It has taken a quarter of a century, but US researchers say their work has finally enabled them to determine to what extent city air pollution impacts on average life expectancy.
The project tracked the change of air quality in 51 American cities since the 1980s.
During that time general life expectancy increased by more than two and half years, much due to improved lifestyles, diet and healthcare.
But the researchers calculated more than 15% of that extra time was due to cleaner air.
"We think about five months of that is due to the improvement of air quality," said Dr Douglas Dockery, head of the Environmental Health Department at Harvard School of Public Health in Boston, which undertook the research.
He added that, due to the relatively clean air in the US, the impact was far larger than anticipated.
Dr Dockery said there were many factors which had an impact on life expectancy.
But he added: "Clean or dirty air is something that is being imposed on you.
"You do have a choice on whether you smoke, drink, exercise or what type of food you eat. But you do not have a choice on what air you breathe."
Dr Dockery believes that if his research was transposed onto the heavily polluted cities of the developing world, such as Beijing or Mexico City, the life expectancy impact would be far greater.
"We would be talking about several years," he said. "Three to four years - a significant change in how long you live for.
"We looked at fine particles that penetrate deep in the lungs, those that are not caught in the nose and the mouth, and directly damage the blood vessels. Most of those come from combustion, from automobiles, diesel trucks and buses and power plants."
Dr Dockery hopes his findings will encourage governments to work towards making air even cleaner over the next 25 years.
Even in Boston, which has comparatively clean city air, pollution levels change suddenly from being safe to highly dangerous.
Bruce Hill, a scientist with the Clean Air Task Force, measured two sets of pollution levels.
One was on a bridge over a highway with only cars and the other over a highway with diesel-powered trucks.
"Just now that truck passed and the levels spiked up to five times higher than they were in the rest of the city," said Hill.
"Now, see, it's gone 25 times higher." From there he went down onto the underground platform of a commuter train station.
"This is bad," he said. "The monitors can't go any higher, meaning the level here could be a hundred time higher than the cleaner air outside.
"Some people commute for five per cent of the day, which is the amount of time they're being exposed to these particles."
Comparable to smoking
Hill describes the damage caused by regularly breathing such air as like living with someone who smokes.
In the long term, he argued, it can cause cancer and cardio-vascular problems. In the short term, it can create asthma attacks and allergies.
Cait Maas, who already has a respiratory problem, lives in an apartment that looks out on a shipping terminal, an oil depot and a multi-lane highway.
A main road near her home is a key route for diesel trucks.
"On a bad day, I can taste the particles. I feel them constricting my airways and I have to cover my mouth so that I can breathe."
Over the next generation, however, it's expected that pollution, especially that created by dangerous diesel particles, will be cut dramatically.
Standard filters are now being fitted to buses. Bio-fuels and cleaner energy in general, brought about by climate change pressures, will make the air safer.