Low levels of exposure to air pollution can still increase the risk of dying early, a study says.
Cars are a major contributor to air pollution
The Imperial College London-led team looked at death rates between 1982 and 1998, comparing them to exposure to black smoke and sulphur dioxide.
Although air pollution had fallen over the decades, the risk of an early death remained, the study of 5,000 UK people in the Thorax journal found.
Experts suggested that pollution may be becoming more toxic.
The researchers looked at death rates during four different time periods - 1982-86, 1986-90, 1990-94 and 1994-98.
They also looked at the exposure to air pollutants by analysing national air quality registers in the 16 years prior to these periods.
They found the levels of black smoke - unburned carbon from traffic and industrial processes - were five times lower for the most recent time period compared to the first one.
Sulphur dioxide was four times lower, the study found.
But the researchers said the risk of death remained fairly constant.
During the four time periods, the increased risk of death for those living in areas with higher levels of pollution than average hovered at or below 10% for both pollutants.
And the risk of early death from respiratory disease actually increased from below 10% in the first time period to 19% for black smoke, and from just above 10% to over 20% for sulphur dioxide.
The figures held true even after adjusting for factors known to increase the chances of an early death, such as social deprivation.
Lead researcher Professor Paul Elliott said: "The findings point to continuing public health risks even at the relatively low levels of black smoke and sulphur dioxide that now occur.
"It is not clear why the risk of early death remains, it could be because the cause of the pollution has changed and it is becoming more toxic."
Dr Richard Russell, of the British Thoracic Society, added: "This is a very interesting paper. There are a number of possible explanations for what is reported.
"There is evidence to suggest the nature of the pollution has changed so that despite levels declining it is more harmful.
"It may also be related to our reaction to the pollution, perhaps over the last 40 years we have become more susceptible.
"There may also be another factor that we simply don't know about. What we need is more research in this area."