Outdoor air pollution has grown 8% globally in the past five years, with billions of people around the world now exposed to dangerous air, according to new data from more than 3,000 cities compiled by the World Health Organisation (WHO).
While all regions are affected, fast-growing cities in the Middle East, south-east Asia and the western Pacific are the most impacted with many showing pollution levels at five to 10 times above WHO recommended levels.
According to the new WHO database, levels of ultra-fine particles of less than 2.5 microns (PM2.5s) are highest in India, which has 16 of the world’s 30 most polluted cities.
China, which has been plagued by air pollution, has improved its air quality since 2011 and now has only five cities in the top 30. Nine other countries, including Pakistan and Iran, have one city each in the worst 30.
For the larger, but slightly less dangerous PM10 particles, India has eight cities in the world’s top 30. Nigeria, Saudi Arabia and Pakistan each have two cities in the top 10. The true figure for the growth in global air pollution is likely to be worse because only a handful of African cities monitor their levels.
The most polluted city in the world, according to the WHO data, is Onitsha, a fast-growing port and transit city in south-eastern Nigeria that recorded levels of nearly 600 micrograms per cubic metre of PM10s - around 30 times the WHO recommended level of 20 micrograms per cubic metre.
Air pollution levels were generally much lower for cities in developed countries with Sydney, New York and London registering 17, 16 and 22 micrograms per cubic metre for PM10s respectively. However, the data only includes measurements for particulates and does not include forms of air pollution such as NO2 and ozone.
“We have a public health emergency in many countries. Urban air pollution continues to rise at an alarming rate, wreaking havoc on human health. It’s dramatic, one of the biggest problems we are facing globally, with terrible future costs to society,” said Dr Maria Neira, director of public health at the WHO in Geneva.
“The cost for countries is enormous. Air pollution affects economies and people’s quality of life. It leads to major chronic diseases and to people ultimately dying,” she said.
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The new data, drawn from city and academic records, shows a rapid deterioration in air quality as low-income cities grow unchecked and populations become unable to escape clouds of smog and soot from transport, industry, construction sites, farming and wood-burning in homes.
Outdoor air pollution causes more than 3m deaths a year - more than malaria and HIV/Aids - and is now the biggest single killer in the world. The toll is expected to double as urban populations increase and car numbers approach 2bn by 2050.
Air pollutants such as sulphates, nitrates and black carbon penetrate deep into the lungs and into the cardiovascular system, posing the greatest risks to human health, says the UN.
“As urban air quality declines, the risk of stroke, heart disease, lung cancer, and chronic and acute respiratory diseases, including asthma, increases for the people who live in them. When dirty air blankets our cities the most vulnerable urban populations - the youngest, oldest and poorest - are the most impacted,” said Flavia Bustreo, WHO assistant director general.
Encouragingly, there is evidence from the WHO data that many cities are addressing air pollution. More than half of the monitored cities in high-income countries and more than one-third of those in low- and middle-income countries reduced their air pollution levels by more than 5% in five years. Delhi, one of the most polluted cities in the world, has banned large diesel cars from going into the city centre.
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Measures taken by cities include reducing industrial smokestack emissions, increasing the use of renewable power sources like solar and wind, and prioritising rapid transit, walking and cycling networks in cities. Many cities are also committed to reducing reducing car traffic and diesel vehicles in particular.
The UN’s third outdoor air pollution database suggests the cleanest cities in the world are generally small, wealthy and situated far from industrial centres. Muonio in Finland, a town above the Arctic circle, has the world’s purest recorded urban air, recording just 2 micrograms per cubic metre of PM2.5 pollution and 4 micrograms per cubic metre of PM10s. It is closely followed by Norman Wells in Canada, Campisábalos in Spain and Converse County, Wyoming in the US.
Of 52 UK towns and cities included in the UN database, Port Talbot in south Wales, a hub for the UK steel industry, is the most polluted, ahead of London, Glasgow, Southampton and Leeds. The cleanest UK city in the WHO list is Inverness, followed by Bournemouth, Newcastle and Sunderland.
The most polluted city in Australia, according to the data, is Geraldton, a major seaport on the west coast, north of Perth. The most polluted city in the United States is the inland city of Visalia-Porterville in California.
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“More than 80% of people living in urban areas that monitor air pollution are exposed to air quality levels that exceed the World Health Organisation limits. While all regions of the world are affected, populations in low-income cities are the most impacted; 98% of cities in low- and middle income countries with more than 100,000 inhabitants do not meet WHO air quality guidelines. However, in high income countries, that percentage decreases to 56%,” said the WHO.
“It is crucial for city and national governments to make urban air quality a health and development priority,” said Dr Carlos Dora, co-ordinator of the WHO’s Interventions for Healthy Environment programme. “When air quality improves, health costs from air pollution related diseases shrink, worker productivity expands and life expectancy grows. Reducing air pollution also brings an added climate bonus, which can become a part of countries’ commitments to the climate treaty.”
Traffic and industrial emissions are largely responsible for severe urban haze in China, but weather also can make air pollution much worse.
For a new study published in the Proceedings of the National Academy of Sciences, researchers collected extensive air samples in Beijing, China’s capital and one of the most heavily polluted areas in the world.
The city is teeming with a large amount of fine particulate matter (called PM) in the atmosphere that eventually results in environmental and health problems, but also in conditions that affect weather and climate.
Weather patterns and emissions from traffic, industrial plants, and aerosol chemical processes combine to produce extremely polluted conditions—forming a thick haze layer that covers the city for days.
“We find in our study that the conditions in Beijing are prone to PM formation, because of highly abundant condensable gases,” says Renyi Zhang, distinguished professor of atmospheric sciences at Texas A&M University.
“Emissions of volatile organic compounds, nitrogen oxides from urban transportation, and sulfur dioxide from regional industry trigger aerosol nucleation and continuous growth over multiple days, leading to particle mass exceeding several hundred micrograms per cubic meter,” he says.
“This efficient aerosol formation process is distinct from that occurring in many other urban centers worldwide, although the chemical composition of Beijing’s aerosols is unremarkable.”
China’s pollution troubles go back three decades, when the country started its rapid modernization.
Of the 20 most polluted cities in the world, China is home to 16 of them. Beijing often exceeds by many times the acceptable air standards set by the US Environmental Protection Agency.
Sometimes the pollution in Beijing can be so bad that visibility is less than 50 feet (15 meters) and the air quality is 40 times higher than acceptable levels established by the World Health Organization, Zhang says.
A study by the Chinese government in 2010 showed that pollution contributed to 1.2 million premature deaths in that country, and that likely, more than 500 million Chinese will have their lives shortened by at least five years because of poor air quality.
Meteorology is a critical factor in how severe the haze is and how densely it forms, Zhang says. “We found that Beijing’s PM episodes occur on a periodic cycle, which is largely driven by wind variations.”
“When the wind is shifted from the south, that’s when trouble starts because most of the factories and power plants are located in the southern region. Under reactively stagnant conditions, the gaseous pollutants from city traffic and surrounding industry react in the air, locally producing a large amount of PM.”
The haze and pollution problem appears to be more intense in fall, spring, and winter months because it rains more in summer and that tends to clean the air by washing out the particles.
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“Beijing’s pollution is similar to other urban cities in China,” Zhang says. “Collectively, those urban areas, along with regional industrial facilities, agricultural activities, and biogenic emissions, throughout the country constitute the dominant sources of large-scale pollution in China, covering thousands of square miles and lasting for many days.”
Facing mounting criticism from the world and its own people, the Chinese government has promised to improve air quality, but imposing the strict controls necessary on the many factories and power plants and reducing car emissions is not easy with China’s booming economy of the last 25 years.
“Our study suggests that targeted regulatory controls over the PM precursor emissions from transportation and regional industry are needed to reduce urban pollution in China,” Zhang points out.
“The formation of severe urban and regional haze in China is attributable to the higher rates but lower standards for air pollutant emissions, which are ultimately linked to its rapidly growing economy, fast urbanization, and a large population.
“Improving air quality in that country will likely have profound consequences on China or even the world economy.”
The Ministry of Science and Technology of China, and collaborative research program between Texas A&M University, the National Natural Science Foundation of China and the Robert A. Welch Foundation.
Source: Texas A&M University
Original Study DOI: 10.1073/pnas.1419604111