These ultrafine particles, or nanoparticles, are released from a wide range of pollution sources, including vehicle exhausts, brake systems and industry. The scientists warn that exposure to these metal-rich particles appears to be directly associated with early and significant heart inflammation and cardiac damage.

“It's especially striking that these findings come from very young children and young adults, indicating how early this significant heart damage is occurring, and how it is then likely that further and greater heart damage will occur in later life,” said one of the lead scientists, Professor Barbara Maher of Lancaster University in the northwest of England.

The researchers from Mexico, the U.K. and U.S. examined the hearts of 72 people aged between three and 32. A total of 63 of the hearts were taken from residents of the metropolitan area of Mexico City, an area with nearly 25 million inhabitants, more than 50,000 industries, and 5.5 million vehicles.

The 63 adults and children had been clinically healthy but died in road traffic accidents. Their hearts were compared against those of nine people who had lived in unspecified areas elsewhere.

The study published in the peer-reviewed journal Environmental Research found the hearts of the Mexico City residents contained up to 22 billion magnetic nanoparticles _ or two to 10 times more ultrafine particles than the control subjects.

“It seems that such particles can very readily transfer from the lungs, into the bloodstream and then into the heart, and into the sub-cellular structures, such as the mitochondria, which are key for supplying the energy needed for the heart muscles to pump,” Professor Maher told AirVisual News.

Furthermore, they were present in the mitochondria of the youngest heart examined – that of a 3-year-old boy.

The researchers say the study “substantially advances the case for global efforts to reduce exposure to particulate matter air pollution,” and offers a case for regulating ultrafine particles.

Professor Lidia Morawska, director of the International Laboratory for Air Quality and Health at the Queensland University of Technology, and an advisor to the World Health Organisation, said the study supports other work linking dirty air to health impacts.

Professor Morawska, who was not connected to the study, said it “has added to the body of literature strongly linking exposure to air pollution and health, and has provided new evidence for Mexico city and on the aspects of air pollution rarely measured (nanoparticles).

“Therefore it provides further support for the urgent need to mitigate air pollution,” she said.

Currently, countries including the U.S. regulate fine particles such as PM2.5, which are already associated with short and long-term health risks, including an increased risk of premature death from heart and lung disease. PM2.5 have a diameter of less than 2.5 microns, or a thirtieth of a human hair, and when breathed in can be absorbed into lung tissue and pass from the lungs into the bloodstream.