High levels of pollution may have contributed to the deaths of thousands of people in England from pneumonia in recent years, a study suggests.
A team at the University of Birmingham examined death rates from the disease and pollution levels in 352 local authorities between 1996 and 2004.
Writing in the Journal of Epidemiology and Community Health, they reported a "strong correlation" between the two.
But the researchers conceded that social factors may also be at play.
Calculations were made by looking at how many deaths there were in each locality in excess of the national average.
These figures were then cross-checked with a range of pollutant levels, including engine exhaust emissions.
In total, 386,374 people died of pneumonia during the eight years examined, but there were significant regional variations. Lewisham in London had the highest number of deaths per head, Berwick-on-Tweed the least.
In the 35 local authorities with the highest rates of pneumonia, there were 14,718 more deaths than the national average.
These areas also tended to see higher rates of some cancers, chronic obstructive pulmonary disease (COPD) and rheumatic heart disease.
"High mortality rates were observed in areas with elevated ambient pollution levels," said Professor George Knox, who wrote the report. "The strongest single effect was an increase in pneumonia deaths."
He added: "Road transport was the chief source of the emissions responsible, although it was not possible to discriminate between the different chemical components".
The team estimated that the annual number of excess deaths - or those which could be attributed to the pollution - could approach those of the 1952 London smog, which killed 4,000 people.
But lung specialists said more detailed research was needed before a clear link between pneumonia and exhaust fumes was declared.
"What this paper does show, however, is that there is clear geographical variation in deaths from pneumonia, lung cancer and COPD," said Richard Hubbard of the British Lung Foundation.
"This would suggest that social factors such as deprivation and smoking, and possibly pollution, are important and that there is great potential to prevent deaths from lung disease."