The vast majority of these cities - 83 - are in India, and all of them have exceeded the World Health Organization's air quality recommendations by over 10 times, according to a report by IQAir, which tracks global air quality, cited by CNN.

The study specifically analyzed fine particles or PM2.5, which is the smallest and most dangerous pollutant.

Only 9% of the over 7,800 cities analyzed globally have air quality that meets the WHO standard, which states that annual average levels of PM2.5 should not exceed 5 micrograms per cubic meter.

"We see that, in every aspect of our lives, air pollution has an impact", said Frank Hammes, CEO of IQAir Global. "And typically, in some of the most polluted countries, pollution is likely to cut between 3 and 6 years off people's lives. And then, prior to that, it will lead to many years of suffering that can be entirely prevented if there is better air quality".

When inhaled, PM2.5 particles travel deep into lung tissue, where they can enter the bloodstream. They come from sources such as fossil fuel combustion, dust storms and forest fires, and have been associated with asthma, heart and lung diseases, cancer and other respiratory illnesses, as well as cognitive deficiencies in children.

Begusarai, a city of half a million in northern India's Bihar state, was last year the world's most polluted city, with an annual average concentration of PM2.5 of 118.9 - 23 times higher than WHO recommendations.

It is followed in the IQAir ranking by the Indian cities of Guwahati, Assam, Delhi and Mullanpur, Punjab.

According to the report, in India, 1.3 billion people, or 96% of the population, live with air quality 7 times higher than the WHO recommended.

Central and South Asia are the regions with the worst results globally, hosting all 4 of the most polluted countries in 2023: Bangladesh, Pakistan, India and Tajikistan.

South Asia is particularly concerning, with 29 of the 30 most polluted cities located in India, Pakistan or Bangladesh. The report ranked major population centers Lahore at 5th, New Delhi at 6th, and Dhaka at 24th.

Hammes said there is unlikely to be any significant improvement in pollution levels in the region without "major changes in energy infrastructure and agricultural practices".

"Another alarming aspect in various regions globally is the overlap between sources of outdoor air pollution and indoor air pollution", he added. "For instance, cooking with unclean fuel can lead to indoor contamination levels significantly exceeding those observed outdoors".

A global issue

IQAir found that 92.5% of the 7,812 locations in 134 countries, regions and territories it analyzed for last year's average air quality exceeded WHO PM2.5 guidelines.

Only 10 countries and territories had "healthy" air quality: Finland, Estonia, Puerto Rico, Australia, New Zealand, Bermuda, Grenada, Iceland, Mauritius and French Polynesia.

Millions of people die each year from health problems related to air pollution. According to a study published in November in the BMJ, air pollution caused by fossil fuels kills 5.1 million people worldwide each year.

Meanwhile, WHO says 6.7 million people die annually due to the combined effects of outdoor and household air pollution.

The man-made climate crisis, driven by fossil fuel burning, plays an "essential" role in influencing air pollution levels, the IQAir report states.

The climate crisis is altering weather patterns, leading to changes in wind and precipitation, affecting pollutant dispersal. Climate change will only worsen pollution as extreme heat becomes more severe and frequent, according to the report.

The climate crisis also leads to more severe wildfires in many regions and longer and more intense pollen seasons, both exacerbating health problems related to air pollution.

"We have such a strong overlap between what is causing our climate crisis and what is causing air pollution", Hammes said. "All that we can do to reduce air pollution will have a remarkable long-term impact and also improve greenhouse gas emissions and vice versa".

Regional rankings

North America was severely affected by wildfires that ravaged Canada from May to October last year. According to the report, in May, the monthly average air pollution in Alberta was nine times higher compared to the same month in 2022.

And for the first time, Canada surpassed the United States in the regional pollution ranking.

American cities like Minneapolis and Detroit were also impacted by wildfires, leading to annual pollution averages increasing by 30% to 50% compared to the previous year. Columbus, Ohio, retained its position as the most polluted major American city in 2023, marking the second consecutive year.

But major cities, such as Portland, Seattle and Los Angeles, saw significant declines in average annual pollution levels, the report said.

In Asia, however, pollution levels largely rebounded in the region.

The report revealed that China reversed a five-year trend of decreasing pollution levels. Although Chinese cities had previously dominated global rankings for the worst air quality, a series of air cleaning policies implemented over the past decade has led to a reversal of this trend.

A study last year found that as a result of this campaign, the average lifespan of Chinese citizens is now 2.2 years longer. But dense smog returned to Beijing last year, where residents saw a 14% increase in the average annual concentration of PM2.5, according to the IQAir report.

China's most polluted city, Hotan, ranked 14th in the IQAir ranking.

In Southeast Asia, only the Philippines recorded a decrease in annual pollution levels compared to the previous year, according to the report.

Indonesia was the most polluted country in the region, with a 20% increase from 2022. Indonesia, Vietnam and Thailand had cities that exceeded WHO PM2.5 guidelines by over 10 times, according to the report.

Last month, Thai authorities ordered government employees to work from home due to unhealthy levels of pollution in the capital Bangkok and surrounding areas, according to Reuters. On March 15th, Chiang Mai, a tourism hotspot, was the most polluted city in the world, as toxic smog from seasonal agricultural burnings blanketed the northern city.

No information from Africa, South America and the Middle East

The report also highlighted a worrying inequality: the lack of monitoring stations in African, South American and Middle Eastern countries, leading to a lack of data on air quality in these regions.

Although Africa saw an improvement in the number of countries included in this year's report compared to previous years, the continent remains largely underrepresented. According to IQAir, only 24 out of 54 African countries had sufficient data available from their monitoring stations.

Seven African countries were among the new locations included in the 2023 ranking, including Burkina Faso, the fifth most polluted country in the world, and Rwanda, 15th.

Several countries that ranked at the top of the most polluted countries list last year were not included for 2023 due to lack of available data.

Among them was Chad, which was the most polluted country in 2022.

"There is so much hidden air pollution still on the planet", Hammes said.

An encouraging aspect is the growing pressure and active involvement from communities, non-governmental organizations (NGOs), businesses and scientists in monitoring air quality.

"In the end, this is wonderful because it really shows governments that people care", said Hammes.

The complete study can be found here.

In Romania, the cities with the highest pollution are: Bucharest and its surrounding areas (Voluntari, Chiajna, Chitila, Buftea, Popești-Leordeni, Glina, Jilava, Otopeni, Rudeni), as well as Cluj-Napoca, Suceava, Sântana de Mureș (Mureș county), Ojdula (Covasna county) or Popricani (Iași county).