Average concentrations of PM2.5 — small airborne particles that damage the lungs — reached 79.9 micrograms per cubic metre in Bangladesh in 2023, and 73.7 micrograms in Pakistan.

The WHO recommends no more than 5 micrograms.

“Because of the climate conditions and the geography (in South Asia), you get this streak of PM2.5 concentrations that just skyrocket because the pollution has nowhere to go,” said Christi Chester Schroeder, air quality science manager at IQAir, a Swiss air monitoring organisation.

“On top of that are factors such as agricultural practices, industry and population density,” she added. “Unfortunately, it really does look like it will get worse before it gets better.”

In 2022, Bangladesh was ranked as having the fifth-worst air quality, and India was eighth.

About 20 per cent of premature deaths in Bangladesh are attributed to air pollution, and related healthcare costs amount to 4pc-5pc of the country’s GDP, said Md Firoz Khan, an air pollution expert at Dhaka’s North South University.

Indian pollution also increased last year, with PM2.5 levels about 11 times higher than the WHO standard. India’s New Delhi was the worst-performing capital city, at 92.7 micrograms.