Scientists do not yet know why, but one theory is that tiny particles and chemicals in polluted air absorbed into the body through the lungs cause damage or inflammation in the brain. The King's College London research found that Londoners who lived in the worst areas in the city for fine particles (known as PM2.5) were 20 per cent more likely to develop dementia within seven years, while those who lived in the worst areas for nitrogen dioxide (NO2) were 40 per cent more likely to develop it, even after accounting for age, class and other lifestyle factors.
It follows a Canadian study of 2.2 million people, which last year found those who lived next to a busy road were 12 per cent more likely to develop dementia.
And earlier this year, a Chinese study suggested that air pollution leads to a decline in brain power, or cognitive function. Researchers who tested people's maths and verbal skills found their average scores decreased over three years of exposure to pollution.
Asthma
Studies have suggested children who are exposed to air pollution at an early age are more likely to develop asthma and lung infections, which can be fatal.
"Air pollution is detrimental to all health, but it can have major implications on the developing child," says Professor Jonathan Grigg of the Royal College of Paediatrics and Child Health.
But pollution has not just been linked to causing asthma - it worsens the condition, too.
Children are most at risk.
A recent report by Global Action Plan on Clean Air Day found that primary and pre-school-age children were exposed to up to 30 per cent more pollution than adults simply because they were closer to the fumes from car exhausts.
Heart attacks and strokes
Breathing in polluted air over many years can cause arteries to become furred up or narrowed, a condition known as atherosclerosis, which can lead to heart attacks or strokes.
People are more likely to suffer heart attacks in the hours after having been exposed to a lot of traffic fumes, according to data from the Myocardial Ischaemia National Audit Project. Studies on children in Europe and Mexico show that those exposed to high levels of fine particles and NO2 have higher blood pressure by the age of 12.
Dr Gary Fuller, of King's College London, says: "What you're breathing in might not just affect you today or tomorrow or over the next year, it might actually cause some permanent, life-changing damage now which might only manifest itself in decades to come."
Children's development
Children can be heavily affected by pollution and the problems it causes can begin in the womb, research increasingly suggests.
Soot particles were found inside the placentas of five women who gave birth to healthy babies at the Royal London Hospital, according to a study published this year. The tiny bits of carbon - typically created by burning fossil fuels - had been breathed in by the pregnant mothers and travelled through their bodies into their placentas.
Scientists don't yet know if the particles can reach babies in the womb, but research has found links between pregnant women exposed to air pollution and problems such as premature births, low birth weights, unexpected deaths of their babies and lung problems in childhood.
Fertility
Pollution can cause problems even before a child is conceived. Doctors suspect pollution is having an effect on sperm quality - the number of sperm cells that are the correct size and shape to fertilise an egg.
A recent study of 6500 Taiwanese men showed exposure to fine particles was linked with a rise in sperm abnormalities. It found that for every increase in the level of pollution particles in the air of 5 micrograms per cubic metre, the number of normal sperm produced by the men dropped by 1.29 per cent.
An earlier Italian study found the quality of sperm of men who worked on motorway tolls - with a high amount of exposure to traffic fumes - was significantly lower than that of men who worked away from busy roads.
