The Nigerian city of Port Harcourt used to be known as "The Garden City" because of its soaring palm trees and green open spaces.

But since late last year, black soot has been falling from the sky, scaring and angering residents of the oil hub who claim nothing is being done to protect their health.

"You hang your clothes and before you know it, they become black. You step on your floor, everywhere is black," Steven Obodekwe, a Port Harcourt resident and environmentalist, told AFP.

The soot appeared in November last year, clouds became a hazy grey and more people were seen wearing protective face masks, said resident Tamuno Alobari.

"It (the soot) is mostly pronounced in the morning hours, especially when you run your hands across your car," he said.

Timi Isiayei said there's no escape from the fine black dust.

"When I brush my teeth in the morning and try to clear my throat, I normally notice a dark phlegm and the same applies when I try to clear my nostrils," he added.

Toxic smog is a phenomenon that has more often been associated with populous developing economies such as India and China.

The government in Rivers state, of which Port Harcourt is the capital, in December urged the public not to panic but last week set up a taskforce to investigate the matter.

The federal environment ministry in the national capital Abuja on Monday declared the air pollution an "emergency situation" and warned residents to shut doors and windows.

- Mystery origins -

The hashtag #StopTheSoot has appeared on Twitter, people are sharing photographs of their hands and feet covered in the dust, and protest marches are being organised.

Burning tyres for scrap copper and illegal oil refineries have both been blamed for the residue.

Oil revenue is central to Nigeria's economy, accounting for some 70 per cent of government earnings and 90 per cent of foreign exchange.

But decades of exploration and spills has polluted the farmlands and fish stocks in the maze of creeks around Port Harcourt and across the Niger Delta region.

Theft of crude oil from pipelines -- or "bunkering" as it is called locally -- sees raw fuel regularly diverted to illegal refineries set up in the bush for conversion into petrol and diesel.

"From investigations carried out so far, we have noticed that it is as a result of years of exploration activities especially from illegal oil bunkering activities," said environment regulation official Emeka Aniamaka.

But the environment ministry suggested another cause after ordering the shutdown of an asphalt-processing plant, saying it was "belching out thick smoke from its operations".

- Health problems -

In 2015 the World Bank said 94 per cent of Nigerians were exposed to air pollution levels that exceed World Health Organization guidelines.

Air quality is worsened by the use of generators to make up for the gaps in supply from the national electricity grid, as well as petrol containing high levels of sulphur.

Toxic smoke also comes from the burning of rubbish -- a legacy of the absence of municipal waste services.

The city of Onitsha, nearly 200 kilometres (125 miles) north of Port Harcourt, was in May last year named as having the world's worst levels of PM10 particles.

PM10 are microscopic particles in the air measuring between 2.5 and 10 thousands of a millimetre or micrometres. They can come from smoke, dust, soot, vehicle exhausts and industries.

Doctors in Port Harcourt say they are seeing the health effects of the soot already, with an increase in consultations for breathing difficulties, including asthma.

Children and the elderly are most at risk. The United Nations Environment Programme (UNEP) estimates some 600,000 people die in Africa every year as a result of air pollution.

Environmentalists are also sounding the alarm, as the soot has been found to contain sulphur dioxide and nitrogen dioxide, which cause acid rain when combined with moisture.

Yet Obodekwe, who works at the non-profit Centre for Environment, Human Rights and Development, says it is unlikely any new regulations will be implemented.

"Many environmental policies are not enforced. There is a lack of political will," he said.

"It's not too important as far as they (the government) are concerned. They don't understand the implications of the attack on the environment, so they don't take it seriously."