A new study reveals that even if the air quality levels satisfy the EU standards, prolonged exposure to the particulate pollution could be enough to increase the risk of premature death.
Published in The Lancet, the paper pointed the finger at fine particles of soot and dust, emissions of which are also stirring a health scare in parts of Asia, especially China.
Scientists led by Rob Beelen of Utrecht University in the Netherlands looked at 22 previously-published studies that monitored the health of 367,000 people in 13 countries in western Europe.
The individuals, recruited in the 1990s, were followed for nearly 14 years. During the time of the study, 29,000 people died, according to the data.
Beelen's team went around to all the study areas to get readings of traffic pollution between 2008 and 2011.
They used these as a basis for calculating the long-term exposure of local residents to two kinds of particulate matter and to two kinds of gas emissions.
They took into account factors such as smoking habits, socio-economic status, physical activity, body-mass index and education that can skew the results.
The biggest source of concern was PM2.5, meaning particles measuring under 2.5 microns, or 2.5 millionths of a metre, across, they found.
Previous research has found PM2.5 is so small that it can lodge deep in the lungs, causing respiratory problems, and may even cross over into the bloodstream.
The risk of early death rose by seven percent with every increase of five microgrammes of PM2.5 per cubic metre, the new study found.
"A difference of five microgrammes can be found between a location at a busy urban road" and a quiet street, Beelen said.
European Union guidelines set down maximum exposure to PM2.5 of 25 microgrammes per cubic metre.
But even in locations where the pollution levels were well below this, there were still higher-than-normal cases of early death.
In an email exchange with AFP, Beelen said that the loss of life expectancy through background exposure to PM2.5 was likely to be "up to a few months".
"Although this does not seem to be much, you have to keep in mind that everybody is exposed to some level of air pollution and that it is not a voluntary exposure, in contrast to, for example, smoking."
The study -- the first of its kind in Europe -- reflects similar findings in investigations in North America.
There was one important difference, though: exposure to PM2.5 was linked to mortality in men, but not in women.
Beelen said the work adds weight to the argument that the EU should toughen up its air pollution standards and adopt the World Health Organisation (WHO) guidelines of 10 microgrammes per cubic metre.
In October, the European Environment Agency (EAA) said that urban levels of PM2.5 had fallen by 16 percent between 2002 and 2011, but many people still lived in areas where exposure breached both EU and the tougher UN marks.
Shanghai on Friday became the latest Chinese city to undergo a pollution alert, with concentrations of PM2.5 24 times higher than UN levels.
Beelen said that such levels were far higher than those in Europe, but he could not venture an opinion as to how dangerous they were.
"Our study focuses on long-term exposure... exposure to daily variation with sometimes high peaks is another research question."
The WHO's International Agency for Research on Cancer (IARC) in October classified outdoor air pollution as a leading cause of cancer, placing it in the riskiest of four categories of sources.
A worldwide study called the Global Burden of Disease found that outdoor air pollution was to blame for 3.2 million deaths per year.