BELGRADE (Reuters) - Despite clear skies and sunshine, residents of the Serbian capital have gasped this week in the acrid air, struggling to breathe in a city whose air pollution readings have approached those of Beijing, Delhi, Lahore and Karachi.

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The Air Visual API application, which compiles data from ground sensors worldwide, ranked Belgrade 16th of the world’s most polluted cities on Thursday.

It stood at 5th place a day earlier.

Just outside the city center, thick smoke has billowed from many fields set ablaze by farmers burning weeds and corn stubble due to a common belief that this is beneficial for the soil.

This has added to the fumes from older diesel cars, domestic heating, industry and nearby open coal pits to create a toxic mist hovering over the city of 2 million.

The average car in Serbia is around 17 years old, according to a Belgrade University study.

On Thursday pollution was particularly bad in the historical part of Belgrade which suffers from chronic traffic congestion and in the outskirts - hit by smoke from burning fields.

The parts of the city overlooking the confluence of the rivers Sava and Danube however, enjoyed a respite brought by a fresh breeze.

According to the European Union Environment Agency’s air quality index the air quality in Belgrade’s city center on Thursday was ranked as harmful.

The poor air quality is a stark reminder of how Serbia must spend around 15 billion euros ($16.69 billion) before it joins the EU to comply with the bloc’s regulations about environment and CO2 emissions.

“(People) are using wood and coal for heating, that heating system must be improved somehow and the flow of traffic must be reduced,” said Jovan Tanasijevic, 75, a pensioner from Belgrade.

Aleksandar Macura, an environmental expert with the RES Foundation think-tank, said burning fields were a source of pollution in recent days, but that domestic heating and industry emitted almost three-quarters of polluting particles.

Serbia’s fiscal advisory council has repeatedly warned that the government should begin investing about 1.3 percent of GDP to tackle environmental problems including air pollution and an acute lack of sewage treatment facilities.