Bolatito Joseph has strong suspicions about what is causing her breathing problems and a mucus build up: a building site near her home and noxious fumes from a rubbish dump close to her church."I inhale a lot of dust as a result of the construction work going in my area," said Joseph, a cleaner from the Akute area of Nigeria's biggest city, Lagos."I worship at a church in Ojota near a rubbish dump and the stench is... injurious to public health," she told AFP.Joseph is not alone in having health problems in Nigeria's cities, where questions are increasingly being asked about whether the air is fit to breathe.In Lagos, in the southwest, the southern oil hub of Port Harcourt and Onitsha in the southeast, Nigerians might be choking to an early death.Diesel generators that compensate for an inadequate electricity grid belch acrid smoke, combining with emissions from old vehicles and traffic gridlock.Unregulated industry and burning waste further turns the air foul.In 2012, the World Health Organisation called air pollution "the world's largest single environmental health risk" and blamed it for about seven million premature deaths globally.Attention has largely focused on efforts to fight pollution in cities such as Beijing and New Delhi, where poor air quality can cause traffic restrictions and school closures.But in Nigeria, the public is largely in the dark, with data lacking and only a handful of scientific studies exploring the phenomenon. Particulate matter In 2012, the WHO said there were 46 750 deaths in Nigeria from air pollution, which has been linked to increased risk of cardiovascular and respiratory diseases, as well as cancer.In megacity Lagos, which is home to some 20 million people, thick plumes of white smoke have been drifting from burning waste at the Olusosun dump site in Ojota for weeks now.Doctors say it has caused a surge in breathing problems.Onitsha, in Anambra state, has been ranked the worst city in the world for PM10s - microscopic particles of dirt, soot, smoke and liquid droplets in the air that can harm human health.In Port Harcourt - once dubbed "The Garden City" because of its palm trees and green open spaces - black soot has fallen from the sky for more than a year.Rafael Navarro, a Venezuelan working for a global oil major, bought an air quality monitor to confirm his suspicions about what he was breathing.He refused to accept local excuses that the poor air was caused by the Harmattan, the seasonal wind that whips up sand and dust from the Sahara desert and dumps them on the Gulf of Guinea."The surprise was that Port Harcourt has many times been in the top five most polluted cities in the world on the AirVisual network," he said."That was quite shocking, as I was not aware that our numbers could have been that bad."Readings since he installed the device in May last year showed huge spikes overnight and in the early morning of PM2.5s, finer particulate matter in the air that are more harmful to health.Illegal oil refineries in the nearby creeks have been blamed: most operate at night to avoid detection from military patrols.Low-cost monitoring Ademola Oduguwa, an electronics and electrical engineering graduate who works in Lagos, said data was key to raising awareness and getting something done.Despite the billions of dollars earned over decades from oil, most Nigerians still live in poverty, making foreign air quality monitors prohibitively expensive.Oduguwa decided to build a low-cost version for $120 - less than half the retail cost of Navarro's device. He installed four at locations across Lagos."More often than not, we have seen very high pollution levels being registered in all locations... above the benchmarks of the World Health Organisation," he said.He now wants to develop his idea to provide concerned city residents with regular air quality readings.Campaigners wearing face masks last week marched in Port Harcourt, calling on the Rivers state government to provide daily data on air pollution and start a clean-up campaign."We are not just going to lie low, fold our arms and allow government to release things that kill us," said #StopTheSoot group leader Tunde Bello.Ako Amadi, an environmentalist, said current pollution levels should be publicly available "like temperature" but political will was lacking.Anyone who raises concerns about the environmental impact of projects or pollution is considered "an enemy of progress", he said. * Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.