Ultrafine particles (UFPs), which are approximately 1,000 times smaller than a human hair, are released during a plane's takeoff and landing.

Their minuscule size means UFPs easily penetrate human tissues, with growing evidence these particles are harmful to people's health.

Yet UFPs remain largely unregulated.

"Tens of millions of Europeans are exposed to increased health risks due to aviation UFPs," said T&E in a report.

"Fortunately, reducing air traffic and improving jet fuel quality can mitigate the problem in the short term, with additional climate benefits," the NGO said, calling for better monitoring and UFP reduction targets.

The Brussels-based NGO analysed UFP concentration levels around Amsterdam-Schiphol airport based on data collected by the National Institute for Public Health and the Environment of the Netherlands (RIVM).

T&E then extrapolated the findings to Europe's 32 busiest airports, assuming that UFP pollution grows with air traffic and is evenly spread around each airport.

It found that 52 million people living in a 20-kilometre radius around the airports are at risk of serious health conditions because of high UFP concentration levels.

In a five-kilometre radius around Amsterdam-Schiphol airport, RIVM researchers found UFP concentrations "between 4,000 to 30,000 particles per cubic centimetre (cm3)," according to T&E.

In city centres, UFP concentration reached between 3,000 and 12,000 particles per cubic centimetre, highlighting "the important contribution of airports to UFP pollution," T&E said.

In February, Airparif, which monitors air quality in the Paris region, recorded UFP concentrations of 23,000 per cm3 at Paris Charles de Gaulle airport.

The watchdog found excessive UFP concentrations associated with air traffic were most notable within five kilometres of the airport, but were outstripped by other sources of the particle beyond 10 km.