According to AZO Cleantech, road traffic is the main source of noise in these cities.

The World Health Organization published some noise-level guidelines, and if they are respected, more than 3.600 deaths caused by ischaemic heart disease could be prevented.

Traffic noise has other unwanted health effects, such as sleep disturbance, annoyance and poor mental health.

Long-term exposure to high traffic noise levels can have more severe outcomes through the release of stress hormones, which increase the heart rate and blood pressure and can lead to depression and anxiety.

Road traffic noise exposure was estimated with the help of noise maps from countries and cities that fall under the European legislative framework, and the results show that nearly half of the 123 million adults from the study were exposed to noise levels that are higher than the recommended levels.

The WHO recommends that the average noise level recorded over 24 hours shouldn't be higher than 53 decibels.

Not all European capitals are the same, however, with cities like Berlin exposing less than 30% of the population to higher than recommended levels of noise, while in Vienna, over 86% of the citizens are exposed to noise level higher than the levels recommended by the WHO.

The study was able to determine that over 11 million people in the documented cities were annoyed by traffic related noise.

They described that the annoyances came when doing daily activities, such as communicating, reading or sleeping.

Sasha Khomenko, researcher at ISGlobal, said that "the available data have only allowed us to analyze the population exposed to more than 55 dB Lden, whereas the WHO-recommended threshold is 53 dB Lden, and we suspect that adverse effects could occur even with exposure to lower noise levels."

Moreover, the team had difficulties when it came to interpreting the data due to the poor quality of some of the maps they went through, and in fact less than 17% of the researched maps ended up being good quality.

The study was part of the European Urban Burden of Disease Project, which analyzed in the past mortality related to air pollution and green space in European cities.

The traffic related noise level from this particular study couldn't allow for a ranking to be made, because of the methodological inconsistencies generated by the poor quality of some maps of the 749 European cities.