According to figures compiled by Swiss air quality monitoring firm IQAir, Chad and Bangladesh were the world's most polluted countries in 2024, with average smog levels more than 15 times higher than WHO guidelines.

IQAir said only Australia, New Zealand, the Bahamas, Barbados, Grenada, Estonia, and Iceland made the grade.

Significant data gaps, especially in Asia and Africa, cloud the worldwide picture, and many developing countries have relied on air quality sensors mounted on U.S. embassy and consulate buildings to track their smog levels.

However, the State Department has recently ended the scheme, citing budget constraints. Last week, more than 17 years of data were removed from the U.S. government's official air quality monitoring site, airnow.gov, which opens a new tab and includes readings collected in Chad.

"Most countries have a few other data sources, but it's going to impact Africa significantly, because oftentimes these are the only sources of publicly available real-time air quality monitoring data," said Christi Chester-Schroeder, IQAir's air quality science manager.

Data concerns meant Chad was excluded from IQAir's 2023 list, but it was also ranked the most polluted country in 2022, plagued by Sahara dust and uncontrolled crop burning.

The country's average concentration of small, hazardous airborne particles known as PM2.5 hit 91.8 micrograms per cubic metre (mg/cu m) last year, slightly higher than in 2022.

The WHO recommends no more than 5 mg/cu m levels, a standard met by only 17% of cities last year.

India, fifth in the smog rankings behind Chad, Bangladesh, Pakistan and the Democratic Republic of Congo, saw average PM2.5 fall 7% on the year to 50.6 mg/cu m.

But it accounted for 12 of the top 20 most polluted cities, with Byrnihat, in a heavily industrialised part of the country's northeast, in first place, registering an average PM2.5 level of 128 mg/cu m.

Chester-Schroeder warned that climate change is driving up pollution. Higher temperatures caused fiercer and lengthier forest fires to sweep through parts of Southeast Asia and South America.

Christa Hasenkopf, director of the Clean Air Program at the University of Chicago's Energy Policy Institute (EPIC), said at least 34 countries will lose access to reliable pollution data after the U.S. program was closed.

Hasenkopf said the State Department scheme improved air quality in the cities where the monitors were placed, boosting life expectancy and even reducing hazard allowances for U.S. diplomats, meaning that it paid for itself.

"(It) is a giant blow to air quality efforts worldwide," she said.