advocates of natural gas promise to dispel the dreary haze that blankets Cairo with a new technology they say is safe, cheap and clean.
They say compressed natural gas, or CNG, is the answer.
"CNG is a fully combustible, non-sulphuric fuel which is completely safe and environment-friendly," said Salah el-Hagaar, professor of mechanical engineering at the American University in Cairo.
Over the past five years, Egypt has become one of the main countries using CNG as a transport fuel.
Today, Egypt ranks eighth out of 46 countries involved in CNG programmes. Other states pushing the new technology include Argentina, Italy, Japan and Brazil.
"Natural gas has fewer emissions than gasoline," said Elhamy Naguib, public awareness manager at the Cairo Air Improvement Project, which was set up in September 1997 and is funded by the US Agency for International Development.
Naguib said CNG emits about 85% fewer pollutants than fossil fuels, adding that vehicle conversion to CNG was a major factor behind a drastic reduction in Cairo's pollution over the past five years.
Egypt has about 24 000 CNG-converted vehicles today, compared to technology leaders Argentina with 400 000 and Italy with 300 000.
Naguib said his project was concentrating on public transport and had already introduced 50 buses running on CNG in Cairo. It planned to target the city's 4 000 transit buses in the near future.
Concerns over CNG-converted cars
But the conversion programmes still face a bumpy ride as critics worry the technology may not be safe, and fear the drawbacks may outweigh the benefits. Some customers have reported faults ranging from minor quirks to fatal incidents.
"If they gave me 100 000 to install CNG, I'd refuse," says Mohamed Abdo, 32-year-old taxi driver and former mechanic.
Abdo said he had converted his car a year ago, but had to sell it soon afterwards because it began to break down regularly.
"I don't believe in conversion. Maybe if the car was originally designed for CNG, then it would be efficient and safe," he explained.
A major complaint about CNG conversion is the size and weight of the gas storage tank, which takes up one third of the car's boot or trunk. It also places a heavy weight on the car's rear tyres, which critics say accelerates their wear and tear.
Some Egyptians also worry that the tanks might explode and lead to fatal accidents, but engineers say the technology is safe.
"These are all rumours," said Mahmoud Elkady, professor of mechanical engineering at al-Azhar university. "The CNG bottle is designed to withstand the high pressure of gas within. The valves are well-designed and are not affected by external temperatures."
An economically viable form of fuel
Experts say the main factor behind the advance of the technology in Egypt so far is its cheap price of 45 piastres ($0.12) per cubic metre of CNG (equivalent to 1 litre) compared to gasoline's price of ?1 per litre.
"I've had no problems with natural gas, it's saved me a lot of money," said Mostafa Kamel, a 36-year-old taxi-driver.
"Customer economics work in our favour," said Frank Chapel, managing director of the Natural Gas Vehicle Company, (NGVC), a partnership set up in 1995 between BP, Egypt Gas and Engineering for Petroleum and Processing Industries (ENPPI) for CNG vehicle conversion and fuelling.
"We started our first fuel station in January 1996 with 200 vehicles," said Chapel, pointing proudly to the long queue of black-and-white taxis waiting to fuel up at the station.
"Now we have 47 to 50 fuelling stations and 24 conversion centres throughout the country."
A typical conversion kit costs ?5 835, a steep price for low to middle-income households. But Chapel said the option of long-term payment packages has also enabled people with limited financial means to convert their vehicles.
More marketing needed
Even with safety assurances and favourable economic incentives, analysts say more awareness needs to be raised about conversion with target customers beyond high-mileage taxi and micro-bus drivers.
Chapel said so far some 85% of converted vehicles were taxis.
"The government should offer incentives to private car owners such as tax exemptions and/or a customs exemption on the conversion kit to lower conversion costs," the American University's Hagaar said.
Experts anticipate the government will do its utmost to promote CNG as part of efforts to exploit Egypt's natural gas resources. They say the conversion to CNG would cut down crude oil consumption and free up Egyptian oil for lucrative export.
Around 1% of natural gas production is currently being converted into CNG.