According to a new research presented at the annual meeting of the American Association for the Advancement of Science, more than 5.5 million people worldwide are dying prematurely every year as a result of air pollution.
Most of these deaths are occurring in the rapidly developing economies of China and India.
The main culprit is the emission of small particles from power plants, factories, vehicle exhausts and from the burning of coal and wood.
The data was compiled as part of the Global Burden of Disease project.
Scientists involved in the initiative say the statistics illustrate how far, and how fast, some nations must travel to improve the air their citizens breathe.
Breathing in tiny liquid or solid particles can increase the risk of heart disease, stroke, respiratory complaints and even cancer. And while developed nations have made great strides in addressing this problem these past few decades, the number of citizens dying as a result of poor air quality in developing countries is still climbing.
According to the study, air pollution causes more deaths than other risk factors like malnutrition, obesity, alcohol and drug abuse, and others. The Global Burden of Disease project puts it as the fourth greatest risk behind high blood pressure, dietary risks and smoking.
In China, there are said to be about 1.6 million deaths a year; in India, it is roughly 1.3 million. This data is from 2013, the most recent year for which it is available.
The key sources of pollution concern are slightly different in each nation, however.
In China, the dominant factor is particle emissions from coal burning.
The project calculates this source alone is responsible for more than 360,000 deaths every year.
Even though China has targets to restrict coal combustion and emissions in the future, it may struggle to bring down the number of deaths because it is acquiring an aging population and these citizens are naturally more susceptible to the illnesses associated with poor air quality.
In India, the problem that draws particular attention is the practice of burning wood, dung, crop residues and other materials for cooking and heating.
This “indoor pollution” causes far more deaths than “outdoor pollution”.
Looking at the broad economic trends in India, the research team says the country runs the risk of having even poorer air quality in the future.
Beijing authorities have issued a second pollution red alert, little more than a week after the first ever such warning.
China’s capital will see hazardous smog from December 19 until December 22, the Beijing Meteorological Service said.
Nationwide, a vast area from Xian in central China to Harbin in the north-east would also be affected, the National Meteorological Center said.
The red alert triggers restrictions on vehicle use, factories and construction work.
The government has promised to take action to address often dangerous levels of pollution.
Meteorological authorities have said that the regional smog is likely to be worse than the last red alert earlier this month, with the PM2.5 pollution level to exceed 500 micrograms per cubic meter.
The smog which hit Beijing on December 8 had peaked just below 300. Residents are encouraged to stay indoors if levels exceed that level.
The World Health Organization (WHO) recommends 25 micrograms per cubic meter as the maximum safe level.
Authorities released a map showing that heavy smog would blanket a swathe of China spanning nearly 1,200 miles, encompassing at least 12 major cities, with Beijing and nearby city Shijiazhuang heaviest hit.
The other cities would experience medium or lesser levels of smog.
The news was greeted with exasperation and worry among Chinese citizens online.
The current four-level alert system was instituted about two years ago, although the red alert had never been issued until this month.
Coal-powered industries and heating systems – in heavy use during the cold Beijing winter – are major contributors to the smog, which is made worse by weather conditions and the city’s geography.
Beijing is bordered to the south and east by industrial areas that generate pollution, and to the north and west by mountains that trap it over the city.
China still depends on coal for more than 60% of its power, despite big investments in renewable energy sources.
Earlier this month China was part of the landmark Paris climate change agreement that set a course for China, and the world, to move away from fossil fuels in the long term.
Beijing authorities have decided to close schools and to stop outdoor construction after the Chinese capital issued its first “red alert” over smog levels.
The red alert is the highest possible, and has not been used in Beijing before, the state-run Xinhua news agency says.
Chinese authorities expect more than three consecutive days of severe smog.
Cars with odd and even number plates will be banned from driving on alternate days.
The alert comes as China, the world’s largest polluter, takes part in talks on carbon emissions in Paris.
Current pollution levels in Beijing are actually lower than last week’s, but the red alert has been placed because of levels expected over the coming days.
The order will last from 07:00 local time on December 8 until 12:00 on December 10, when a cold front is expected to arrive and clear the smog.
China’s CCTV news channel reported at the weekend that some parts of Beijing had visibility of only 660ft.
Coal-powered industries and heating systems, as well as vehicle emissions and dust from construction sites, all contribute to the smog which has been exacerbated by humidity and a lack of wind.
At 18:00 local time on December 7, the air pollution monitor operated by the US Embassy in Beijing reported that the intensity of the poisonous, tiny particles of PM 2.5 was 10 times above the recommended limit.
The level in Beijing reached more than 256 micrograms per cubic meter in some of the worst-affected areas. The World Health Organization considers 25 micrograms per cubic meter to be a safe level.
Activists said the level hit 1,400 micrograms per cubic meter in the north-east city of Shenyang last month, saying it was the worst seen in China.
Last week, activists from Greenpeace had urged the Chinese government to declare a red alert. Another Chinese city, Nanjing, issued a red alert in December 2013.
On November 30, Beijing issued an orange alert – the second-highest of the four-tier system adopted in 2013.
Correspondents say Chinese officials had been unwilling to commit to hard targets on reducing carbon emissions, but have now realized that the dependence on fossil fuels has to stop.
Chinese President Xi Jinping vowed to take action on the emissions at the current global climate change talks in Paris.
Chinese authorities have a removed from websites Under the Dome, a popular documentary which highlights China’s severe pollution problem.
Under the Dome explains the social and health costs of pollution, and was watched by more than 100 million people online, sparking debates.
It was removed just two days after Premier Li Keqiang called pollution a blight on people’s lives.
Li Keqiang had promised to fight it with all the government’s might.
The environmental issue has dominated the current session of the Chinese parliament, the National People’s Congress, in Beijing.
The newly appointed environmental protection minister, Chen Jining, had praised Under The Dome, telling reporters it should “encourage efforts by individuals to improve air quality”.
The huge popularity of an impassioned, independent film on the issue appears to have made the communist authorities nervous, correspondents say.
Under the Dome, a year-long investigation of pollution in China, had garnered more than 100 million views in less than 48 hours.
Made by renowned investigative journalist Chai Jing and funded with her own money, the film sharply criticizes the Chinese state’s lax environmental laws.
Standing in front of an audience in a simple white shirt and jeans, Chai Jing speaks plainly throughout the 103-minute video, which features a year-long investigation of China’s noxious pollution problem.
At times, the documentary is deeply personal. Near the start of the documentary, Chai Jing interviews a 6-year-old living in the coal-mining province of Shanxi, one of the most polluted places on earth.
“Have you ever seen stars?” Chai Jing asks.
“No,” replies the girl.
“Have you ever seen a blue sky?”
“I have seen a sky that’s a little bit blue,” the girl tells her.
“But have you ever seen white clouds?”
“No,” the girl sighs.
As of March 7, Under the Dome was no longer available on popular Chinese mainland video sites.
A link on the Youku website that previously led to the video now prompts the message: “We’re very sorry, Youku was unable to find the page you requested.”
China operates the world’s most formidable online censorship machine, known as the Great Firewall.
Some social media users in China voiced frustration at the removal of the film.
Under the Dome is still available on YouTube with English subtitles.